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Kazakh – British Technical University
UDС 541.13; 546.22; 546.221; 621.2.035.2 Retaining manuscript rights
TOKTAR GULMIRA
Obtainment inorganic compound of sulfur by electrochemical methods
6D072000 – Chemical technology of inorganic compounds
Dissertation for the degree of
Doctor of Philosophy (PhD)
Supervisor: the winner of the state
prize of the Republic of Kazakhstan
Corresponding Member National
Academy of Sciences.,
d.c.s., Professor Bayeshov A.B
Scientific adviser: Director Faculty of
Science and Engineering,
Manchester Metropolitan University, UK
PhD, Professor Graig E. Banks
Republic of Kazakhstan
Almaty, 2016
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CONTENT
DEFINITIONS.........…………………………………………………………... 4
SYMBOLS AND ABBREVIATIONS................................................................5
INTERDUTION..................................................................................................6
1 SULFUR AND ITS COMPOUNDS PRESENT ISSUES, PHYSIC-
CHEMICAL PROPERTIES AND THEIR IMPACT ON THE
ENVIRONMENT……………………………………………………………………9
1.1 Physic-chemical properties of sulfur and its compounds................................9
1.2 The distribution sulfur in the nature and its application.................................14
1.3 Electrochemical properties of sulfur and its compounds...............................14
1.4 Е-pH diagram of sulfuric- water system.........................................................25
1.5 Sulfur content Oil and sulfur-containing waste sources impact on the
environment.................................................................................................................32
2 EXPERIMENTAL METHODS AND IMPLEMENTATION OF
TECHNOLOGY........................................................................................................35
2.1 Methods of potentiodynamic polarization curves recording...........................35
2.2 Preparation of current conductor sulfur-graphite composite electrodes and the
methodology of electrolysis.........................................................................................37
2.3 Used reagents, drugs and analysis of the obtained products..........................41
2.4 Determination of sulfur-containing compounds by physico-chemical
methods........................................................................................................................42
3 ELECTROCHEMICAL PROPERTIES OF ANODE POLARIZED
SULFUR ELECTRODE’S IN THE SOLUTION OF HYDROCHLORIC
ACID………………………………………………………………………………...48
4 ELECTROCHEMICAL PROPERTY OF SULFUR IN SODIUM
CHLORIDE, AND CARBONATE SOLUTION…………………………………...52
4.1 Dissolution of anode polarized elemental sulfur in sodium chloride solution.52
4.2 Electrochemical properties of dissolution elemental sulfur by formation of
sulfate-ions in sodium carbonate solution....................................................................55
5 ELECTROCHEMICAL PROPERTIES OF ELEMENTAL SULFUR IN
ALKALINE SOLUTIONS.........................................................................................59
5.1 Cathodic electrochemical property of elemental sulfur dissolved in sodium
hydroxide solution.......................................................................................................59
5.2 In alkaline solution dissolved elemental sulfur oxidation with the formation of
sulfate ions...................................................................................................................62
5.3 Investigation of electrochemical properties of elemental sulfur preliminary
dissolved in alkaline solution by recording the anodic and cathodic potentiodynamic
polarization curves.......................................................................................................67
5.4 Obtainment of monosulfide and investigate its electrochemical properties by
unloading polarization curves......................................................................................71
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6 RECEIVE OF COPPER SULFIDE AND ITS ELECTROCHEMICAL
BEHAVIOR...............................................................................................................83
7 CREATION OF CHEMICAL POWER SOURCE BY USING
OXIDATION REACTIONS ON THE SULFUR COMPOSITION
ELECTRODE............................................................................................................89
7.1 The regularities of formation of motive force in the galvanic pair of "sulfur-
graphite" - "lead dioxide".............................................................................................89
CONCLUSION..................................................................................................95
REFERENCES..................................................................................................97
ADDITIONAL A -Auther Certificate………………………………….......106
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DEFINITIONS
In this dissertation following terms and definitions are used:
Anode – electrode in an electrochemical cell on which the oxidation reaction
occurs.
Cathode– electrode in an electrochemical cell on which the reduction reaction
occurs.
Electrochemistry– a field of chemistry that focuses on the interchange between
electrical and chemical energy.
Sulfide – an inorganic anion of sulfur with the chemical formula S2−. It
contributes no color to sulfide salts. As it is classified as a strong base, even dilute
solutions of salts such as sodium sulfide (Na2S) are corrosive and can attack the skin.
Sulfide is the simplest sulfur anion.
Electrolysis – the decomposition of a substance by means of electric current. This
method pushes a redox reaction toward the non-spontaneous side.
Electrolytic cell – electrochemical cell that is being pushed toward the non-
spontaneous direction by electrolysis.
Electromotive force, EMF (or cell potential) – difference of potential energy of
electrons between the two electrodes.
Flotation agents– reagents that selectively adsorbed on the mineral surface,
which must be translated into a lather, and giving the particles hydrophobic properties
Oxidation– lose of electrons, can occur only in combination with reduction.
Reduction– gain of electrons, can occur only in combination with oxidation.
Redox reaction– shorthand for reduction-oxidation reaction.
Voltaic cell or galvanic cell– an electrochemical cell that uses redox reaction to
produce electricity spontaneously
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SYMBOLS AND ABBREVIATIONS
Іa – Anodic current;
Іc – Cathodic current;
I – Current, А
J – Current density, А/m2;
Еа – Anodic potential, V;
Еc – Cathodic potential, V;
ƞ – Current efficiency, %;
t – Electrolyze time, hour;
С – Solution concentration;
V – Scan rate, mV/sek;
Еа – Activation energy, kJ/mol;
E⦵ – Standard electrode potential, V;
Е – Electrode potential, V;
T – Thermodynamic temperature, К;
R – Universal gas constant;
XR – X-ray analysis;
IR – Infrared spectra;
mt – Theoretical mass;
F – Number of Faraday;
n – Number of electrode;
k – Electrochemical equivalent, mEq/l;
mp – Practical mass, g.
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INTRODUCTION
The actuality of work. All countries, including our republic, there are many
problems in the field of full and effective use of natural resources and the application
of wastes as a raw material.
Most of the Kazakhstan oil is considered that possesses medium or high-sulfur
content. Large quantities of by-product sulfur are being produced as a result of the
removal of hydrogen sulfide from the oil and gas produced in the region. And also the
elemental sulfur which accumulates in the territory during the purification is as
agarbage. At the same time, sulfur compounds contain fuels and lubricants impact on
corrosivon exposure of internal combustion motors, to reduce its capacity and the
cleanliness of the environment. This is not only on the territory of our country, but also
causing global environmental problems all aroung the world. Accumulated elemental
sulfur in the oil and gas industry of the republic is not being found fully used. Sulfur is
used in the production of 30 thousand names of production. This phosphate fertilizer,
paper, rubber, asphalt, paint, textiles, plastic and even cosmetics. It is also used in the
nuclear industry, through the production of sulfuric acid, which is used for leaching of
uranium ore. It should also be noted that one of the indicators of industrialization of
any country is the production of sulphuric acid. In this regard, to create simple methods
to obtain variety sulfur compounds necessary for the production is a very important
and one of the actual problems. A detailed knowledge of the physical and chemical
properties of sulfur is essential in the theoretical basis to produce optimized sulfur
compounds
The objective of work and tasks. The purpose of the work: Comprehensive study
on electrochemical properties of polarized sulfur in alkaline, acid and neutral mediums
and to investigate synthesis methods of important inorganic compounds.
Depending on the purpose of the work is expected to accomplish the following
tasks:
- to study the effect of main electrochemical paremeters(current density,
electrolyte concentration, electrolysis duration) for the anodic dissolution of sulfur
composite electrode during the polarization;
- depending on the state of the electrochemical polarization, to determine the
current output of generated electrolysis products;
- to investigate electrochemical properties of polysulfide solution obtained by
elemental sulfur preliminarily dissolved in alkaline solution by the method
ofelectrochemical polarization and removing the anodic and anodic-cathodic
potentiodynamic polarization curve;
- to obtain sodium monosulfide and study its electrochemical properties by
unloading of potentiodynamic polarization curves;
- to receive copper sulfide and to study the influence of various parameters
(current density, electrolyte concentration, electrolysis duration) for the formation of
copper sulfide;
- to study the possibilities to use sulfur-graphite composite electrode as a negative
electrode of galvanic element on the basis of its oxidation reaction.
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The objective of study. To propose a new method to obtain inorganic sulfur
compounds by using electrochemical proces.
Connection with the plan of basic scientific research. The dissertational work
is carried out as a part of a research project of 2012-2014 y., funded by the Committee
of Science of Ministry of Education and Science of RK «By electrochemical
technology processing to obtain sodium sulfide from the sulfur waste» and 2015-2017
y., «Development of electrochemical technology to producing a new generation of
modern flotation reagent – calcium sulfide and luminophores – sulphides of zinc and
cadmium from sulfur with manufacturing and the creation of its pilot installation»
(Electrochemical labarotory at Institute of Fuel, Catalysis and Electrochemistry after
named D.V Sokolsky)
Scientific novelty of the thesis. – In this thesis for the first time, anodic oxidation
of elemental sulfur in alkaline, acidic and neutral medium investigated
comprehensively by using specially prepared electric conducted composite sulfur-
graphite electrodes;
- for the first time, the results of the analysis by the method of IR spectroscopy
for the product was given which formed by dissolving elemental sulfur powder in a
solution of sodium hydroxide, this obtained product’s the formation regularities of
sulfate ions on the anode side and sulfide ions on the cathode side were studied by the
method of electrolysis in the electrode space allocated electrolyzer;
- electrochemical properties of preliminary elemental sulfur dissolved in a
solution of sodium hydroxide electrolyte for the first time investigated by removing the
anodic and anodic-cathodic potentiodynamic polarization curves, the result shown that
oxidation of polysulfide ions to elemental sulfur was stage process;
- proposed a new the method of obtaining sodium monosulfide and its
electrochemical properties for the first time researched by unloading potentiodynamic
polarization curves, and identified that oxidation of monosulfide ions to elemental
sulfur was complicated process and which carried out in diffusion mode;
- a new and simple method to get copper sulfide powder was proposed, for this
powder were done X-ray and elemental analysis, as a result of elemental analysis
identified that the composition of the powder contained 63.63% copper and 31.03% of
the sulfur;
- the possibility to use composite sulfur- graphite electrode as negative electrode
were considered during the obtainment chemical power, there "sulfur-lead dioxide "
galvanic element be provided 1050 can mV of electromotive force;
- for the first time shown that elemental sulfur can be used to get electric current.
The novelty of this method was protected by innovation patent of RK №. 31177.
The theoretical value and practical significance. According to the carried out
research works results for the frist time shown that from sulfur can synthes in the
national economy widely used inorganic compounds of sulfur which in the Republic
of Kazakhstan as waste have been delisted into the environment. At the same time was
shown that elemental sulfur can be used to obtain electric current.
Accuracy of the results and conclusions which given doctoral thesis. Research
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work modern, high-precision measuremention by the methods of I-160 IM, photo
calorimeter, Autolab PGSTAT 302N (potencostat galvanostat) installations and
physico-chemical methods (IR) and element analysis are confirmed and formulated
electrochemical kinetic calculations.
Personal contribution of the author. In the work carried out in collaboration
and included in the thesis is to carrying out research and analysis of literature data, the
experimental solution of set tasks, interpretation and generalization of the results and
publication of research results in the form scientific conferences and scientific papers.
Approbation of practical results of the work. The main results of work were
presented at the following international conferences, seminars and forums: "Innovative
development and relevance of science in modern Kazakhstan" VIII International
Scientific Conference (Almaty, 2014), "International Conference on Chemical
Engineering and Advanced Materials" (China, 2016), "The new science: from idea to
results" international Scientific Conference (Russia, 2016).
Publications. The main content of the dissertation was published in 11 works in
the open press, which include:
- 1 inovation patent Republic of Kazakhstan;
- 6 articles published in journals recommended by Committee on Control of
Education and Science of the Ministry of Education and Science of Republic of
Kazakhstan;
- 1 articles published in international journals having non-zero impact factor
included into the Scopus databases;
-3 materials and theses in international, national, scientific meetings and
conferences (one of them is registered into the Thomson Reuters and Scopus
databases).
Content and structure of the dissertation. The thesis consists of introduction,
7 chapters, conclusions, list of literature sources. In the thesis, consisting of 106 pages,
including 60 figures and 6 tables. List of bibliographic literature consists of 149
domestic and foreign literature sources.
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1 SULFUR AND ITS COMPOUNDS PRESENT ISSUES, PHYSIC-
CHEMICAL PROPERTIES AND THEIR IMPACT ON THE ENVIRONMENT
Now a day, in Kazakhstan oil and gas industry, not only increased in the direction
of oil and gas production, but also towards the creation and implementation of
innovative technologies for the processing of its waste have been developped. Sulfuric
acid is charged from sulfur, through the acid could carried out the process of leaching
of important elements to the solution from the ore, fertilizers and explosive substances
are taken. Many of the world's oil processing factories the huge amount of elemental
sulfur are collected as an additional product [1].
Production and processing of sulfuric oil and gas condensate raw materials, as
well as, by increasing of deep cleaning of sulfur from oil, elemental sulfur’s formation
size is grown sharply. That is why, to find ecological clean and safe way for long-term
storage of accumulated sulfur and to obtain its important compounds are today’s issue
[2].
There are some disadvantages of modern technologies of processing of generated
wastes during the oil and gas production, including an undevelopped domestic
technologies on profoundly cleaning of sulfur from oil; application of imported
catalysts; modern experimental installations used less for industrial research. One way
to solve the caused problem– to use high technologies for oil and gas processing,
including the development of technologies for the processing of sulfur which is the
most extensive residual was occurred during oil and gas production [3].
Understanding of physical and chemical properties of sulfur, knowledge of
molecular and crystal structure, which is very important to create its various industrial
processing technologies.
In order to solve this serious problem, necessary to comprehensively study
properties of elemental sulfur and its compounds. As well as, causes the necessity
research of electrochemical properties of sulfur and its compounds. Based on the
investigation of anodic-cathodic reation mechanisms accompanied on the sulfur
electrode are leads to determine the possibilities of creation new methods.
1.1 Physic-chemical properties of sulfur and its compounds
Sulfur (sulfur) is chemical element in VI group on the periodic system with
symbol S and atomic number 16 and atomic mass 32.06. Sulfur has 25 known isotopes,
four of which are stable 32S(95.084%), 33S(0.74%), 34S(4.16%), 36S(0.016%) [4]. These
electrons are divided into three layers, electronic formula of sulfur: 1s22s22p63s23p4.
Sulfur II, IV, VI can be valent, and oxidation state of sulfur - 2, 0, +4 and +6.
Radius of sulfur atom 0.104 nm. Ions of radius 0,170 nm (coordination number equal
to 4). Energy of sulfur atoms from S0 - to S6+ (10.36, 23.35, 34.8, 47.3, 72.5 and 88.0
eV). Elemental sulfur occurs naturally as the element (native sulfur), but most
commonly occurs in combined forms as sulfide and sulfate minerals. The amount of
sulfur weight on the earth 0.05%, on the sea water 0.08-0.09% [5,6 ].
Physical properties of sulfur. Elemental sulfur is a bright yellow crystalline solid
at room temperature. hydrophobic sulfur insoluble in water, soluble in benzene, carbon
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and that is fragile solid. It melts at 119 °C, boils at 444.6 °C. External valences layers
of sulfur atom have two independent electronic that is why they can communicate. The
nature could occurs in the form free mode and minerals of sulfide (pyrite, galena,
antimonite, etc.), sulfate (gypsum, anhydrite, barite, mirabilis, etc.). Several crystalline
forms of sulfur has been known, including stable form are rhombic α-sulfur and
monoclinic β-sulfur. The density of sulfur is about 2 g·cm−3 for example which twice
heavy than water [9]. Poor conducted heat and current, thermal conductivity is 0.208
W / (m • deg). The number of atoms in the molecule sulfur gradually reduced when
heated sulfur: S8→ S6→ S4→ S2, finally, at temperatures above 2000 °C, sulfur steam
appears in the individual atoms. Allowed to cool this process reversed and occurs the
polymerization phenomenon [10].
According to the scientific work [11] were mentioned that allotropic states of
sulfur allocated three groups: cyclo-octasulfur, polymeric sulfur, intermediate sulfur.
Including well-known crystal allotropic modifications of cyclo-octasulfur are rhombic
S (to 95.62 оС) and monoclinic -β (95.6-119.3 оС). Rhombic modification of sulfur
transferred to monoklinge release heat as follows:
Sα → Sβ + 2,4 kcal. (1.1)
Three modifications of liquid state sulfur are known S S + S 12, if cool it
down quickly, metastable modification may formatted. Depending on the ways to
obtain solid sulfur permanent on a metastable state micro impurities basis may be
consist eight ring S8 cycles modifications. Between them poor connections is
determined low electrical conductivity of sulfur.
Chemical properties of sulfur. Sulfur in their compounds shows from -2 to +6
oxidation degrees. Sulfur reacts with nearly all the elements exception of gold,
platinum, iridium, nitrogen, tellurium, iodine and the noble gases [13-16], as well as
the reactions metals are released a very large amount of heat. With oxygen formatted
several oxide [17].
Redox potentials of sulfur passage from one state to another, Depending on the
medium are equal following values:
S-2 → S0 → S+4 → S+6 (1.2)
Acidic medium, В: +0.14, +0.15, +0.17 (1.3)
Alkaline medium, В: -0.48, -0.61, -0.91 (1.4)
Sulfur insoluble in aqueous medium, but its same modification soluble in organic
solution (toluene, benzene) and carbon sulfur, liquid ammonia NH3.
Colorless and pungent smell of sulfur oxides is obtained by burning of sulfur in
the air:
S + O2= SO2 (1.5)
Basically, spectral analysis identified that the process of oxidation of sulfur to
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sulfur dioxide was serial and proceeds with generating of intermediate products (sulfur
oxide S2O2, molecular sulfur S2, free sulfur atoms S and free radicals of sulfur oxide
SO).
Sulfur plays a role of reduction with interaction of other non-metallic:
S + 3F2= SF6 (1.6)
On Harold work was shown that interaction of sulfur alloy with chlorine formed
sulfur dichloride and dithiodichloride 18:
2S + Cl2= S2Cl2 (1.7)
S + Cl2= SCl2 (1.8)
When heated sulfur reacts with phosphorus formed P2S5 phosphorus sulfide 19:
5S + 2P → P2S5 (1.9)
Moreover, when heated by adding hydrogen obtained H2S hydrogen sulfide and
(H2Sn) sulfan:
H2 +S = H2S (1.10)
Sulfur could react with many metals by releasing high amount of heat, for
example: iron, copper, etc. sulfur reduced by reacting metals formed their sulfides [20]:
Fe + S = FeS (1.11)
2Na + S = Na2S (1.12)
Obtained sulfides do not stable, in contrast, composition can be variable. As well
as the composition of the calcium sulfide changed from CaS to CaS5. And polysulfides
obtained in the form of CaSn and Na2Sn, and sulfan is formed by interaction of
hydrochloric acid (value of n can be between 1-10).
Sulfur dissolved in concentrated nitric acid, and concentrated sulfuric acid
changed sulfur (IV) oxide [21-24]:
S + 6HNO3 = H2SO4 + 6NO2 + 2H2O (1.13)
S + 2H2SO4 = 3SO2 + 2H2O (1.14)
And liquefied nitrogen, hydrochloric and sulfuric acids do not react with sulfur at
room temperature, elemental sulfur disproportioned alkaline solution or boiled hot
water.
Research of Shulek [25, 26], between sulfur and boiling water accompanied
hydrolysis reaction by the following formula:
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2S +2H2O →H2S +H2SO2 (1.15)
When the pH ˂ 7, disintegration of H2SO2 the following formula:
H2SO2→SO2+S +2H2O (1.16)
When the pH˃7:
2H2SO2→S2S32−+ H2O+2H+ (1.17)
Near neutral solutions, at temperature to 100 °C and the following reaction
occurred:
4S +3H2O →2 H2S+ S2S32−+2H+ (1.18)
When the temperature is higher than 100 °C, in Bendassoli [27,] work shown that
the reaction of the sulfur with water will be held in two stages, in the first stage occurred
H2S and SO2:
3S + 2H2O ↔ H2S + SO2 (1.19)
In the second stage at 400 °C occurs disintegration of H2S:
H2S ↔ 2H2 + S2 (1.20)
Work [28] was told that the main product the reaction of sulfur with water at
temperature is higher than 260 °C were ions of sulfide and sulfite.
3S + 3H2O ↔ 2H2S + H2SO3 (1.21)
Here is equilibrium concentration of H2S at 25 °C, only 10-7 g-ion/l, experimental
research cause very difficult and does not give a unique information.
According to the data [29] solid sulfur reacts with sodium hydroxide, polysulfide
and sodium thiosulfate reacted as follows:
(2n+4)S + 6NaOH →2Na2Sn+1 + Na2S2O3 + 3H2O (1.22)
Around temperature of 100 °C sulfur will react aqueous solution of sodium
hydroxide and disproportionated [30]:
1/2S8 +4OH- → S2 +2HS- + 3H2O (1.23)
In this direction, Schuler and Keresh were contributed a lot and [25, 31] work
shown disproportion reaction of sulfur and sodium hydroxide occurs by following:
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2nS +6OH- → S2O32−+ 2S𝑛−1
2− +3H2O (1.24)
By following reaction may formed ions of polysulfide and sulfite:
9S +6OH- → 2S42−+ SO3
2− + 3H2O (1.25)
3S + 6NaOH = 2Na2S + Na2SO3 + 3H2O (1.26)
With increasing of sodium hydroxide concentration the ratio of S𝑛2−/S2O3
2−
decreases. This could shows by following stoichiometric equation:
6S +3OH- →1/2 S2O32− + S5
2− + 3/2 H2O (1.27)
6S +6OH- → S2O32− + S2
2− + 3H2O (1.28)
Sulfur reacts with sulfide could formed polysulfide-ions:
Na2S + (n-1)S = Na2Sn (1.29)
Reacts sulfur with sodium sulphite, a compound of sodium thiosulfate (Na2S2O3)
is formed:
Na2SO3 + S = Na2S2O3 (1.30)
Heated sulfur could reacts with nearly all other elements including gold, platinum,
iridium, nitrogen, tellurium, iodine and the noble gases.
Also many of sulfur oxides are well known. From stable sulfur oxide obtained
(sulfur gaz, sulfur anhydride, sulfur oxide (IV)) and from sulfur trioxide SO3 (sulfur
gaz, sulfur anhydride, sulfur oxide (VI)) were taken except that could got unstable
oxides S2O, (SO2 sent smoldering current) and S8O (hydrogen sulfide (H2S) interact
with SOCl2) are charged. And peroxides (SO4 and S2O7) were formed discharge of SO2
flame by a mixture of oxygen or oxidation of SO2 with ozone. Acidic sulfur dioxide
(SO2) corresponds to the average strength unstable sulfuric acid (H2SO3):
H2O + SO2 → H2SO3 (1.31)
In addition, sulfur trioxide acid, SO3 accordance with the strong sulfuric acid:
SO3 + H2O → H2SO4 (1.32)
As well as sulfur, sulfuric acid corresponds to two series of salts: acide
(hydrosulfides NaHSO3, Ca(HSO3)2 and hydrosulfates KHSO4, NaHSO4 etc.) and
medium (sulfides Na2SO3, K2SO3 and sulfates CaSO4, Fe2(SO4)3).
Low dispertion sulfur at room tempearture could react with alkaline and formed
thiosulfate and thiofulfide.
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Sulfur reacts with many organic compounds such as saturated hydrocarbons. The
reaction of sulfur with olefins very important which used for vulcanization of natural
and synthetic caoutchouc [32].
1.2 The distribution sulfur in the nature and its application. Sulfur was known
in China since the 6th century BC, "pure sulfur" the name as natural mineral means
"combustible stone". In 1777, Antoine Lavoisier proved that sulfur is not compound
just a simple element. Now a days, the sulfur-containing medicines fights against
meningitis bacteria and sulfur containing cream helps fight skin diseases. In 1839,
Charles Goody a mixture of rubber and sulfur spill fire accidentally. Goody received
a "burning" rubber called by the name of Roman God - the owner of the fire Volcano.
In 1867, underground ore sulfur was opened Louisiana and Texas [33].
The prevalence of sulfur in nature took 16th place among all the elements. It
consists in the earth's crust, seawater and even the composition of meteorites [34].
Sulfur occurs widely across the globe and high sulfur oil and gas processing
locations. For example, sulfur could meets in the USA, Canada, as well as Asian
countries. Canada is the largest sulfur exported country and the largest imported
country is china.
In recent years, sulfur reserves in storage has increased gradually. That is, in 1999,
the world warehouses motionless elemental sulfur was 22 million. As well as sulfur
market depending on the type, quality and transfer fee of product. International trade
sulfur, in the last 15 years mainly increased 20% by expense of granulated and liquid
sulfur trading [35].
Today, Kazakhstan reserves of sulfur takes second place in the world. Once upon
a time "Tengizchevroil" united enterprise collected more than12 million ton of sulfur.
By 2020, there is a risk of amount of sulfur increases several times in the Republic of
Kazakhstan [36]. Sulfur mainly used in the chemical industry to obtain sulfuric acid,
as well as paper, rubber, in creation of matches, in textile fabric bleaching, drugs,
cosmetics drugs preparation, plastic, explosive substances, in agricultural crops very
important biogenic substance and widely used for receiving toxic chemicals [37-39].
1.3 Electrochemical properties of sulfur and its compounds
Sulfur dielectric, so they do not carry electrical current. Sulfur is insoluble in
water, so its polarography properties investigated in anhydrous organic solvents [40].
Sulfur is stable oxidizing agents of aqueous and acid solutions. However, it is
unstable in alkaline solution that could disproportionated HS-, S2-, and formed other
oxidant products.
During the electrochemical practices of chalcogenes, the reaction occurs slowly
and only in the presence of heat in concentrated alkaline medium. Alkaline metals and
earth metals soluble in water although same of their sulfides insoluble but they soluble
in various rank of acid medium. The possibility of oxidation of sulfur was determined
by the method polarography. Sulfide ions electrochemical oxidized in aqueous
solutions depends on their experimental conditions the formation possibility of
elemental sulfurs – polysufide, sulfate, thiosulfate and dithionite were identified.
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Polarography condition thiosulfate, sulfoxides and sulfon are considered organic active
compound.
Electrochemical properties of elemental sulfur with formation of polysulfide
anions is provided comprehensively in organic solvents (DMF, DMA, DMSO, CH3CN,
etc.) and mmonium liquids [42].
Interpretation of this process significant results achieved dimethylformamide
(DMF) solution [43]. Electrochemical properties of sulfur investigated in alkaline
medium (NaOH + H2O solution) [44].
In sulfur-sulfide system’s electrochemical properties research, the main work
directed on to receive chloride sulfur-based batteries in sulfur-polysulfide and chloride
alloys. Because of poorly dissolution of elemental sulfur in water, to study its
electrochemical reduction do not allowed in aquatic medium. However,
electrochemical regularities of sulfide (HS- , S2-) and polysulfide ions in aqueous
solution in less studied than anhydrous solution.
Anyway, A number of carried out research to study electrochemical behaviours
of sulfur aqueous sulfur-sulfide system, sulfur redox regularities were considered by
absorption of sulfur and its compounds on variety of substrate electrodes and on the
basis of formation of sulfur compounds in solution [45-47].
Electrochemical properties of sulfur-polysulfide are widely studied in organic
solution and same of salt alloys.
Electrochemical properties of sulfur and polysulfides studied in ionic liquids. On
authors’ [48] work studied in details on reduction of sulfur and proved that reduction
process is occurred by change of one or more electrons each reduction peak
accompanied with chemical modification and said presence of this process directly
depend on solution, reference electrolyte and nature of electrode. Reduction S8 of in
organic solution (THF, DMF, DMSO, or CH3CN), when use different electrodes
(Pt, graphite, glass, carbon and Au), usually carried out with two electronic stage. An
additional, smaller wave situated between the two main reduction peaks has been
observation and most authors [49-51] showed the presence of acidic impurities:
The first reduction peak for S8, its 2 electron losing by following reaction:
S8 + 2e- →S82− (1.33)
Often proposed to be shown by following reaction:
4S82−→ 4S6
2−+ S8 (1.34)
S62−→S3
.− (1.35)
Kim and Park [52] also suggest formation of S42− and S3
2− (by 1.36 and 1.37
reaction) based on UV-vis spectro-electrochemical experiments.
2S82− → 2S4
2− + S8 (1.36)
S82− + S6
2− → 2S32− + S8 (1.37)
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The second reduction wave has been attributed to different processes, including
the 2-electron reduction of 𝑆82− to 𝑆8
4− which may dissociate into𝑆42− (1.38, 1.39
reaction) or the reduction of 𝑆62− or 𝑆3
.− to 𝑆32−(1.40, 1.41 reaction) [53].
S82− + 2e- → 2S8
2− (1.38)
S84−→ 2S4
2− (1.39)
S62− + 2e- → 2S3
2− (1.40)
S3.− + e- → S3
2− (1.41)
In chloroamuminate (AlCl3/NaCl ), the electrochemistry of elemental sulfur is
further complicated by the formation of chlorinated species such as S2CI2 and SCI3+,
normally, four oxidation and four reduction waves observed in same system [54-56]. Nevertheless,
reduction to 𝑆82− is also proposed to occur at the first voltammetric wave in AlCl3/NaCl
melts. It has also been shown that elemental sulfur can be easily reduced to other sulfur species by
dissolving it at certain temperature in same phosphonium and imidazolium inic liquids [57].
Figure 1.1 – UV-vis spectra of solution of N2S6 and N2S4 in (a) [C4 mim][DCA]
and (b) DMSO
According to the result, the spectrum of N2S4 shows an absorption band band at c
nm whereas that of N2S6 depicts a band at nm with a shoulder at ca. 350nm. These date
agree well the absorption spectra reported for the two dianions S42− and S6
2− in organic
solvents (table 1.1).
Table 1.1 - Absorption maxima (λmax/nm) assigned to sulfur species observed during
the electrochemical reduction of S8 in molecular solvents (DMSO and DMF; Literature
Values) and λ max obtained in this work for [C4mim][DCA]
Solvent S2− S2
2− S32− S4
2− S52− S6
2−
S72− S8
2− S3.− S4
.− Rep
Page 17
17
Сontinuation table 1.1
1 2 3 4 5 6 7
8 9 10 11 12
DMSO 435 505 618 13
420 475 492 618 14
260 350,
450
340,
450
379,
490
610 770 17
DMF 498 617 18
250 280 334 420 435 442,
450 470,490 (S8
2−liner),
355 (S82−
cyclic) 600
˷700 20
[C4mim][DCA 440 350,
460
620 In this
work
Oxidation of sulfide ions. During the anodic oxidation of sulfide ions in ionic
solution, depending on the circumstances formed elemental sulfur, polysulfide, sulfate,
dithionate and thiosulfate ions. Bohnholtzer and Heinrich [58] are carried works on
smooth platinum electrode wire identified that discharge of S2- ions occurred by
formation of polysulfide and elemental sulfur, shown that sulfur easily dissolves in
polysulfide solution. Grischer [59] found that sulfur melted in a Na2S2 solution with
the highest rate. He [60] oxidation of S2- ions was studied by removing polarzation
curves. On the research of Bohnholtzer and Heinrich were identified three potential
area: a – from 0- to +0.9 V; b – from +0.9-to +2.1 V; с – from +1.6-to +2.5 V. In this
three area formed different products. ''А'' area formed only polysulfides, and are of ''b''
– obtained polysulfide-, sulfate- , thiosulfate- ions, there output of ploysulfide
decreased by the increasing, but output of sulfate is grown; ''с'' area formed only sulfate
ions. There is no formation of ions dithionate ions.
Grischer believes that following electrode reactions accompanied:
On the potentials of in the area ''a'':
SX2− → xS + 2e (1.42)
And
SX2− + S → SX−1
2− (1.43)
On the area of ''b'':
S2- + 6OH- → SO32− + 6e + 3H2O (1.44)
SX2− + 6OH- → S2O3
2−+ 6e + 3H2O + (x-2) S (1.45)
Page 18
18
The next reactions:
2 SO32−→ S2O6
2−+ 2e (1.46)
SO32−+ S → S2O3
2− (1.47)
2 S2O32− → S4O6
2−+ 2e (1.48)
4S4O62- +6OH → 5S2O6
2−+ 2S2O62−- + 3H2O (1.49)
S3O62- + 2OH- → S2O3
2− + SO42−+ + H2O (1.50)
In the area of '' c '' potential is defined by the following reactions:
SO32−+ 2OH- → SO4
2− + 2e + H2O (1.51)
S2O32−+ 2OH- → SO4
2−+ 2e + H2O + S (1.52)
On the work of A. Baeshov, and Blazhako the anode oxidation of sulphide-ions
were studied on the electrode of platinum and copper group metals [61-64]. The
polarization curves were taken these mentioned metals on stationary and rotating disk
electrodes under argon atmosphere. By previously research shown that sulfide ions in
1M NaOH solution oxdaized on platinum electrode and the oxidation is observed from
+1.0 V positive potential and was said that cathodic polarization curve of copper and
silver electrodes in sulfide-alkaline solution only only one current maximum recorded
which for copper electrodes at 0.9 V and silver electrodes - 0.65V.
Kuit university scientists [65],oxidation of sulfide ions on ploycrystalic platinum
electrode studied by cyclic voltametry, An electrolyte of 3.5% NaCl containing of
sulfide ions was used as the testing medium.
Accprding to the research, the following anodic reactions are expected to occur in
the sulfide system, resulting in the formation and removal of sulfur layer:
HS- + OH- → S + H2O + 2e- (1.53)
S2-→ S + 2e- (1.54)
2HS- + 2OH- →S22− + 2 H2O + 2e- (1.55)
HS- + 9OH- →SО42−+ 5H2O + 8e (1.56)
S + HS- + OH- →S22− + H2O (1.57)
S + S22− →S3
2− (1.58)
Page 19
19
Figure 1.2 – Effect of HS- ions concentration on the cyclic voltammograms
(polycrystalline platinum at scan rate 10 mV/s)
Study the effect of pH sulphide containing electrolyte prodominate species HS- is
in this electrolyte values from 9 to 12. It is shown that anode current taken cycle
voltomogramms in the presence of sulfide ions increased significantly than
involvement of sulfide ions solution (figure1.2), which explained by auther anodic
oxidation of sulfide ions.
In the presence of sulfide ions, only anodic currents are measured in the forward
sweep while the magnitude of currents in the reverse sweep in negligibly small. This
indicates that the products of the anodic reaction in the forward sweep do not undergo
reduction in the reverse sweep and have passivated the platinum surface. Three features
appear in the forward sweep, marked a, b and c at potentials of -0.1, 0.475 and 1.0 V,
respectively. While features a and c readily defined, feature b is less clear. a small
shoulder at point (b) is formed which may be due to the starting point of the deposition
of elemental sulfur. The starting of formation of white thin layer adherent to the
electrode surface was observed exactly at this potential. Such formation causes the
reduction in the size of effective surface of the Pt electrode. The formation of large
peak afterward at point (c) (see figure 1.2) can be explained on the basis of the
oxidation of the deposited sulfur according to the equation (equation 1.61) to form
soluble sulfate ions. The phenomenon of periodic formation and dissolution of sulfur
element on the Pt electrode could be represented as follows:
Pt + HS- + OH- → PtS + H2O + 2e- (1.59)
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20
PtS + HSx− → Pt + HSx−1
− (1.60)
PtS + 8OH- →Pt +4 H2O + SO42− + 6e- (1.61)
The above mechanism is substantiated by XPS performed on the electrodes
surface after potentiostatic experiments. Many XPS spectra of polycrystalline platinum
polarized at different potentials in sulfide polluted salt water were measured. An
illustrative example of these spectra is shown in figure-1.3. It shows XPS spectrum of
platinum electrode polarized at a potential of 1.0 V for 60 minutes 3.5% NaCI + 0.15
M HS-. A sharp S2p peak appears at 163.2 eV, which is characteristic of the presence
of elemental sulfur (S) on the electrode. A small shoulder appears at 169.2 eV which
indicates the presence of sulfate (SO42−) ions.
Figure 1.3 – XPS spectrum of a polycrystalline platinum electrode after
potentiostatic polarization at 1.00 V (Ag/AgCl) for 1 hour in presence of 3.5 % NaCl
solution containing 0.15 M HS- at 25 °C
Cyclic voltamograms of polycrystalline platinum show negligibly small currents
in the absence of sulfide ions compared to those measured in its presence, at all
potentials. These anodic currents are indeed resulting from the oxidation of the sulfide
ions. The magnitude of currents measured in the reverse sweep is much less than those
measured in the forward sweep which reveals that the reaction products in the forward
sweep have passivated the platinum surface. Three current peaks appear in the forward
sweep at potentials of -0.1, 0.475 and 1.0 V, respectively. The first peak indicates the
possibility of the formation of platinum sulfide and polysulfide. The second peak may
be due to the starting point of the deposition of elemental sulfur. A white thin layer
Page 21
21
adherent to the electrode surface starts exactly at this potential. The third peak can be
explained on the basis of the oxidation of the deposited sulfur to form soluble sulfate
ions.
Oxidation and reduction of elemmental sulfur. [66] In authors work studied
reduction of sulfur suspension, and they shown that on lead and zinc cathode
unreduction in sulfuric acid and alkaline solutions. But in netural and alkaline solution
on graphite reduced with output of 55-75%. Current density does not affect for this
process. The reduction in acid solutions as state of suspension of elemental sulfur,
widely used for obtaining hydrogen sulfide [67, 68].
In the study oxidation of sulfur on platinum, nickel and stainless steel electrodes,
the results of polarization curves of on the electrodes identified that the oxidation of
sulfur was implemented to oxygen separation potential. Which, on platinum electrode
"plus" 1.13 V, on stainless steel "plus" 0.86 V and on nickel - "plus" 0.80V. In the
presence of dispersed sulfur, the distribution of oxygen voltage increased, comparison
with the testing solution should be noted that the shift to the right side which explained
that gradually passivation of electrode surface and absorption of sulfur on the electrode.
As a result of polarization curves proved that the oxidation potential of sulfur
powder due to the electrode material.
On the basis of standard redox potentials on the anodic oxidation of sulfur to
sulfite and sulfate ions reactions could be assumed by following reaction:
S + 8OH˗ - 6e- ↔SO42− + 3H2O, E0 = - 0,660V (1.62)
S + 8OH˗ - 6e- ↔SO42− + 4H2O, E0 = - 0,753V (1.63)
The formation of sulfite and sulfate ions after anodic polarization of dispersed
sulfur, electrolysis products confirmed by the results of chemical analysis.
The effects of alkali concentration for the oxidation process sulfur studied on
platinum electrode. With increase of alkaline concentration oxygen separation potential
moves toward the negative side, that is why when concentration of alkaline higher than
5 M oxidation of sulfur wasn’t observed.
Baeshov and his scientific staff [69] predict when sulfur powdered, its particles
are charged negatively by this verified following, if immediately polarize after crushing
of sulfur on cathode, on polarization curve on the lead electrode its reduction clear
waves weren’t observed. Which explained negatively charged sulfur particles alienated
from cathodic polarized electrode and as a result, dispersion of sulfur on the surface of
the cathode electrode would not adsorbed.
But in the condition of anodic polarization, sulfur particles on the positively
charged anode surface because of involved in electrostatic it can be seen its oxidation
occurs easily.
After neutralization of negative charge of dispersed sulfur, its particles reduction
speed increased that displayed as clear maximum on polarization curve. In the literature
[70] has confirming data about sulfur could have a strong negative charge after
grinding.
Oxidation and reduction of thiosulfate. In previously works have been made
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22
prognosis that thiosulfate oxidized separated oxygen from anode.
Professor the electrochemical state of thiosulfate studied by taking voltametric
curves on P-5827 potentiostat under potentiodynamic regime[71]. As a electrode
material were used platinum, copper, nickel and stainless steel. As a reference electrode
was used silver chloride electrode and as a counter electrode was used graphite rod.
There work potentials are given by the participation of normal hydrogen electrode.
When thiosulfate ion anodic oxidized on platinum nickel, stainless steel and
copper, oxidation waves were registered all mentioned electrodes polarization curves.
Second waves was appeared at the potential of +0.7 V and the studied compound’s
anodic oxidation results can see visually by the surface of copper electrode lined with
the black sediment and identified that was copper (II) oxide. This phenomenon cites a
decision the possiblity of oxidation of thiosulfate ions on copper electrode
accompanied by complex mechanisiom.
Muhmoud [72] was studied the electrochemical reduction of sodium thiosulfate
in aqueous media on platinum electrode. Na2S2O3 solution containing sulfuric acid
(H2SO4) at pH =3 the cathodic reduction was observed around -0.6 V. And in this
electrochemical reduction reaction can be determined to know the mechanism of the
reaction by the following equation:
/𝐸𝑝-𝐸𝑝/2/= 1.857𝑅𝑇
𝑎𝑛𝑓 =
47.7
𝑎𝑛 mV (1.64)
The influence of electrolyte concentration for the reduction of thiosulfate anion
was studied in the range of 0.001-0.0075 M (figure 1.4). There is a slight decrease in
the reduction peak current density which explained by a weaker dissociation at higher
concentrations of thiosulfate anion.
In addition, at low pH levels, a colloidal sulfur is generated which is distributed
and adsorbed on the electrode surface. This retards the electron transfer process at these
concentrations.
The effect of temperature on electrochemical reduction of thiosulfate anion was
investigated for 0.005 M in the temperature range between 20 and 50 °C. The peak
current was increased by the increasing of temperature (figure 1.5). This indicates that
the temperature increase causes the electroreduction process to take place more easily
on the Pt- electrode surface, due to the decrease in the activation energy.
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23
Figure 1.4 – Dependence of the reduction peak current on the concentration of
thiosulfate anion at v=20 mV/s at pH= 3 and t=22 °C
Figure 1.5 – LSV of 0.005M Na2S2O3 on Pt- electrode at pH= 3 Scane rate
5mV/s at different temperatures
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24
The oxidation of thiosulfate was studied by using the method of cycle
voltogramma [73]. According to the resolute shown that the oxidation process was
sensitive value of pH and scan rate (figure 1.6).
Figure 1.6 – LSV of 0.5 mol.l-1 of thiosulfate solution on various pH value
Seen from Figure, when the pH 5 or 6 on the cyclic voltomogramma was
registered three oxidation waves which 0.05 V, 0.58V and 1.02V. Increase in pH value
with the decreasing scan rate, oxidation peak on 0.05V was observed. When the pH
value is 8 ~ 9, can be seen in three obvious oxidation peak, approximately 0.05V, 0.91V
and 1.22V. If increase scan rate the values of the cyclic voltomagromma curves closed
one to another and the system will disappeared However, when increase the pH value
than 10, registered the disappearance of third oxidation peak (figure 1.7). As seen figure 1.7 oxidaton of thiosulfate ions in acid and alkaline medioum have
big. The oxidation kinetics of sulfur was difficult, so interm oxidation of thiosulfate
ions not only depended on with the presence of thiosulfate ions but also due to close
reaction medium, pH value of the products, disproportion and decomposition reaction
of thiosulfate ions.
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25
Figure 1.7 – LSV of 0.5 mol.l-1 of thiosulfate solution at pH=10
1.4 E-pH diagram of sulfuric- water system. A comprehensive survey of the
classical electrochemical facts for sulfur, selenium, and tellurium, as documented until
about 1970, can be found in the reviews of Zhdanov [74], where from we cite the lists
of standard and formal potentials for aqueous solutions, in tables 1.2 many of these
potentials have been calculated thermodynamically since the experimental
determinations are few. The listed data are largely drawn from the monograph by
Pourbaix. In these tables, the redox systems are assorted by decreasing formal valency
of chalcogen in the oxidized state, while at a given valency of the oxidized state they
appear in the order of decreasing valency of sulfur in the reduced state.
Latimer [75] has compiled useful aqueous redox transition potential diagrams that
are convenient as a quick guide in practical problems and for perceiving the oxidation–
reduction properties of some sulfur hydride and sulfur oxide species. Standard
potentials of chalcogens in non-aqueous media are generally not known. As tandard
approach for the theoretical presentation of electrochemical equilibria is the use of
Pourbaix [76], or potential–pH predominance area diagrams, which incorporate
chemical and electrochemical thermodynamics simultaneously in a straight forward
manner. These diagrams comprise an extremely useful, yet fundamental, starting point
for the study of electrochemical systems. Certainly, the ability to predict, understand,
and ultimately control electrochemical reactions requires also knowledge of process
kinetics. Reproduced below are the potential–pH diagrams for the chalcogen–water
systems will serve as a guide to all subsequent discussion on aqueous systems used for
electrochemical preparations of metal chalcogenides. The diagrams represent, almost
exclusively, the important “valencies” of the chalcogens, namely –2, +4, and +6. The
diagrams are valid only in the absence of substances with which the respective
chalcogen can form soluble complexes or insoluble salts.
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26
It is considered useful to include here the potential–pH diagram for some redox
systems related to oxygen (figure 1.8) [77].
Table 1.2 – Standard potentials of sulfur in Aqueous solution
Half-reaction Standard or formal potential
(E0, V at 25◦C)
1
2
S2O82− + 2e- → 2SO4
2−
+ 2.01
S2O82- + 2H+ + 2e- → 2HSO4
2−
+2.123
2SO42−+ 4H+ + 2e- > S2O6
2−+ 2H2O
-0.22
SO42− + 4H+ + 2e- — H2SO3 + H2O
+0.17
SO42- + 4H+ + 2e- — SO2 + 2H2O +0.138
SO42-+ H2O + 2e- — SO3
2- + 2OH-
-0.93
HSO42− + 7H+ + 6e- → S(s) + 4H2O
+0.339
SO42- + 8H+ + 6e- → S(s) + 4H2O
+0.357
HSO42- + 9H+ + 8e- → H2S (aq) + 4H2O
+0.289
SO4- + 10H+ + 8e- → H2S (aq) + 4 H2O
+0.303
SO4- + 9H+ + 8e- → HS- +4H2O
+0.252
SO4- + 8H+ + 8e- → S2- + 4H2O
+0.149
SO4- + 10H+ + 8e- → H2S (g) + 4H2O
+0.311
S2O62− + 4H+ + 2e- →2H2SO3
+0.57
S2O62− + 2H+ + 2e- → 2H2SO3
−
+0.455
3H2SO3 + 2e- → S2O62−+ 3H2O
+0.30
2 H2SO3 + H+ + 2e- → HS2O42− + 2H2O -0.08, -0.056
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27
Сontinuation table 1.2
1 2
S2O62− + 2e- → 2SO3
2-
+0.026
2SO32-+ 2 H2O + 2e+ → S2O4
2−+ 4 OH-
-1.12
2SO32- + 4H+ + 2e- → S2O4
2− + 2H2O
+0.416
2HSO3- + 3H+ + 2e- → HS2O6
2−+
2H2O
+0.060
2HSO3− + 2H+ + 2e- → S2O4
2− + 2H2O
-0.013, -0.009
4 H2SO3 + 4H+ + 6e- → S4O62− + 6H2O
+0.51
4HSO3− + 8H+ + 6e- → S4O6
2− + 6H2O
+0.581
4SO2(g) + 4H+ + 6e- → S4O62− + 2H2O
+0.510
2 H2SO3 + 2H+ + 4e- → S2O32− + 3H2O
+0.40
2S2O32− + 3H2O + 4e- → S2O3
2−+ 6OH-
-0.58
2 S2O32− + 6H+ + 4e- → S2O3
2− + 3H2O
+0.705
H2SO3 + 2H+ + 2e- → H2SO2 + H2O
< +0.4
2HSO3− + 4H+ + 4e- → S2O3
2− + 3H2O
+0.491
5H2SO3 + 8H+ + 10e- → S5O62−+ 9H2O
+0.41
H2SO3 + 4H+ + 4e- →S(s) + 3H2O
+0.45
SO32- + 3H2O + 4e- → S(s) + 6OH-
-0.66
SO2(g) + 4H+ + 4e- →S(s) + 2H2O +0.451, +0.470
S4O62− + 2e- →2S2O3
2−
+0.08, +0.219, -0.10
S4O62− + 12H+ + 10e- → 4S(s) + 6H2O +0.416
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28
Сontinuation table 1.2
1 2
H2SO2 + 2H+ + 2e- → S(s) + 2H2O
>+0.5
S2O32− + 6H+ + 4e- → 2S(s) + 3H2O
+0.465
S5O62− + 12H+ + 10e- → 5S(s) + 6H2O
+0.484
SO(g) + 2H+ + 2e- → S(s) + H2O
+ 1.507
5 S2O32−+30H+ +24e- → 2S5
2- + 15 H2O
+0.331
S2O32− + 8H+ + 8e- → 2HS- + 3H2O
+0.200
S2O32− + 6H+ + 8e- → 2S2- + 3H2O
-0.006
S2Cl2 + 2e- → 2S(s) + 2Cl-
+ 1.23
5S(s) + 2e- → S2-
-0.340, -0.315
4S(s) + 2e- → S42-
-0.33
S(s) + 2H+ + 2e- → H2S(aq)
+0.141
S(s) + 2H+ + 2e- → H2S (g)
+0.171
S(s) + H+ + 2e- → HS-
-0.065
S(s) + H2O + 2e- → HS- + OH- -0.52
S52-- + 5H+ + 8e- → 5HS- +0.003
S52- + 10H+ + 8e- → 5H2S(g)
+0.299
3S42- + 2e- → 4S3
2-
-0.478
S(s) + 2e → S2- A large number of
measurements and calculated
values are available. These vary
from−0.48 to−0.58 V, but most are
closer to−0.48 V
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29
Сontinuation table 1.2
1
2
S42- + 2e- → S2- + S3
2-
-0.52
S42- + 4H+ + 6e- → 4HS-
+0.033
2S32- + 2e- → 3S2
2-
-0.506
S32-+ 2e- → S2- + S2
2-
-0.49
S32- + 3H+ + 4e- →3HS-
+0.097
S22- + 2e- → 2S2-
-0.48, -0.524
S22− + 2H+ + 2e- → 2HS-
+0.298
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30
Figure 1.8 – Potential–pH diagram for the stable equilibria of the system sulfur–
water at 25◦C
HS-, S2-, S, HSO4− , SO4
2− and S2O82−, which contain sulfur only in the oxidation
states –2 and +6 (aside from the solid element); other species, such as thiosulfates,
dithionites, sulfites, and polythionates arei n “false” equilibrium in aqueous solution.
Note also that the persulfates (S2O82−) are unstable in water, so that if the equilibria
were attained, only the remaining six forms would be present in solution. The limits of
the domains of relative predominance of the dissolved substances included in the
Pourbaix diagram (plus solid sulfur) regard the following homogeneous and
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31
heterogeneous (solid/liquid, gas/liquid) equilibria, involving redox and non-redox
processes:
Limits of the domains
of relative predominance of
dissolved substances
(1’) H2S/HS-
(2') HS-/S2-
(52’) H2S(g)/H2S(aq)
(53’) H2S (g)/HS-
(11') HSO-4 /SO2-
redox equilibrium
(41') S2O82− + 2e- → 2SO4
2−
(40') S2O82− + 2H+ + 2e- → 2HSO4
−
(50') 2HSO4−+ 7H+ + 6e- → S(s) + 4H2O
(51’) SO42− + 8H+ + 6e- → S(s) + 4H2O
(20’) HSO4- + 9H+ + 8e- → H2S(aq) + 4H2O
(21’) SO42−+ 10H+ + 8e- → H2S (aq) + 4H2O
(22') SO42− + 9H+ + 8e- → HS- + 4H2O
(23') SO42− + 8H+ + 8e- → S2- + 4H2O
(58') SO42− + 10H+ + 8e → H2S (g) + 4H2O
(42') S(s) + 2H+ + 2e- → H2S (aq)
(60) S(s) + 2H + 2e- → H2S (g)
(43’) S(s) + H+ + 2e- → HS-
Solid substances considered: S (sulfur, light yellow, orthorhombic). Dissolved (in
aquo) sulfur substances considered: H2S (hydrogen sulfide, colorless), HS- (hydrogen
sulfide ion, colorless), S2- (sulfide ion, colorless), S22− (disulfide ion, orange), 𝑆3
2−
(trisulfide ion, orange), S42− (tetrasulfide ion, orange), S5
2− (pentasulfide ion, orange),
H2S2O3 (thiosulfuric acid, colorless), HS2O3− (acid thiosulfate ion, colorless), S2O3
2−
(thiosulfate ion, colorless), S5O62− (pentathionate ion, colorless), S4O6
2− (tetrathionate
ion, colorless), HS2O4−(acid dithionite ion, colorless), S2O4
2− (dithionite ion, colorless),
S3O62− (trithionate ion, colorless), H2SO3 (sulfurous acid, colorless), HSO3
− (bisulfite
ion, colorless), SO32− (sulfite ion, colorless), S2O6
2− (dithionate ion, colorless), H2SO4
(sulfuric acid,colorless), HSO4− (bisulfate ion,colorless), SO4
2− (sulfate ion, colorless),
S2O82− (dipersulfate ion, colorless).
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1.5 Sulfur content Oil and sulfur-containing waste sources impact on the
environment
At the present time, one of the main issues – suspension of environments’
exposure consequences with painful disasters. For instance, prevention of the
environment and efficient application of natural resources is very important. As well
as, one of the main causes of environmental pollution is the rapid development of
industries, as a result of their actions, the recovery of natural cyclicity ability is
seriously affected. Any industry have technogenic impact on our environment. One of
the examples is oil production that is considered the main source of income of the
country economy [78].
Among the 55 oil-producing countries in the world the Republic of Kazakhstan is
placed on the 12th according to the hydrocarbon reserves. The volume of oil produced
in our country includes one of the seventeen all oil, which produced in the CIS.
However, depending on geological features, our country’s oil is deemed medium and
high sulfur containing oil. That is why, during the cleaning process of hydrogen sulfide
from crude oil, elementary sulfur is formed as an additional product and is accumulated
as a waste in the territory of production. These chemicals lead to eyes and inflammation
of the mucous of upper respiratory tract, diseases of skin and other gastrointestinal tract
also causes irritation, moreover, sulfur is not stored in the air. The famous scientist,
academician, same researcher believed that sulfur is meteorological and under the
influence of temporary factors which cracked by erosion and further only accelerated
the destruction process. By the way, a large amount of hydrogen sulfide, sulfur dioxide
and polysulfide is allocated and has ecological negative impact to the environment.
It is known that, organic and inorganic compounds of sulfur are always present in
all living organisms. It is an important biogenic element, but according to the negative
impact on the environment and human sulfur and its compounds are on the first places.
Thus about 96% of sulfur emitted from enterprises, release into the atmosphere as
sulfur dioxide SO2. In the atmosphere, sulfur dioxide reacts with water vapor and
formes acid solution, which then falls as acid rain. Once in the soil, acidic water inhibits
development of soil fauna and plants [79].
Among other anthropogenic emissions there are many toxic sulfur compounds and
from them it is necessary to note that inhalation of hydrogen sulfide causes rapid
dulling of reactions to its unpleasant odor and can lead to serious poisoning, even with
fatal consequences. MPC hydrogen sulfide in the air of working premises 10 mg/m3,
in the atmosphere 0.008 mg/m3. The toxic properties of sulfur and its some compounds
are listed in table 1.3 [80].
Table 1.3 – The toxic properties of sulfur and its compounds
Toxic effects compounds MPC, mg/m3
MPC (Dust 8 hours a day) S 2,0
MPC (8 hours a day) H2S 10
Irritation of the eyes and nose H2S 4
Mortally for 1 - 3 hours H2S 800
MPC (Dust 8 hours a day) SO2 10
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Сontinuation table 1.3
1 2 3
Perception threshold SO2 3
Irritation of the eyes and nose SO2 20-50
Mortally for 1 - 3 hours SO2 5000
In the development of oil fields that contains sulfur, waste pollution of the
environment is happening. The presence of sulfur in commercial oil is forbidden
because, it is a catalyst poison, in addition, sulfur compounds, present in petroleum,
dramatically deteriorate operational qualities of fuels and oils and cause corrosion of
equipment.
Maximum allowable concentration of sulfur compounds in soil and permissible
levels of their content on the hazard indicators are shown in the table 1.4 [81].
Table 1.4 – MPC of sulfur compounds (mg/kg) in soil and permissible levels of their
content on the indicators of harmfulness
Substance MPC, mg/kg
soil
considered
background
indicators of harmfulness
translocation
(accumulatio
n in plants),
mg/kg
migratory general -
sanitary,
mg/kg aqueous,
mg/kg
airy,
mg/kg
The total content
Hydrogen
sulfide
0.4 160.0 140.0 0.4 160.0
Elemental
sulfur
160.0 180.0 380.0 - 160.0
Sulfuric
acid
160.0 180.0
380.0 - 160.0
Main formation source of sulfur – the burning of fuels and production of metals
generalization from sulfur ore. On the basis of these enterprises, a large amount of
sulfur oxides spreads in the air. Sulfur oxides in the biosphere mixed with various forms
of natural sulfur sources.
As well as non-ferrous metallurgy industry, oil refining, synthesis of inorganic
and organic substances and more the field of industries the diversity of the nature of
pollutants released into the atmosphere was shown in table 1.5.
The composition of the waste from the productions released into the air varies, they
contain hundreds of gases and chemical compounds that can be in the form of vapors
and aerosols.
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Table 1.5 – The nature of the air pollutants in the field of chemistry
Produced substance The composition of the air pollution
Sulfuric acid
NO, NO2, SO2, SO3, H2SO4
SO2, SO3, H2SO4
The method of nitrous NH3, (NH (SO3NH4)2, H2SO4
Contact method H2SO4, HF
Sulfa Amin acid H2S, Cl2, SO2, (CH3)2S
Superphosphate SO2, H2S, P2O5, karbofos dust
The main sources of sulfur and its compounds from the environment shown that
oil, petrochemistry and chemical enterprises and among the sulfur compounds the
hydrogen sulfide provided the effect to the environment. Especially, hydrogen sulfide’s
impact on the environment causes significant changes in the nature every day and it is
shown that it disturbs factor of the equilibrium and structure of geological system
metabolism.
That is why, it is very important to create simple methods to obtain necessary
sulfur’s different compounds and that is one of the most topical issues.
It is well known from the literature, in earlier times, elemental sulfur was
considered as one of the basic chemical raw material in the manufacturing industry
and the national economy than- coal, oil, lime, salt and at the present time in chemical
industry it is widely used for the production of sulfuric acid [82].
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2 EXPERIMENTAL METHODS AND IMPLEMENTATION OF
TECHNOLOGY
2.1 Methods of potentiodynamic polarization curves recording
Electrochemical oxidation-reduction in aquoues solution was investigated by
recording polarization curves under potentiodinamic regime and electrolysis under
galvanostatic conditions.
To recording potentiodynamic polarization curves was used "Autolab"
potentiostat brands of PGSTAT 302N. "Autolab" potentiostat / galvanostat carried out
three electrode cell glasses with thermostat (figure 2.1), which is known to be applied
for corrosion studies, bio-electrochemistry, in the study of rechargeable batteries and
many other areas. External manifestations of potentiostat was shown in figure 2.1. The
surface of the instrument panelwas placed cell for working, counter and reference
electrode. With (U, E, I) signs marked additional cell is used to identify the relationship
of the device with potentiostat. The central cell of the surface panel devoted to check
the device (determined by C + R and W) and these cells are not applied for
electrochemical measurement. While working left side of surface of panel, red or green
diodes indicates the working status of the device.
Back of panel is located cell which separates and connects the device that
connected to the network cell and zero model is connected to the cable and computer
and potentiostat is connectted with each other. At the same time, back of planes is
located air-cooled radiator. Management of device is fully implemented by a private
computer program. Calibration was carried out once a month.
Before working, potentiostat was connected to the mains during the 20-30
minutes. When the potentiostat is ready to work, electrodes are immersed in a cell
where filled with electrolyte then electrochemical measurements are carried out.
Device management and all functions of processing of the measurement results
selected at the beginning of the program.
As a working electrode rhodium (S = 0.04 cm2) is used, which placed on the
conductive fluoroplastic (figure 2.2). As comparative electrode used silver chloride
(E0= +0.203 V) and as auxiliary electrode served platinum electrode. Potential values
are comparable with silver chloride electrode.
Before each practice electrodes always smoothed microns emery paper, then
washed with distilled water, implemented through wiping with a filter paper. The
potential of the working electrode measured by maximum close placing on surface of
lugine capillaries.
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1 - "Autolab" potentiostat; 2 - electrochemical cell; 3 – working electrode (Rd);
4 – reference electrode(silver chloride); 5- platinum electrode (counter electrode); 6
– bridge
Figure 2.2 – General view of potentiodynamic polarization curves recording
installations
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Before the cell experiment is washed by a large amount of water, distilled water
after that washed with working solution.
Before each experiment installations according to the operational objectives
were prepared for the work.
Figure 2.2 – Construction of rhodium electrode for unloading polarization
curves
In many cases, during the electrochemical reactions, the nature of the electrode
polarization determined by studying the effect of temperature on the rate of
electrochemical reactions [83]. The effect of temperature on the rate of electrochemical
process calculated by similar to the equation of Arrhenius:
lgi = B - A
2.3RT (2.1)
Where: i - the current density; B - constant; A- effective activation energy.
Activation energy of the electrochemical reaction calculated by a straight-line
figure lgi-1/T depends on the value of the angular coefficient.
2.2 Preparation of current conductor sulfuric-graphite composite electrodes
and methodology of electrolysis
Electrochemical studies was carried out with a glass electrolyzer in galvanostatic
condition. Installations for the study electrochemical properties of elemental sulfur in
different solutions and principal schemes to receive sulfate compound with anodic
polarized sulfur-graphite composite electrode (which processed by sulfur waste) were
shown on figure 2.3 and 2.6.
The manufacturing method for sulfur and graphite or sulfur composite electrodes
are as follows:
Preliminary graphite and sulfur ground up to 25-50 mcm (figure 2.4). These
powders used for the preparation of electrodes (50% of graphite powder, 50% of sulfur
powder,). For this, in advance 1:1 ratio measured graphite and sulfur granules were
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powdered individual in a special container, then first, sulfur powder on the electrical
heating stove in the fume hoods at 120-130 °C melted by heating then put graphite
powder on the top of melted sulfur and mixed thoroughly, the end this obtained mixture
poured special normalization and cooled at room temperature for 24 hour [84].
1- Sulfur graphite electrode, 2-cation membrane, 3- graphite rod
Figure 2.3 – The principal technological scheme to obtaining sulfur compounds
based on the processing of waste sulfuric
Sulfur-graphite composite electrode could create a different form (plate cylinder).
Developed electrodes with this proposed method, electrolysis can be carried out
50-300 A/m2 current density. During electrolysis sulfur accompanied by
electrochemical way then held to the solution and graphite particles precipitated the
bottom of the electrolyzer. As a result, sulfur and graphite electrode surface is updated.
Deposited graphite powders collected and washed which can be used for against
preparation of the electrode after dried.
We proposed method of preparation of sulfuric graphite electrodes has following
features:
- the technology of manufacturing of sulfur-graphite electrodes is very simple;
- the electrode will be prepared under atmospheric conditions;
- sulfur - graphite electrodes shows very high electrochemical activity;
- electrodes could be made in laboratory conditions without using special,
complex installations;
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- deposited graphite powder again can be used for the preparation of sulfur-
graphite composite electrode that is why consumption of graphite not much;
Graphite piece, crumbs are industrial waste. Such waste widely used for the
preparation of sulfur-graphite composite electrode. From the literature well known coal
didn’t conduct electricity and some types will be conducted a very small amount. But
graphite is a good conductor. In our research during the perperation of sulfur-graphite
composite electrode carried out comperehernsively study shown instade of graphite
can use its crumbs. And such a waste source in our republic enough. Already from
Temirtau city productions produced a large amount graphite crumbs collected with out
finding application.
In this study used sulfur-graphite composite electrode method’s novelty was
protected innovation patent of RK (№ 21327) [85].
Experiment carried out electrolyzer (1) with a capacity of 200 where allocated by
MK-40 (4) cation membrane (figure 2.6). Electrode polarization (2-3) during electrolysis
is connected to current power (7). In electrochemical chain, in order to regulate and
measurement of the current strength to chain connected ammeter (6), rheostat (5) and key
(8).
Depending on the conditions of the electrolysis sulfur and graphite electrodes could
be polarized as anodic and cathodic. As an additional electrode was used pure graphite rod
or titanium.
By special method prepared composite sulfur graphite electrode’s micrograph was
shown in figure 2.5. Pictures were taken by METHAN-1 micrographical MFN-12
microscope. Obtained micrograph of the surface of the composite sulfur-graphite
electrode, shows that the sulfur and graphite is uniformly distributed by forming a
homogeneous mass on the electrode surface.
Figure 2.4 – Manufacturing of sulfur-graphite composite electrodes
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40
(a)
(b)
Figure 2.5 – A micrograph of the composite sulfur-graphite electrode increase (a)
100 and (b) 200 times
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1 -Electrolyzer; 2- cathode; 3 - the anode; 4 - MK-40 cation membrane;
5 rheostat; 6 - ampermeter; 7 - power supply; 8 - key
Figure 2.6 – Installation for study electrochemical properties of elemental sulfur in
aqueous solution
2.3 Used reagents, drugs and analysis of the obtained products As a result of the electrolysis obtained products were used physico-chemical and
chemical methods. In this work, for analysis iodometry, chelatometry and potentiometric
methods were applied. After the electrolysis formed products: sulfide-, polysulfide-,
sulfate-ions and thiosulfate-ion analysis defined by titration and chelatometry methods
[86-92].
Determing sulfide-, sulfite- and thiosulfate- ions all together in waste water, first of
all sulfide ions deposited in the form of ZnS, CdS by zinc or cadmium salts. To avoid
tanning of sulfite ions with oxygen in the air added glycerol to the solution. In order to
determine sulfide-, sulfite-, thiosulfate-ions, sulfide ions precipitate by adding a solution of
zinc carbonate and cadmium carbonate suspension. This sediment filtered and washed.
The filter paper with a sediment pure into the 250 ml conical flask then 25-50 ml of
0.01 N iodine solution and to create an acidic medium dilute hydrochloric acid were added.
The filter paper is grinded glass rod. After that, excessive amounts of iodine in the
presence of starch titrated 0.01 N sodium thiosulfate solution.
Obtained sulfur compounds by electrochemical way was identified with X-ray and
physical-chemical methods. X-ray studies was taken in DRON-4 diffractometer. As a
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result of the electrolysis formed product’s current output calculated by the following
formula:
ƞ =𝑚𝑡ℎ𝑒𝑜𝑟
𝑚𝑝𝑟𝑎𝑐𝑡 100% (2.2)
There:
mtheor ˗ practical weight of formed substance during carrying out the certain the size
of the electric current, g;
mpract - product theoretical weight which calculated by Faraday's law, g
In order to determine current output, theoretical weight substances calculated by
Faraday's law:
Mtheor = Iqτ (2.3)
where: I - current, A; q- electrochemical equivalent, g .A/ hr; t- time, hours.
q = (2.4)
where:
A - the atomic mass of the element; n - the corresponding valence of the element;
F - Faraday number.
All obtained experimental values were processed by mathematical statistics
method [89]. To make sure the accuracy of the experiment results which were repeated
at least 3-4 times.
2.4 determination of sulfur-containing compounds by physico-chemical
methods
As a result of the electrolysis, sulfur ions are formed during the analysis is
conducted. Sulfide- and polysulfide- ions were identified by [86-92] given method.
Sulfide ions with zinc and cadmium salts were infused as ZnS and CdS. By adding a
solution of glycerin (to prevent oxidation of ions with the oxygen in the air) tincture of
sulfide filtered. Sulfide ions in the tincture were identified by iodometric method.
In order to determine sulfide and polysulfide ions in 250 ml conical flask was poured
50-100ml preliminary neutralized testing solution. Then added 10ml of glycerol and
up to 150ml diluted with distilled water. After that 20ml of zinc carbonate suspension
added and filtered. Filtered sediment washed with hot water, in 200 ml volumetric
measuring flask filtrate cooled and diluted with distilled water
2.4.1 Determination of ions– sulfide, sulfite, sulfate, thiosulfate and polysulfide
Determination of sulfide ions. Sulfide ions were identified by the iodometric
method. The analysis is carried out for the following reaction:
:
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S2- + I2 → 2I- + S↓ (2.5)
In 250 ml flask cone put the sediment filter and 25-50 ml a solution of iodine
added then acided 5ml hydrochloric acid with the ratio of 1:9. With the glass stick by
mixing filter excessive amounts of iodine, in the presence of 0.5% starch titrated with
0.01 N of sodium thiosulfate.
Amount of sulfide ions (x), mg / l will be found by the following formula:
X =(ak1−bk2)∙16,03∙0,01∙1000
𝑉 (2.6)
There:
A- the amount of added iodine solution, ml;
k1 – correction coefficient that applied to the concentration of 0.01N iodine
solution;
k2 – correction coefficient that used to the concentration of 0.01N thiosulfate
solution;
b - exploded volume of 0.01N sodium thiosulfate solution for titration process,
ml;
V - the amount of testing electrolyte, ml; 16.03 – for 1ml 0.01N iodine solution
calculated sulfide ions equivalent size.
On direct current dissolution process of sulfur electrode with the formation of
sulfide and polysulfide ions accompanied by the following reaction:
S0 + 2e S2- (2.7)
nS + 2e S𝑛2− (2.8)
Identification of thiosulfate ions. Took a certain number of aliquots from
allocated filtrate to 250 ml cone-shaped flask over the top added 5 ml 40% of
formaldehyde and 10 ml 10% of acetic acid, thiosulfate ions titrated with 0,01N iodine
solution in the presence of 1-2 ml 0.5% of starch until the color of solution changed to
blue. Used the amount of iodine for the titration accordance with the amount of
thiosulfate. So the amount of thiosulfate is determined by the following reaction:
Na2Sn+1 + nNa2SO3 → Na2S + nNa2SO23 (2.9)
НСНО +SO32−+ Н+ → СН2ОН SO3
2− (2.10)
2S2O32−+ I2 → 2I- + S4 (2.11)
Amount of thiosulfate ions (x), mg / l will be found by the following formula:
X=ak∙200∙0,01∙112,1∙1000
V V1 (2.12)
There:
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а – the amount of iodine for the titration, ml;
k – correction coefficient that applied to the concentration of 0.01N iodine
solution;
V – the amount of electrolyte for analysis, ml;
V1 – the amount of an aliquot, ml;
200 – volume of flasks, ml;
0.01 – normal concentration of the titration solution;
112.1 – for 1ml 0.01N iodine solution calculated thiosulfate ions equivalent size.
Definition of polysulfide ions. In order to determine polysulfide ions the testing
solution is heated 10 minutes up to 50-60 °C in the presence of sulfite.
Interaction reaction of polysulfide with SO32− ions:
S𝑛2− + (n-1) SO3
2− So + (n+1)S2O32− (2.13)
After that according to the above-mentioned method sulfides will be separated in
the form of zinc sulfide. To filtrate added 5 ml of 40% formaldehyde ( to associate with
sulfite ions) and 10 ml 10% of acetic acid, the thiosulfate ions titrated with iodine
solution, in the presence of 1-2 ml of 0.5% starch until color of solution changed to
blue.
Determination of sulfite ions. To 250 ml volume cone-shaped flask add a certain
volume of (25-50ml) iodine solution then poured 5-10 ml 10% of acetic acid and the
amount of the sulfide separated filtrate, top of them insert 1-2 ml of starch solution then
titrated with 0.01 N solution of sodium thiosulfate. Spent the amount of iodine for
titration process corresponds to the total size of S- and S2O32− ions in the solution.
Oxidation process of SO32− - ions carried out by following reaction:
SO32− + H2O + I2 SO4
2− + 2H+ + 2I- (2.14)
Amount of sulfite ions (x), mg/l will be found by the following formula:
X=(bk1−ck2−ak1)200∙40,03∙1000
V V1 (2.15)
There:
а - the volume of 0.01 N iodine solution for the determination of sulfite ions, ml;
b – the volume of 0.01 N iodine solution added for analysis, ml;
с –the volume of sodium thiosulfate solution used for reverse titration, ml;
k1 - correction coefficient that applied to the concentration of 0.01N iodine
solution;
k2 - correction coefficient that applied to the concentration of 0.01N sodium
thiosulfate solution;
V – the studied solution volume, ml;
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V1– the volume of aliquot to determine the solution was cleaned from the filter,
ml; 200 - the volume of flask, ml; 40.03- equivalent of sulfite.
Determination of sulfate ions. To take 5 - 25 mg SO42− ions conained aliquot,
poured into the 250 ml conical flask, and diluted with distilled water up to 100 ml. To
this solution added 2 ml of hydrochloric acid with ratio of 1: 1 then heated, over them
poured 3ml hot BaCl2 solution, mixed for one minute and heated for an hour until it
boils. Obtained sediment is carried out the filtration washed carefully with distilled
water, then dried and burned in the temperature of 800 °С. The received product have
measured, according to differences weight of BaSO4 have found.
Amount of sulfate ions (x), mg/l will be found by the following formula:
X=m∙0,4116∙1000
48,03 𝑉 (2.16)
There:
m - mass of BaSO4, mg;
V – The volume of sample is taken for analysis, ml;
0.4116 - BaSO4 's coefficient is calculated the ions of SO42− ;
48.03 – equivalent of SO42−ions.
2.4.2 Determining of the concentration of sulfide ions by the application of
ionomer
While studying electrochemical properetis of elemental sulfur in aqueous
solution, after electrolysis obtained polysulfide ions concentration on the cathodic side
was identified by the method of ion selective electrode [93].
Ionomers laboratory I-160MI intended for measurement indicator of the activity
of hydrogen ions (pH), and other monovalent and divalent cation ions (pX), as well as
mass, molar concentration and the mass fraction of ions (cX), redox potential
(Eh),electromotive force (EMF) of the electrode system and the temperature of aqueous
solutions.
In the measurement of cation activity in the solution and its redox potential were
used electrode system composed of detective and auxiliar electrode. The potential of
the measuring electrode depends on the content of specific form of ions in solution,
called potential generators. The potential of the reference electrode is not dependent on
the composition of the solution and it serves as a reference when measuring the
electromotive force (EMF) and developed electrode system.
The measurement results in terms of concentration cX (for all ions except H+)
depending on the selected dimension is determined by the formulas of 2.17-2.19:
cX = (10-рХ) К, (2.17)
Where, cX - concentration, mol/l; K - activity coefficient. Depending on the ionic
strength of the sample solution. On the device of I-160MI K- is assumed to be 1.
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cX’ = М (10-рХ) К, (2.18)
Where cX '- concentration, g/l; M - molar mass of the ion, g/mol;
cX’’ = (10-рХ) К/|n|, (2.19)
Where cX '' – concentration, mol equivalents / liter; n - ion charge.
Monovalent cations, including H+, n=1;
Monovalent anions, n = - 1;
Divalent cations, n = 2;
Divalent anions, n= - 2;
15 minutes before the useing of the installation should be turn on and need to
prepare practice. In order to turn off facility need to print "off" button for 1-2 seconds.
The amount of the product formed on the cathodic side determined by ion-
selective electrodes [94, 95]. Mentioned electrode based on the definition of certain
amount of ion concentration, identified each ion has a special electrodes, corresponding
electrode to the each ion is considered selective. The range of possibilities of
determining the amount of the ion is very wide. Which in the range of 10-5-10-1 mol/l,
very simple and fast, in this work ionomer’s type model of I-160 MI used.
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1-Determining electrode (sulfide- ion selective electrode- xc-sgl.-001, 02-58);
2- auxiliary electrode (electrode-AgCI which filled with 0.3 M KCI); 3-thermo
sensor TDL-1000; 4-bridges; 5-beaker; 6- the testing solution (containing sulfide
ions); 7-ionomer
Figure 2.7 – Installation for the determination of the amount of sulfide ions by
using ion-selective electrodes
2.4.3 Used reagent and the identification of chemical compounds
The following reagents were used in our research works: elemental sulfur
according to the name of "clean", graphite powder (25.0 microns) according to the
name of "pure for analysis"; hydrochloric acid (p = 1.19 g/cm3), 10% acetic acid (p =
1.07 g/cm3), sodium hydroxide, sodium carbonate, sodium sulphite, zinc sulfate, zinc
acetate, 0.1 N iodine fiksanaly, sodium thiosulfate fiksanaly, 40% formaldehyde,
glycerol, starch, filter paper (blue ribbon). Bidistilled water used to prepare solutions.
In order to determine obtained products as a result of the electrolysis chemical
and physico-chemical (X-ray, microscopes, IR spectrum), chemical analysis methods
and equipment were used.
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3 ELECTROCHEMICAL PROPERTIES OF ANODE POLARIZED
SULFUR ELECTRODE’S IN THE SOLUTION OF HYDROCHLORIC ACID
Oil production in the Republic of Kazakhstan mainly developed in the western region
of country – Mangistau, Atyrau, west Kazakhstan and Aktobe, as well as Kyzylorda
regions which is located the southern of country. And produced oil composition directly
related to the geological features of the fields [96], Kazakhstan oil and gas is rich in
hydrogen sulfide (H2S), which is removed as elemental sulfur during the production
process. As a result, huge quantities of by-product sulfur are produced and stored in
large stockpiles. According to the data, at the Kashagan Field by the processing of
crude oil, every year million ton of sulfur may be collected into the environment. At
the same time, has an information that during the processing of crude oil for many
years, approximately 12 million tons of sulfur was accumulated at the Tengiz field.
In the last, this accumulated sulfur in the country have been sold to foreign
countries by very cheap price. Sulphur international experts noted that Kazakhstan is
among the 10 largest exporters of sulphur. Processing of sulfur and obtaining its
compounds by certain technologies was not implemented because of that is
economically inefficient. To resolve this problem, today’s one of the key issue is to
create new methods for processing of elementary sulfur and getting its usefull
compounds [97]. That is why, in order to obtaining a variety of sulfur compounds there
hardship is arisesed in comprehensive study its electrochemical properties. Elemental
sulfur is isolator and insoluble in water and acid. Therefore, its electrochemical
properties in aqueous medium are very poorly studied [98].
previous results of experimental studies identified that electric conductive sulfur
electrode, in practice at room temperature in aqueous solutions didn’t insoluble if
electrode unpolarized condition.
Electrolysis was hold in electrolytic cell with capacity of 200 mL where the space
of electrode was allocated with MK-40 cationite membrane. As a anodic and cathodic
electrode were used 70 cm2 sulfur-graphite and 64.5 cm2 graphite electrode. Electric
conducted sulfur-containing composite electrode developed with a special method
which provided by ourselves.
The research work carried out the laboratory of "Electrochemical Technologies"
at “Institute of Fuel, Catalysis and Electrochemistry after named D.V. Sokolsky” as a
continuation of [99-108] the scientific papers in the field of investigation of the
electrochemical properties of elemental sulfur.
For the main research, 50 g/l of НCI solution was used as an electrolyte. The
amount of sulfate ions from electrolysis was determined by quantitative analysis
method [109].
The main factor to influenced for the reactions direction and speed which carried
out on the electrode – current density. That is why, influence of current density for the
formation of sulfate ions was investigated in the range of 50-250 А/m2 at room
temperature, in 50g/l hydrochloric acid solution.
During the electrolysis on the anodic side sulfur could oxidized sulfite and sulfate
ions.
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S + 3H2O – 4е → SO32− + 4Н+ E0 = +0.450 V (3.1)
S + 4H2O – 6е → SO42− + 8Н+ E0 = +0.359 V (3.2)
2Cl-(aq) - 2e → Cl2(g) E0 =+1.359V (3.3)
At the time of electrolysis on the cathode occurs discharge of hydrogen ions:
2Н+ + 2e− = H2 E0 = 0.0V (3.4)
С(НCl) = 50 g/l; t = 1,0 hour; T = 25 ºС
Figure 3.1– The influence of current density in anodic polarized sulfur-graphite
composite electrode to formation of sulfate ions current efficiency
As seen in Figure 3.1, the formation of sulfate ions current efficiency by the
increasing of current density decreased gradually. This can be explained additional
process – due to the increase in the proportion of chloride ions discharge. This procces
speeds quicker than complicated oxidation process of sulfur,that is why main oxidation
output reduce due to the separation of chlorine gaz.
Following research, the concentration of HCl for the formation of sulfate ions
current efficiency was investigated. In this stage, according to the previous results 50
A/m2 was chosen as optimal current density.
Figure 3.2- shows, by the increasing of the concentration of HCl identified that
the formation of sulfate ions current efficiency was raised gradually. Apparently, an
increase of the concentration chloride ions could increased sulfur’s activity which
allows to conclude that the opportunity of the oxidation process of sulfur atoms was
given by chlorine:
S + 3Cl2 +4H2O → H2SO4 + 6HCl (3.5)
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As well as, by using the optimal values in this experiments, influence of
electrolysis duration (0.25-1.5 hours) for the formation of sulfate ions current
efficiency was investigated (figure 3.3).
The anodic polarized sulfur-graphite electrode oxidation process with presence of
sulfate ions higher current efficiency was appeared start time of electrolysis. This
phenomenon can be assumed that electrolysis products sulfate ions diffusion limitation.
Diffusion is the net movement of molecules or atoms from a region of high
concentration (or high chemical potential) to a region of low concentration (or low
chemical potential). Which is depnde on time, and illustrated that on the initial time of
oxidation procces given the high current efficiency of sulfate ions, and by the
increasing of electrolysis duration the formed sulfate ions mass transported lower
concentration region, as a resulte reduced the number of molecules to react.
I = 50A/m2; t = 1 hr; T = 25ºС
Figure 3.2 – The influence of HCl concentrations to formation of sulfate ions
current output by anodic polarized composite sulfur-graphite electrode
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I= 50 А/m2; С (НCl) = 110 g/l; T = 25 ºС
Figure 3.3 – The influence of electrolysis duration in anodic polarized sulfur-
graphite composite electrode to formation of sulfate ions current efficiency
Conclusion of section 3
In conclusion, at the anodic polarization were identified that, in HCl solution
sulfur which consisted in electrical conducted composite electrode could oxidized with
high current output by formation of sulfate ions. The influence of different parameters
(the current density, the concentration of hydrochloric acid, the duration of electrolysis)
for electrochemical behaviour of sulfur-graphite electrode were shown at optimal
condition sulfate ions formation current output was 41.5%.
By the increasing of chloride ions amount in the solution that leads growth of
activation for the oxidation of anodic polarized sulfur.
The results of the study – can serve as the basis for make new ways to obtaining
sulfur compounds that widely used in the national economy.
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4 ELECTROCHEMICAL PROPERTY OF SULFUR IN SODIUM
CHLORIDE AND CARBONATE SOLUTION
4.1 Dissolution of anodic polarized elemental sulfur in sodium chloride solution
At present, by the developing of oil production, the problem of environmental
pollution by industrial waste has been increased. In Kazakhstan, the amount of sulfur in
the oil is higher than other oil-producing countries [110]. Therefore, in the processing of
oil and oil products one of the main waste is –sulfur.
If left unresolved, the potential environmental and economic liabilities associated
with the stored sulfur will pose an increasing risk for the international oil & gas oil
companies operating in Kazakhstan.
Ability to understand the different properties of sulfur, which is considered one
of the chalcogen will help in creation of its electrochemical technology [111].
Data about electrochemical properties of various sulfur compounds were shown
on the researcher’s scientific works and monographs and reviewed in systematic basis
were given in a lot of scientific data [76,117]. But, detailed information about
electrochemical properties of elemental sulfur wasn’t exist. As well as, now concluded
that sulfur with low active could not melt under the influence of an electric shock,
because this element is a dielectric and do not conducted the electrical current. But it
has been well known since ancient times that under the influence of an electric current
could dissolve many metals and obtainment of their compounds. The dissolution of the
sulfur in aqueous solutions comprehensively studied with prepare of a sulfur-
containing composite electrode by using this method for sulfur.
Moreover, this study as the electrode material specially prepared conductive
composite sulfur electrode used too.
As well as, current conductor composite sulfur electrode gives electrochemical
activity to dielectric sulfur. By anodic polarization of sulfur electrode shown that the
possibility to get sulfate compounds which is one of the most useful compounds of
sulfur.
During the research, the influence of various parameters for the anodic polarized
sulfur-graphite composite electrode’s oxidation with formation of sulfate ions in NaCl
aqueous solution ( current density, NaCl concentration solution, electrolysis duration)
were studied and the optimal condition for the formation of sulfate ions was
investigated.
The effect on the generation of sulfate ions on the anodic current density was
carried out in the range of 50-250 A/m2 at room temperature in 50 g/l NaCl solution
(figure 4.1).
The highest current efficiency of sulfate ions was shown on lower current density
(50-100 А/m2), and by the increasing of current density to 250 А/m2, which is
decreased current output gradually. A decrease in current yield is due to additional
process. It depends charging of these hydroxyl ions with the formation of oxygen.
During electrolysis the following reactions may occur on the the anode:
S + 6OH- -4e → SO32− + 3H2O E0 = -0.660 V (4.1)
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S + 8OH- -6e → + 4H2O E0 = -0.753 V (4.2)
2Н2О -4е → О2+4Н+ (4.3)
And formed chloride gaz:
2Cl- -2e → Cl2 (4.4)
On the cathode occurs separation of hydrogen and reduction of sulfur process:
nS +2e→S𝑛2−… nS2- (4.5)
2Н2О +2е → H2+2OН- (4.6)
С(NaCl) = 50 g/l; t = 1,0 hour; T = 25 ºС
Figure 4.1– The influence of current density in anodic polarized sulfur-graphite
composite electrode to formation of sulfate ions current efficiency
When current density on the sulfur electrode was 50 A/m2, the current yield
generation of sulfate ions indicated 48% and at 250 A/m2 current density equal to
12.5%. The reduction of formed sulfate ions curren output in the increasing of current
density on the electrode explained by the generation of additional process oxygen gaz.
Following study, the concentration of NaCl for the formation of sulfate ions
current efficiency was researched. This research stage based on the results of the past
analysis as optimal current density of 50 A/m2 was chosen. As seen from figure 4.2
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identified that the increase in the concentration of NaCl solution, there was a step rise
in the formation of sulfate ions current output. Oxidation of sulfur with the formation
of SO42− ions exceed 100% current yield accompanied can be interpreted as the reaction
of sulfur’s disproportion (equation 1.25).
Disproportion reaction usually carried out in alkaline solution which well known.
Apparently, this reaction can take place in NaCl solution too. According to this reaction
formed sulfite ions on the anode oxidized by giving one electrode which creates an
opportunity to increase the current yield.
J = 50A/m2; t = 1 hour; T = 25ºС
Figure 4.2–The influence of NaCl concentrations to formation of sulfate ions
current output by anodic polarized composite sulfur-graphite electrode
With the increasing of the concentration of sodium chloride the current output in
the formation of sulfate ions soared. Based on the achieved data, the following research
as the most effective concentration of NaCl 150 g/l were selected.
The effect of the electrolysis of duration for the formation of sulphate ions current
efficiency shown in figure 4.3.
The anodic polarized sulfur-graphite electrode oxidation process with formation
of sulfate ions higest current output was appeared initial time of electrolysis.
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I= 50 А/m2; С (NaCl) = 150 g/l; T = 25 ºС
Figure 4.3–The influence of electrolysis duration in anodic polarized sulfur-
graphite composite electrode to formation of sulfate ions current efficiency
4.2 Electrochemical properties of dissolution elemental sulfur by formation
of sulfate-ions in sodium carbonate solution
At the present time, with the increase in oil production causes a lot of
environmental problems. In other words, oil processed in the country have a sulfur, for
example fuel (gasoline, solaria) compliance with the sulfur compounds which exposure
to engine corrosion and will have a material impact on the destruction of the
environment [112-113]. Preliminary studies, shows that anodic polarized conductive
composite sulfur electrode in sodium carbonate solution melted intensively with the
formation of sulfite and sulfate ions.
Electrolysis carried out under galvanostatic condition in electrolyzer where
electrodes spaces separated MK-40 cation membrane. Sulfur composite electrode made
by proposed our own special ways. The effects of different parameters (current density,
electrolyte concentration and duration of electrolysis) for anodic oxidation the sulfur
that contented in the composition electrode in sodium carbonate solution is considered.
As an anode was used electric conducted sulfur electrode and as a cathode was used
graphite rod [114].
During electrolysis at the oxidation of elemental sulfur anodic side takes places
following reactions:
S + 6OH- -4e → SO32− + 3H2O E0 = -0.660 V (4.8)
S + 8OH- -6e → SO42− + 4H2O E0 = -0.753 V (4.9)
2Н2О -4е → О2+4Н+ E0 = +1.228V (4.10)
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The effect of anodic current density for faradaic efficiency of generated sulfate
ions was studied in the range of 50-250 А/m2 at room temperature in 53.0 g/l sodium
carbonate solution.
If we pay attention, as seen in figure 4.4 sulfate ions highest current yield was
registered when lower current density on the anode. By the increasing of current
density, the current output of formatted sulfate ions decreased. Current efficiency was
135 % when current density on the composite electrode was 50 А/m2, and the current
density is 250 А/m2 which is current efficiency equal to 35.5 %. At higher current
density obtained lower current output explained by the increase in the share of
oxidation of hydroxide ions.
In the process of electrolysis the effect of the concentration of sodium carbonate
for the formation of sulphate ions the current yield was seen in figure 4.5. Electrolysis
carried out in the range of 26.5-212.0 g/l concentration of sodium carbonate solution.
С(Na2CO3) = 53.0 g/l; t = 1,0 hour; T = 25 ºС
Figure 4.4 – The influence of current density in anodic polarized sulfur-graphite
composite electrode for the formation of sulfate ions current efficiency
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I = 50A/m2; t = 1 hour; T = 25ºС
Figure 4.2 – The influence of Na2CO3 concentrations for the formation of sulfate
ions current output by anodic polarized composite sulfur-graphite electrode
J= 50 А/m2; С (Na2CO3) = 53.0 g/l; T = 25 ºС
Figure 4.6 – The influence of electrolysis duration in anodic polarized sulfur-
graphite composite electrode to formation of sulfate ions current efficiency
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By the increasing of the electrolyte concentration, there is a reduction in the
formation of sulfate ions current yield gradually, which consist of 142.2% when its
concentration 26.5 g/l, if increase the concentration to 212 g/l the current efficiency is
reduced by up to 121.5%.
The influence of electrolysis duration for anodic polarized sulfur-graphite
composite electrode’s oxidation in sodium carbonate solution was shown in figure.4.6.
Electrolysis was accomplished interval of 0.25-2.0 hour.
Anodic dissolved sulfur’s current output reduced by the increasing of the
electrolysis time, initial time of process oxidized elemental sulfur’s current efficiency
with formation of sulfate ions was 144.1%, if electrolysis time extended by two hours,
its value will be reduced to 46.7%.
Conclusion of section 4
In conclusion, in NaCl solution were determined that the anodic polarization
sulfur composite electrode oxidized with high current efficiency, this leads to create
simple methods to obtain inorganic compunds of elemental sulfur.
Current output of sulfate ions formed in sodium carbonate solution higher than
100% duo to the chemical reaction carried along with electrochemical process.
To compare anodic oxidation of sulfur in these two medium, in sodium chloride
obtained slfate ions purity was higher than in carbonate solution because its chemical
properties smilar to alkaline solution. In the alkaline sulfurs’ small amount is dissolved
by disproportion reaction.
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5 ELECTROCHEMICAL PROPERTIES OF ELEMENTAL SULFUR IN
ALKALINE SOLUTIONS
5.1 Cathodic electrochemical property of elemental sulfur dissolved in
sodium hydroxide solution
The presence of aggressive sulfur compounds in the ground mass of hydrocarbons
in the most fields of Western Kazakhstan create difficulties in production,
transportation, storage and processing, which makes desulfurization of petroleum and
petroleum products particularly relevant issue.
Free sulfur is formed during petroleum and sulfur-containing gases refining
process as a result of oxidation by the conventional technology. Even with the partial
realization, the stock of sulfur increases. Sulfur piles are a growing threat to the
ecological security of the region.The sulfur was produced as byproduct of efforts to
meet environmental requirements that limit the emissions of sulfur dioxide into the
atmosphere [115].
literature is known that in the chemical industry applicable range of sulfur is very
wide [116]. A byproduct of oil production-sulfur still can not find a full-fledged
consumer collected as a waste. Therefore, to create simple methods for the production
of various compounds of sulfur is very important. The reason is very little research
electrochemical properties of elemental sulfur in inorganic environment because of
sulfur isolator, insoluble in water and acid [117].
In this work for the first time in alkaline solution dissolved sulfur’s chemical and
electrochemical properties are considered, and as a resulte of electrolysis cathodic and
anodic side generated in mining process widly used flotation reagent- sodium sulfide
and in soda production used raw material -sodium sulfate’s obtainment studied
comprehensively.
In order to study chemical behavior of elemental sulfur in alkaline solution,
grinded 50 g/l sulfur powder dissoloved in sodium hydroxide solution in the range of
20-200 g/l at the temperature of 90 ºС with mechanical mixer.
According to the literary dates [118] were shown that element sulfur reacted with
hydroxide-ions based in different mechanisms and disproportionated by following
reaction:
nS +6OH- → S𝑛2− +SO3
2− +3H2O (5.1)
2S +6OH- → S2- +SO32−+3H2O (5.2)
3S+6OH- → S2- +S2O32−+3H2O (5.3)
Therefore, when sulfur powder react with sodium hydroxide solution which could
dissoloved with the formation of sulfide, polysulfide, thiosulfate and sulfite ions. In the
composition of polysulfide ions has sulfur’s ad-atoms. Its end between 2 and 6 of that
is well known from the literature. It seems such a solution can be said sulfur alkaline
suspension solution. A part of elemental sulfur dissolved 40 g/l of sodium hydroxide
solution's electrochemical properties studied comprehensively.
For the first time the effect of current density, NaON and sulfur concentrations,
the duration of electrolysis for in alkaline solution dissolved elemental sulfur’s
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cathodic restoration with formation of sulfide ions were studied.
This research first of all, the effect of current density for the formation of sulfate
and polysulfide ions current yield studied interval of 50-250 A/m2 at room temperature.
During electrolysis on the cathode occur the reactions of hydrogen separation and
reduction of sulfur:
S𝑛2− + 2e- = S2- +S𝑛−1
2− →∙∙∙→nS2- (5.4)
2Н2О +2е → H2+2OН- (5.5)
The product size obtained on the cathode side determined by the ion selective
electrode [119]. This electrode based on determination of the amount of a specific ion
concentration, for identified each ions used an own special electrode, which in
accordance with considered selective.
Sulfur in the alkaline solution will be different valence state. In alkaline solution
dissolved sulfur when reduced on cathode or oxidazed on the anodic, the current output
will be calculated duo to the element sulfur have been involved in electrochemical
reactions.
50 g/l S+40g/lNaОН; t = 1,0 сағ; T = 25 ºС
Figure 5.1– The influence of current density on the electrode in cathodic
polarized sulfur powder dissolved alkaline solution formed sulfide ions current
efficiency
On the cathode side is formed sulfide ions as a result of electrolysis. Generation
of sulfide ions current output decreased, according to the research the highest current
efficiency was shown on 100 A/m2. The reason that the current output of sulfide ions
higher than 100% explained by active separation of hydrogen gas with the formation
of sulfide ions.
The effect of the sodium hydroxide concentration for sulfide ions current yield as
a result of electrolysis formed is shown in figure 5.2.
The influence of alkaline concentration identified that in sulfur alkaline
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suspension solution doesn’t effect seriously on the formation of sulfide ions current
output.
Using the optimal values obtained in previous research, the effect of electrolysis
duration for the generated sulfide ions current efficiency was studied in the range of 1-
5 hour. The result was shown in figure 5.3.
50 g/l S+40 g/l NaOH; J=50 A/m2;T = 25 ºС
Figure 5.2 – The influence of sodium hydroxide concentration for the cathodic
polarized sulfur powder dissolved alkaline solution formed sulfide ions current
efficiency
50 g/l S+40 g/l NaOH; J=50 A/m2;T = 25 ºС
Figure 5.3 – The influence of electrolysis duration for the cathodic polarized
sulfur powder dissolved alkaline solution formed sulfide ions current efficiency
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According to the study, by the increasing of electrolysis duration, formed sulfide
ions current output at cathodic polarization reduced from 43% to 26%.
Latest studies investigated that the effect of sulfur concentration in the product
which formed on the cathode side (figure 5.4).
The results of the research was determined that by the increasing of the sulfur
concentration on the cathode formed sulfide ions current yield rose from 8% to 67%.
40 g/l NaOH;J= 50 А/m2; t = 1,0сағ; T = 25 ºС
Figure 5.4 – The influence of sulfur concentration for the cathodic polarized
sulfur powder dissolved alkaline solution formed sulfide ions current efficiency
5.2 In alkaline solution dissolved elemental sulfur oxidation with the
formation of sulfate ions
The essential problem is the presence of hydrogen sulfide in the fields of Zhanajol
and Tengiz as well. The lack of processing and sulfur application techniques leads to
serious environmental problem. Sulfur clusters are mostly accumulated in the process
of petroleum refining. With an annual capacity of 3 million tons of crude oil a stable
daily produces about 1,000 tons of sulfur. The inevitable consequence is the
technological impact of accumulated elemental sulfur and hydrogen sulfide on the
environment. Сurrently, In the Western Kazakhstan accumulated sulfur impacts on
climatic conditions of this area (extreme changes in temperature, wind, etc.), by the
day the sulfur pollution of such a large area which will can cause environmental
problems not only in Western Kazakhstan, but also in a global level.
Therefore, at the present time, is one of the most important issues is long-term
conservation of sulfur formed in the oil-gas industry and to consider new ways of
rational use in agriculture, medicine, veterinary [120].
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A theoretical basis of to obtain sulfur compounds is to learn detailed knowledge
of the physical and chemical properties of sulfur [121].
In this work, for the first time the chemical and electrochemical properties of
sulfur in alkaline medium was studied and as a result of the electrolysis in the anode
space formed sodium sulfate compound’s optimal ways to obtain (used as a raw
material in the process of obtaining soda) were discussed comprehensively.
In order to study the chemical properties of elemental sulfur in sodium hydroxide
solution, the powdery sulfur preliminary dissolved in alkaline solution.
Literary sources [122, 123] has an information about the interaction of elemental
sulfur powder with sodium hydroxide solution occurs by a complex mechanism. Sulfur
dissolved alkaline solution have been made IR- spectroscopic analysis.
The results of the study shows that thiosulfate ions dissolved with the formation
of SO32−ions, however only in 40 g/l of sodium hydroxide solution identified that the
sodium thiosulfate ions comparatively intense dissolved than other concentration
(figure 5.5).
The following research, sulfur dissolved 40 g/l of sodium hydroxide solution
electrochemical properties were discussed in detail. By the method of electrolysis, the
influence of various parameters for the anodic oxidation of pre-prepared solution of
sulfur suspension with the formation of sulfate ions were investigated. They are:
current density, NaON and sulfur concentrations, duration of electrolysis. The optimal
condition of formed sulfate ions were discussed.
Figure 5.5 – IR spectroscopic analysis of consisting of 50 g dissolved sulfur
(volume of one liter) in 40 g/l of sodium hydroxide solution
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In the electrolysis as the cathode 54 cm2 stainless steel and as anode 57 cm2
graphite electrodes were used.
The main influencing to the direction and speed for the accompanied reactions on
the electrode is current density. So first, the effect of current density on the electrode
for the formation of sulfate ions current output was studied in the range of 50-250 A/m2
at room temperature (figure 5.6).
Sulfur powder dissolved in the sodium hydroxide solution according to the last
section, prepared electrolyte poured into the anode and cathode side of electrolyzer.
During the electrolysis on the anode side may occurs following reactions:
SO32−+ 2OH- -2e → SO4
2− + H2O (5.5)
S𝑛2− -2e → nSº (5.6)
S+ 8OH- -6e → SO42− + 4H2O (5.7)
S2O32−+ 6ОН- -4е = SO3
2−+ H2O (5.8)
4ОН- -4е → О2+2H2O (5.9)
40 g/l NaOH + 50 g/l S;J=50 A/m2; t = 1.0 сағ; T = 25 ºС
Figure 5.6 – The influence of current density on the electrode inanodic polarized
sulfur powder dissolved alkaline solution formed sulfate ions current efficiency
Anode space of electrolyzer the formation of sulfate ions current output decreased
by the increasing of current density. This depends on additional process discharge
hydroxyl ions with the separation of oxygen. When anodic current density was 50
A/m2, the formation of sulfate ions the current yield does not exceed of 82.5 %, and at
250 A/m2 which less than 30%.
The increase in current density, active distribution of oxygen gas in the anode
leads to a reduction in the formation of sulfate ions current yield.
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As a result of the electrolysis, the effect of the concentration of sodium hydroxide
for the formed sulfate ion current yield was shown in figure. 5.7.
As shown in the figure, we have discussed the effect of the alkaline concentration
for the formation of sulfate ions current yield, generated sulfate ions current output
minimum value was 22.5% at 20 g/l, at the 40 g/l NaOH solution gives highest current
efficiency which equal to 82.5%.
Previously received optimal result for the anodic oxidation of elemental sulfur,
the effect of electrolysis duration for the obtained sulfate ions current efficiency was
studied in the range of 1-5 hour. The result was shown in figure 5.8.
As a result of the study given that by the increasing electrolysis every time, sulfate
ions formation of current yield reduced from 81% to 12%.
50 g/l S; J=50 A/m2; t = 1.0 сағ; T = 25 ºС
Figure 5.7 – The influence of sodium hydroxide concentration for the anodic
polarized sulfur powder dissolved alkaline solution formed sulfate ions current
efficiency
As a result of the study given that by the increasing electrolysis every time, sulfate
ions formation of current yield reduced from 81% to 12%.
In the latest research, the effect of sulfur concentration for anodic side formed
product was investigated (figure 5.9).
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50 g/l S+40 g/l NaOH; J = 50 A/m2; T = 25 ºС
Figure 5.8 – The influence of electrolysis duration for the anodic polarized
sulfur powder dissolved alkaline solution formed sulfate ions current efficiency
40 g/l NaOH; J= 50 А/m2; t = 1,0сағ; T = 25 ºС
Figure 5.9 – The influence of sulfur concentration for the anodic polarized sulfur
powder dissolved alkaline solution formed sulfate ions current efficiency
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According to the result identified that by the increasing sulfur concentration,
anodic side formed sulfate ions current efficiency decreased gradually. This
phenomenon explained by the growth in sulfur amount leads to increase the proportion
of polysulfide ions. Oxidation of sulfur to sulfate ions used 6 electrons but oxidation
to sulfide ions needed 8 electrons.
5.3 Investigation of electrochemical properties of elemental sulfur
preliminary dissolved in alkaline solution by recording the anodic and cathodic
potentiodynamic polarization curves
A comprehensive analysis of the polarization curves forms and investigation of
its dependence on the concentration, temperature and other physical and chemical
parameters, allows you to get detailed information about the processes kinetics and
nature which carried out on the surface of the electrode.
In order to study, electrochemical properties of elemental sulfur in sodium
hydroxide solution, in the range of 10-50 g/l sulfur powder under vent cupboard at 90
S temperature while stirring with the help of a mechanical mixer dissolved in 40 g/l
aqueous solution of sodium hydroxide.
For the purpose of deep understanding the redox properties of the elemental sulfur
dissolved in alkaline solution were taken "anode" and "anode-cathode" polarization
curves with solution where different sulfur concentration of has been dissolved in 40
g/l solution of sodium hydroxide.
Preliminary sodium hydroxide dissolved sulfur electrolyte captured the anodic
potentiodynamic curves results was shown in figure 5.10.
V=50mV/s; t=25 oС; 1) С= 40g/l NaOH 2) С= 40g/l NaOH + 10 g/l S; 3) 40g/l
NaOH + 20 g/l S; 4) С= 40g/l NaOH + 30 g/l S; 5) С= 40g/l NaOH + 50 g/l S
Figure 5.10 – The anode potentiodynamic polarization curve of elemental sulfur
dissolved alkaline solution on rhodium electrode (a-polysulfide-ions oxidation
maximum (Imax) dependence on the concentration of sulfur)
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On the anodic polarization curve of testing solution (1M sodium hydroxide) on
rhodium electrode where registered only oxidation of hydroxide-ions with formation
of oxygen (figure 5.10, curve1).
4OH- -4e → O2 +2H2O (5.10)
And this alkaline solution by the increasing of sulfur concentration in the interval
of 0-50 g/l and its its potential forced positive in the direction, on the anode
polyarogram "minus" 0.6V- "plus" 0.2V were registered two or three oxidation waves
and maximum (figure 5.10, curves 2-5). This phenomenon can be explained that
oxidation of polysulfide-ions to sulfur was a stage process.
S𝑛2−– 2e → nSo (5.11)
In alkaline solution by the increasing of sulfur concentration, the intensity
distribution of oxygen increased too. It will be decreased the intensity of oxidation of
polysulfide-ions and its shown on that maximum value of potential shifted to cathodic
side. If potential value of rhodium electrode further shifte to "plus" 1.0V side, on the
polarograma registered oxidation wave of sulfur to sulfite ions.
Alkaline solution in advance dissolved elemental sulfur’s electrochemical
properties was studied by taking anode-cathode cyclic poteniodinamic polarization
curves (figure 5.11).
V=50mV/s, T=25 oС;С; 1) 40g/l NaOH; 2) 40g/l NaOH + 10 g/l S
Figure 5.11 – The anode-cathodic cyclic potentiodynamic polarization curves of
elemental sulfur dissolved alkaline solution on rhodium electrode
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Anodic-cathodic cyclic polarization when potential value of rhodium electrode
shifted on anodic side in the range of «minus» 0.6 and «plus» 0.2 V, as shown figure
5.10 there were registered stage oxidation peak of polysulfide-ions to elemental sulfur.
Oxidation waves of active sulfur to sulfite-ions among «plus» 0.75V– «plus» 1.0V
potential was registered on polyogramma (equation 5.12).
S + 6OH- - 4e ↔ SO32− + 3H2O (5.12)
When shifted potential of rodium electrode to cathodic side, at «minus» 0.25 V
formed sulfite ions re-reduction current wasn’t resgistered, which indicates a small
amount the sulfite and sulfate ions were formed and this irreversible reaction.
if shift rhodium electrode potential to more negative value, there is a possibility
of its restoration with the formation of polysulfide ions according to equation 5.11 due
to the reduction of sulfur to plysulfide is carried out with high speed, on the rhodium
electrode was not shown the separation of hydrogen gas.
The results of polarization curves, oxidation of sulfite ions to elemental sulfur
occurs until oxygen separation potential, in this work on the rhodium electrode,
potential observed in the territory of "plus" 1.13V. Oxygen separation of intensity
increased by formed sulfur atoms on the surface of electrode.
Anodic side of anodic-cathodic cyclic polarization, until oxygen formation
potential wasn’t registered oxidation of sulfate-ions. But the results of the special case
of Galvano static electrolysis was identified that sulfite-ions in the electrolyte during
the electrolysis was oxidized to sulfate-ions with active oxygen which formed on the
anodic side.
SO32−+2OH- -2е→SO4
2−+Н2О Ео = 0.05V (5.13)
SO32− + O2 + H2O → SO4
2− + OH- (5.14)
The potential value shifted towards cathode side from "minus" 0.5V potentials
polysulfide-ions in the solution reducted to monosulfide-ions(figure 5.11, 2-curve).
𝑆62−+2e →S2- + 𝑆5
2−+2е → ···→6 (5.13)
But polysulfide ions ions to monosulfide ions stage reduction on the
polyarogramma wasn’t registered.
On the cathode potential as above, for the first time there is no separation of
hydrogen gas, consequently, the current is losing by the formation of monosulfide-ions
according to equation 5.13. Only after a certain period of time is divided into hydrogen
gas.
By the increasing of scans rate, on the rhodium electrode a growth anodic
oxidation current maximum of polysulfide-ions to the elemental sulfur was observed
on the polarograma (figure 5.12), which that oxidation of polysulfide ions occurs in
diffusion regime.
By the increasing of scan rate, current maximum is grown too, such the connection
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between the scan rate and the limited current size’s proportional growth identified that
oxidation of polysulfide ions carried out with diffusion mode.
40g/l NaOH + 10g/l S; T=25 oС; mV/s: 1) 25; 2)50; 3)100; 4) 150; 5) 200
Figure 5.12 – The influence of scan rate on the anodic potentiodynamic
polarization curves of elemental sulfur dissolved alkaline solution on rhodium
electrode (a- oxidation maximum (Imax) of sulfur dependence on (mV/s) the scan rate)
The effect of temperature for the anodic potentiodynamic polarization curve of in
alkaline solution dissolved elemental sulfur on rhodium electrode were investigated in
the range of 25-65 oС (figure 5.13).
With increasing of electrolyte temperature, the peak of anodic maximum current
on the voltage curve exceed too, maximum potential value was shifted towards anodic
side.
From the effects of temperature dependences, the effective activation energy of
the anodic oxidation process for elemental sulfur dissolved in alkaline solution on
rhodium electrode with dependence of lgIip - 1/T was calculated by Gorbachev [124]
method (figure 5.14), its value equal to 13.67 kJ/mol, which indicated the oxidation
reaction of sulfur occurred in diffusion mode.
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40g/l NaOH + 10g/l S; t, 0С: 1) 25; 2) 35; 3) 55; 4)65
Figure 5.13 –The effect of temperature for the anodic potentiodynamic
polarization curves of elemental sulfur dissolved alkaline solution on rhodium
electrode (oxidation maximum (Imax) of sulfur dependence on the temperature (oС) of
electrolyte)
Figure 5.14 – In alkaline solution dissolved elemental sulfur’s lgI value
dependence on temperature (1/Т∙103)
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5.4 Obtainment of monosulfide and investigate its electrochemical properties
by unloading polarization curves
According tothe range of anthropogenous influence to the environment and from
here arised danger level, decontamination of environmentally harmful substances, the
development of various technological processes and considering its new ways one of
the essential issues of the day. Development of new technologies to obtain eco-friendly
and wasteless product, electrochemical methods have been taken an important place.
In our country during the desulphurization stage of oil, large amounts of contained
toxic byproduct elemental sulfur will produced. The accumulation of large amounts of
sulfur emissions have caused serious environmental problems. Therefore, harmful
sulfur emission change into the commodity products that improved the economic
performance of the industry, and lead to create opportunities for the solution of
environmental problems [125,126].
At present obtaining of sulfur compounds with known methods are complicated,
expensive and is not in accordance with the requirements of the environmental
aspects.Therefore, to find a simple, inexpensive and efficient methods for synthesis
of the inorganic compounds of sulfur are today’s the main issues.
Based on this research can be made the method of obtaining the alkali metal
sulfide compounds from sulfur emissions. The principal technological schemes from
oil production generated sulfur emissions processing is shown in figure 5.16, 5.18.
X-ray phase analysis for the nature of sodium sulfide obtained by electrochemical
way was carried out with the help of the American ASTM card-indexes (figure 6.17).
The parameters of of the diffraction line intervals of sodium sulfide on the
rentgenogramma corresponds to the values of American card files (3.21 А0; 2.80 А0;
2.98 А0; 2.62 А0; 1.89А0). As well as was determined that the diffraction maximums
accordance with the crystal lattice structure of Na2S • 9H2O.
In this scientific work, preliminary dissolved sulfur powder in alkaline solution
electrolyte, for the first time, under electrolyze on the cathodic side obtained flotation
reagents-monosulfide which have been widely used in mining and its electrochemical
property studied by the method of removing the potentiodynamic polarization curves.
In order to study electrochemical property of elemental sulfur, measured between
1-10 g of sulfur powder dissolved in 40 g/l aqueous solution of sodium hydroxide at
90 0C temperature and mixed with a mechanical agitator. When sulfur completely
dissolved in the sodium hydroxide the сolour of solution will be changed orange, then
stopped hearting process and sent to cooling in water bath.
Various sulfur ions obtained the solution poured into the cathode side of
electrolytic cell with capacity of 200 ml where the space of electrode was allocated
with MK-40 cationite membrane. As an anodic and cathodic electrode were used 57
cm2 graphite and 54 cm2 titanium electrode. Electrolysis was carried out 3-4 hours,
during the electrolysis polysulfide’s ions orange-yellow color in the electrolyte to be
held gradually to colorless state. It identified that polysulfide and other ions in the
solution gradually passing to the monosulfide ions [127]:
Sn2−+ 2e → S2- + S𝑛−1
2− → ……+nS2- (5.14)
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2SO32− + 4e + 3H2O →𝑆2O3
2−+ 6OH- (5.15)
S2O32− +8e + H2O →2S2- + 6OH- (5.16)
Completed monosulfide solution pured into special containers and is sent to the
customer or after evaporation process could be obtained sodium sulfide crystals.
1- titanium electrode, 2- cationite membrane, 3-graphite electrode
Figure 5.15 – Electrolyzer used for receive monosulfide
One of the main methods for the investigation of electrochemical reactions of
mechanisms and kinetics is considered to take a voltage curves and analysis which
was characterized the relationship between the current density with electrode potential.
Polarization curves allows to get detailed information about the nature of the reactions
occurring on electrodes [128].
Sulfur dissolved sodium hydroxide solution at the room temperature poured into
the cathode space of electrolysis, which polarized with cathodic current, and identified
that the platinum electrode "red-ox" potential value by the time changed in the form of
a wave and shiftd in the territory of negative potentials (figure 5.19). Hence, in the red-
ox system of S - 𝑆𝑛2− , polysulfide ions gradually passage to monosulfide ions, thevinert
platinum electrode potential value is moved toward the negative potential values.
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Figure 5.16 – The principal scheme of obtainig of sodium monosulfide by
processing of sulfurwaste
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1-thermostat; 2-mixer; 3 tripod; 4-current source; 5-MK-40 cation membrane; 6
- anode; 7 - cathode; 8-mass-exchange apparatus
Figure 5.18 – The fundamental technological scheme of obtainig of sodium
monosulfide by processing of sulfurwaste
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С= (40 g/l NaOH + 7 g/l S)
Figure 5.19 – During the electrolysis (S-𝑆𝑛2−) ions red-ox potential value
dependence on the time
Measured potential value dependence on time during electrolysis shown that the
passage of polysulfide ions to monosulfide was another confirmation of cathodic
electrode process that accompanied by complex stages.
In order to deeper understanding of the oxidation properties of obtained
monosulfide ions in alkaline medium after electrolysis was studied by removing anodic
and anodic-cathodic polarization curves in alkaline different amounts of sulfur
dissolved solutions on rhodium electrode (figure 5.20).
In figure 5.20 was shown alkaline monosulfide solution’s anode-cathode cycle
potentiodynamic polarization curves on rhodium electrode after the electrolysis.
On 40 g/l NaOH solution’s anodic-cathodic potentiodynamic cyclic polarization
curve on rhodium electrode were registered only oxygen and hydrogen gases
generation current.
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V=50mV/s; T=250С; 1)С= 40 g/l NaOH ; 2) С= 40 g/l NaOH+7 g/l S2-
Figure 5.20 – Anode-cathode potentiodynamic cyclic polarization curves of
monosulfide solution on the rhodium electrode
And the potential value of the rhodium electrode submerged in the electrolyte
have monosulfide ions shifted towards anodic side, in the potential area «plus» 0.1V -
«plus» 1.2V, on the polyarogram (figure 5.20, curve-2), newly formed sulfur’s anodic
oxidation wave to sulfite ions fixed clearly (5.17-reactions).
S + 6OH- - 4e →S𝑂32−+3H2O E0= -0.660V (5.17)
In figure 5.19 oxidation of monosulfide ions to elemental sulfur is not observed,
but at low current density captured polarization curves, specially at high temperatures,
in the potential space "minus" 0.5 V "plus" 0.2V were registered two or three waves of
oxidation (figure 5.21). This wave can be judged the monosulfide ions’s stage
oxidation related with the formation of disulfide, polysulfide further elemental sulfur.
The rhodium electrode potential shifted towards anode side, the first monosulfide-
ion is oxidized to elemental sulfur atom by taking two electrons, at the same time which
joined with other monosulfide and formed disulfide-ion, while this gradually formed
𝑆62−- ploysulfide ions, then sulfur atoms.
6𝑆2−-2e→𝑆0 +5𝑆2−→ 𝑆22−+4S0– 2e →…𝑆6
2−-2e→ 6𝑆0 (5.18)
On the polyarogram does not registered all of the stage oxidation waves of
monosulfide ions. In the potentials territory of "Plus" 1.2 V on polyarogram was
registered oxygen gas separation current. As you can see the curvature of the
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polarizing, monosulfide ions in an alkaline solution oxygen gas is divided by high
voltage.
The potential of electrode forced from the anodic to cathodic potentials area,
reduction wave of formed products were not registered, only at "minus" 1.2 V potential
there is a current of hydrogen gas separation.
An enlarged scale of anodic potentiodynamic polarization curves of rhodium
electrode's in sulfur dissolved alkaline solution is can be seen from figure 5.21, it is
shown on the polyarogram the anodic oxidation waves of monosulfide ions to the
elements sulfur and its the number of maximums are increased gradually.
40g/l NaOH + 7 g/l S; T, 0С: 1-25; 2-35; 3-45; 4-55; 5-65;
Figure 5.21 – Anodic potentiodynamic polarization curves of rhodium electrode
in monosulfide solution from "minus" 0.5 to "plus" 0.28 V
On the polyarogram was registered that by the increasing scan rate the oxidation
peak elemental sulfur to sulfite ions increased (figure 5.23). This explained oxidation
of sulfur could take a place in diffusion regime.
The effect of temperature for the anodic potentiodynamic polarization curve of
monosulfide ions in alkaline solution on rhodium electrode were investigated in the
range of 25-65 °C (figure 5.24).
It’s visible to see an increase in the number of oxidation waves due to the increase
in temperature, on polyarogram registered in the territory of "minus" 0.4V, "plus" 0.1V
and "plus" 1.0 V. 1st peak shows oxidation of monosulfide ions up disulfide ions, next
peak up to "plus" 0.8V waves it is due to polysulfide ions to the formation of elemental
sulfur in stages. In the territory "plus" 1.2V -1.4V potentials can be explained by
oxidation of sulfite ions to sulfate ions.
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V=50mV/s; T=250С; 1) С= 40g/l NaOH +1 g/l S2- ; 2) С= 40g/l NaOH + 5 g/l
S2-; 3) С= 40g/l NaOH + 7 g/l S2- ; 4) С= 40g/l NaOH + 10 g/l S2- ; 5) С= 40g/l NaOH
+ 20 g/l S2-
Figure 5.22 – Anodic potentiodynamic polarization curves various amount of
sulfur contained monosulfide solution on rhodium electrode
40g/l NaOH + 7 g/l S2-; T=25ºС; v, mV/s: 1-25; 2-50; 3-100; 4-150; 5- 200
Figure 5.23 – The influence of scan rate on the anodic potentiodynamic
polarization curve of monosulfide ions in alkaline solution on rhodium electrode (a-
oxidation maximum (Imax) of sulfur dependence on (mV/s) the scan rate)
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40g/l NaOH + 10g/l S; T=250С; t, 0С: 1) 25; 2) 35; 3) 55; 4)65
Figure 5.24 – The effect of temperature for the anodic potentiodynamic
polarization curves of alkaline monosulfide solution on rhodium electrode (oxidation
maximum (Imax) of sulfur dependence on the temperature (0С) of electrolyte
Сурет 5.25 – lgI value of monosulfide ions in electrolyte dependence on
temperature (1/Т∙103)
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From the effects of temperature dependences on the stage oxidation of alkaline
monosulfide solution ions was calculated activation energy of the process (figure 5.25),
which value equal to 13.43 kJ/mol, that shown the oxidation reaction of monosulfide
on the rhodium electrode occurred in diffusion mode.
Conclusion of section 5
Conclusion, from alkaline sulfuric suspension electrolyte made by the way of
chemical dissolution of elemental sulfur determined that as a result of the electrolysis
could be obtain sulfide ions with a high current yield. The amount of sulfide ions were
identified by the ion selective electrode.
For the first time, through the dissolution of elemental sulfur made alkaline sulfur
suspension electrolytes’ the oxidation regularities with formation of sulfate ions were
investigated by electrolysis method. The amount of sulfate ions from electrolysis was
determined by quantitative analysis method.
For the first investigation of the electrochemical properties of elemental sulfur
dissolved in alkaline solution using the method of removing the potentiodynamic
polarization curves on rhodium electrode. Formed polysulfide-ions by dissolving
sulfur in sodium hydroxide solution, at cathodic polarization reduced to monosulfide-
ions, but it shows anodic polarization until the formation of oxygen, which oxidized
elemental sulfur to sulfite-ions.
The various amount of elemental sulfur dissolved in alkaline solution as a result
of electrolysis obtained monosulfide ions electrochemical behavior for the first time
studied by method of removing anodic and anodic-cathodic the potentiodynamic
polarization curves. Anodic oxidation of monosulfide ions to sulfite and sulfate ions
could occurred with stage formation of intermediate product of disulfide, polysulfide
and elemental sulfur.
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6 RECEIVE OF COPPER SULFIDE AND ITS ELECTROCHEMICAL
BEHAVIOR
At present, in the synthesis of the inorganic compounds of non-ferrous metals,
using of electrolysis with stable and the industrial AC frequency currents gives
effective results so that is has been used widely in many industries. Also, in the
processing of non-ferrous metals of sulfur ore a large quantities of chalcogen are
produced as waste, therefore, to obtain there useful compounds, concentrate and
protection of the environment from poisoning methods should be considered
comprehensively [129]. By electrochemical methods creation of a non-waste
technology and improvement of its measures are considered as a effective way to solve
a number of environmental issues. The carried out results of the research work show
the effective opportunities in the creation of simple methods to obtain many of metal
salts [130]. In the republic, the main source of income for the country's economy
considered – oil refining, with the increase in the production area, one of the top arising
issue is undeveloped elemental sulfur’s open accumulation. In order to increase rational
use of natural resources of elemental sulfur, professor Baeshova and his scientific staff
has been done the number of studies in the direction of "to reveive metal sulfide" [131-
135], contributed to the elimination of dependence on imported flotation reagents
which applied in the field of mining and processing. It is well known that there are a
number of advantages in inorganic compounds of metals produced by electrolysis
[136].
The main aim of the proposed work, to study the influence of various parameters
for the reduction process of copper (II) ions with sulfite ions in the aqueous solutions,
which: current density on the cathode, copper (II) and sulfite ions concentration, the
concentration of sulfur acid, electrolysis duration.
Preliminary studies identified that in acid aqueous medium the copper (II) and
sulfite ions reduced along with on cathode by the formation of copper sulfide powders.
In the process of electrolysis first of all, the effect of current density on the
titanium electrode for the formation of copper sulfide powders current yield was
investigated at intervals between150 – 300 A/m2. Electrolysis was carried out in
electrode space retained 150 ml electrolyzer. As a cathode 6 cm2 titanium and as a
anode 9.2 cm2 copper electrodes were used. For the main research, as a electrolyte was
used the mixed solution of 10 g/l of sodium sulphite, 7.5 g/l of copper (II) sulphate and
50 g /l sulfuric acid. After electrolysis, the fromed powder is filtered, and washed with
distilled water, then processed with sulfuric acid solution, against
Filtered out and rinse with distilled water, at the end dried. The obtained powder
searched by the method of X-ray and identified that black copper sulfide (CuS) powder
is formed.
During the electrolysis the copper (II) and sulfur (IV) ions can be reducted on the
cathode by following reactions:
Cu2+ + 2e → Cuo Eo= + 0.34 V (6.1)
SO32-+ 4e + 6H+ = So + 3H2O E0 = + 0.45 V (6.2)
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The newly formed active sulfur and copper atoms can quickly interact with each
other and created copper sulfide:
Cu + S → CuS (6.3)
2Cu + S → Cu2S (6.4)
10g/l Na2SO3+ 7.5g/l CuSO4+ 50g/l H2SO4,T= 25 0C, t= 1hour
Figure 6.1 – The influence of current density for the formation of copper sulfide
powder
As seen in figure 6.1, by the increasing of current density, the formation of copper
sulfide current output decreased. This explained by the increasing of additional
hydrogen separation share with a reaction of 6.5:
2Н+ + 2е → Н2 (6.5)
In figure 6.2, the effect of the concentration of copper (II) ions for the formation
copper sulfide powders current yield was discussed.
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10g/l Na2SO3 + 50g/l H2SO4,Jk= 200 А/m2,T= 25 0C, t= 1hour
Figure 6.2 – The influence of copper (II) ions concentration for the formation of
copper sulfide powder
As shown, with increasing of copper (II) ions, the formation of sulfide powder
current efficiency at the first rose to 100 % then higher concentration which reduced.
This is illustrated by high concentration, the copper ions formed copper (II) sulfide
along with the copper powder. At high concentrations additional elemental copper
powders formed so obtained powder dissolved in diluted sulfuric acid then inflated by
the air. After that copper sulfide filtered again and rinsed distilled water, dried, then
the weight was measured. Consequently, by the growing copper (II) ion concentration,
the formation of the pure copper powder share started increase. Again, it should be
noted that copper (II) ions concentration’s rise to create an opportunity in the
generation of Cu2S compounds.
The effect of sodium sulfite concentration for the formation of copper sulfide
powders current yield was studied (figure 6.3).
Sodium sulfite concentration’s optimal condition were observed at 10 g/l. This
phenomenon explained by high concentration of sodium sulfite during electrolysis with
copper sulfide additional substances, mainly due to the formation of elemental sulfur.
The effect of sulfuric acid concentration for the formation of the copper sulfide
powders current yield are shown in table 6.1. As seen determined that the concentration
of sulfuric acid does not affect the current yield.
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J=200 А/m2, С =7.5g/l CuSO4 + 50g/l H2SO4, t=1hour, T= 25 оC
Figure 6.3 – The influence of sodium sulfite concentration for the formation of
copper sulfide powder
Table 6.1– the influence of sulfuric acid for the formation of copper sulfide powder
current efficiency
С(Н2SO4) 50 100 150 200
Ƞ, % 97.2 93.2 93.4 93.6
In the final research, the effect of electrolysis duration for the formation of copper
sulfide powder was studied (figure 6.4). On the basis of the achieved results identified
that by the time, for the generation of copper powder current output at the 1st hour
which reached to 93%, but after four hour of electrolysis its value reduced to 27%.
Copper (I) sulfide powders roentgenogram was shown in figure 6.6. The results
of the study showed that 1.67; 1.86; 1.94; 2.37; 2.62; 2.88; 3.00; 3.19; 3.33 reflexes -
CuxS (ASTM 23-957), (1.96> x> 1.86). In addition, when the obtained powder was
sent for elemental analysis too, which shown the powder contained 63.63% copper,
and 31.03% of the sulfur.
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J= 200 А/m2, С= 10g/l Na2SO3+ 7.5 g/l CuSO4+50 g/l H2SO4, T= 25 оC
Figure 6.4 – The influence of electrolysis duration for the formation of copper
sulfide powder
Figure 6.5 – Copper(II) sulfide powders roentgenogram
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Figure 6.6 – Copper(I) sulfide powders roentgenogram
Conclusion of section 6
The main parameters of copper sulfide particles output current were studied such
as. The density of current the ions of copper (II), the consent ration of sulphate sodium
sulfide, the time duration. The highest output of copper sulfite was shown in optimal
condition
The results of the research, two-valent ions of copper and four-valent sulfur ions,
when reduced along with in sulfuric acid solutiom on the titanium electrode shown the
formation of copper sulfide powder. Based on obtained indicators can be create the
receipt method of copper sulfide. The main product of the electrolysis were copper
sulfide CuS and the Cu2S powder.
Тhe obtained powder was analysed with X-rays and which determined that
powder was the copper sulphade. Аt the same time,when we sent the powder to
elemental analysis was identified that the composition of powder contained with
63.63% of copper 31.03% of sulfur.
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7 CREATION OF CHEMICAL POWER SOURCE BY USING
OXIDATION REACTIONS ON THE SULFUR COMPOSITION ELECTRODE
7.1 The regularities of the formation of motive force in the galvanic pair of
"sulfur-graphite" - "lead dioxide"
All over the world, including in our country, one of the main directions of the
strategy for the industrial and innovative development is power supply of the
population. XXI century in the world, especially in developing countries, rapidly
growing in useing of electricity. Global consumption energy in every fifty years, the
limit of growth will be expected to increase more than twice. This growth due to the
growth of population, economic development and increase in use of electricity. Of
course, energy should be manufactured by without decreasing the environmental
situation. for a long time has been used coal, oil, natural gas and other energy sources
exhaustion or reduction of fund, in addition the harmful effects on the environment is
growing by the day.
At present, there is a shortage of energy in the world and in order to get rid of it,
there have been done a lot of work. Including the environmental problems effective
solutions is a non-waste technology that was popular modernity topics [137].
Currently, in the world more than 500 chemical power sources (CPS)
electrochemical was systems, only about 40-50 of them carried out the practical side.
All over the world annually produced number of batteries the accumulator exceeds
more than billion. One of the world's largest producers in the production of various
types of power sources and great contribution to the energy sector are: VARTA,
Hawker Batteries Group, FIAMM, Benning, Chloride Power Electronics, American
Power Conversion, Best Power, ABZ Aggregate-Bau GmbH, SDMO, Toshiba,
Siemens, Duracell and etc. [138-142].
According to the principle of work CPS divided into three groups: primary,
secondary and fuel cells. Primary electrodes CPS or galvanic cells constitute of active
substances after a full consuming power sources stops there work. Secondary CPS
(batteries) after consuming the active substances (row) again charged with an electric
current. And primary chemical power sources can be attributed a reserve elements. The
main difference will be which is unemployed condition for a long time. Its three main
reasons: electrodes can be isolated electrolyte, can be solid state, chemically inert, or
do not participate at all. Reserve battery will be ready to work only when they are faced
with the state of activation [143-145].
Today is one of the main directions to increase functional index of modern
chemical power sources, They are differ from high values of EMF and SCC, non
negative impact of electrode material on the environment and efficiency in the
economic context [146].
In this work, for first time as a negative electrode was used composite of sulfur-
graphite. In our country at the present day, from oil production more than one million
tonnes of elemental sulfur was accumulated [148].
In this regard, to obatain the chemical power sources in the use of elemental sulfur
has a special significance and could be one of the solutions in solving the
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environmental problem.
In this proposed work as a chemical current source submerged in a solution of
sulfuric acid in the galvanic pair of "sulfur-graphite" and "lead dioxide" at between the
electrodes presented electro motive forces formation of regularities was investigated
[149]. For example:
(С)S | H2SO4 | PbO2
Phenomenon of the formation electric motive force in these galvanic pair was
studied. Research was carried out with capacity of 100 ml glass electrolyzer. This
dishes is filled with sulfuric acid, as electrode "lead dioxide" and "sulfur-graphite"
composite wereused. Sulfur and graphite electrodes - serves as the negative pole of the
galvanic cell and lead dioxide the positive pole. Experimental installation is shown in
figure 7.1.
Electrodes directly connected to the voltmeter and the electromotive force (EMF)
values to be measured constantly. And after the period of time (10 minutes) ammeter
connected to the chain, the short circuit current (SCC) identified then which put off
again. Lead dioxide electrodes prepared by placing in the middle of plastic cylinder
with small holes all the side which was filled lead sulphate through the anode
polarization under condition of current I = 0.13A, EMF E = 3.1V and t = 20 min.
Preparation of composition sulfur-graphite electrodes has been done through [147,148]
professor A.B Baeshov with his disciples proposed methods.
1-lead sulfate powder; 2-lead electrode; 3-sulfur-graphite composite electrode;
4-sulfuric acid solution; 5-аmmeter; 6-voltmeter
Figure 7.1– Installation scheme for the study of the phenomence of the
formation of electric current in "(C) S - PbO2" galvanic pair
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During the research, the influence of sulfuric acid solution concentrations and
time for the formation regularity of electro motive force and short circuit current in the
galvanic pair of "sulfur-graphite" and "lead dioxide" at between the electrodes was
investigated.
Galvanic pairs of "(C) S - PbO2" between the electrodes, the electro motive force
(a) and short circuit current (b) valus changes by the time was shown in figure 7.2, 7.3.
According to the results of the experiment identified that EMF provides the maximum
value of 1050 mV, and initially the value of SCC is equal to 40mA further its value
will begin to decline.
The influence of sulfuric acid concentration for the formation of short circuit
current and electro motive force was studied in the range of 25-200 g/l (figure 7.4, 7.5).
When increasethe concentration of sulfuric acid from 25 - 125 g/L the values of
EMF and SCC increased dramatically, and at higher concentrations shown reduction.
By increasing of sulfuric acid concentration, a rise in the values of EMF and SCC
can be explained by the increase in the electrical conductivity of the electrolyte. A
decrease in high concentration of acid which can be illustrated by inhibition of
elemental sulfur’s (equation 7.1) oxidation reaction.
On sulfur electrode takes place following reaction, sulfur oxidized and ions will
be held to the solution [149]:
S + 3H2O – 4e → H2SO3 + 4Н+ Е0 = 0.450 V (7.1)
The electrons by the external chain from sulfur - graphite electrode through the
lead dioxide (PbO2) where the basis of the reduction reaction lead sulfate is formed:
PbO2+SO42−+ 4Н++2е = PbSO4+ 2Н2О Е0 = 1.682 V (7.2)
In terms of the theory the value of EMF generated between the two electrodes
must be as follows:
Е = Е1 – Е2 = 1,682 –0,450 = 1.232 V
As well, in this we proposed galvanic element, oxidation reaction of composition
sulfur graphite electrode (equation 7.1) and reduction reaction of lead dioxide is carried
out by (equation 7.2). Consequently, sulfur galvanic cell plays the role of negative
charged electrode and PbO2–is positively charged
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Figure7.2 – The quantity of EMF formed between the electrodes in "(C) S -
PbO2" galvanic pair changes by the time: (100 g/l H2SO4)
Figure 7.3 – The amount of SCC formed between the electrodes in "(C) S -
PbO2" galvanic pair changes by the time: (100 g/l H2SO4)
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Figure7.4 – The quantity of EMF formed between the electrodes in "(C) S -
PbO2" galvanic pair depence on the concentration: (t=10 min)
Figure7.5 – The quantity of SCC formed between the electrodes in "(C) S -
PbO2" galvanic pair depence on the concentration: (t=10 min)
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Conclusion of section 7
In the sulfuric acid solution "sulfur and lead dioxide" galvanic pair can be used as
a chemical power source in laboratories. This shown galvanic pair, for the first time
element sulfur can be used to obtain electric current.
Galvanic pairs of "(C) S - PbO2" between the electrodes, the electro motive force
and short circuit current valus changes by the time and concentration of electrolyte
were investigated. Maximum values of SCC and EMF concentrations sulfuric acid in
the range of 50-125 g/l registered. The value EMF is 1050mV and SCC value equal to
40 mA.
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CONCLUSION
In this dissertational work, for the first time by DC polarized sulfur-graphite
electrodes’ electrochemical anodic dissolution in aqueous solutions of hydrochloric
acid and neutral (Na2CO3, NaCl) and preliminary sodium hydroxide dissolved sulfur’s
redox regularities were studied. For electrochemical process the effects of current
density on the electrode, solution concentration and the duration of the electrolysis
were discussed. Based on the concluded literature review and carried experiment
results, the possibilities of synthesis of inorganic compounds of sulfur by
electrochemical way was shown.
According to the results of conducted comprehensive study the following
conclusions have been drawn:
- electrochemical properties of elemental sulfur preliminary dissolved in alkaline
solution studied on rhodium electrode by unloading anodic and anodic-cathodic
potentiodynamic polarization curves. According to the polyogramms, on the anode
oxidation of polysulfide ions carried out with stage process. By temperature- kinetic
method the effective activation energy of the anodic oxidation process of polysulfide
ions was identified, which is equal to 13.67 kJ/mol, this illustrated oxidation of
polysulfide ions took away with restriction of diffusion.
- elemental sulfur preliminary dissolved in alkaline solution formed ions which
electrochemical behaviors for the first time was investigated. Under optimal condition
sulfate ions current yield on the anode was 81%, and at the cathode polarization formed
sulfide ions current yield was equal to 46.7%.
- in alkaline solution in advance sulfur dissolved and the generated ions by
cathodic polarization was obtained monosulfide solution, and for the first time its
electrochemical properties comprehensively studied by unloading anodic and
cyclicpotentiodynamic polarization curves. Activation energy of oxidation process,
with dependence of lgIip - 1/T was calculated by Gorbachev’s method its value equal
to 13.43 kJ/mol.
-in alkaline medium the polysulfide consist solution polarized on cathodic side
and its "red-ox" potential value measured on inert platinum electrode. Over time, for
the first time identified that the red-ox potential varied six forms of wave. These
research results identified in polysulfide ions 𝑆𝑛2−- the value of “n” equal to six.
Determined that, by the time on the cathode side yellow coloured polysulfide ions, due
to the formation of monosulfide ions its changed colorless solution.
- anodic properties of sulfur-containing composite electrode were studied.
Shown that the sulfur oxidized with formation of sulfate ions. For the first time at
optimal condition sulfur in hydrochloric acid solution current output by the formation
of sulfate ions achieve up to 52%.
- two-valence ions of copper and four-valence sulfur ions, when reduced along
with in sulfuric acid solution on the titanium electrode identified the formation of
copper sulfide powder. Optimal conditions the formation of copper sulfide powders
were determined;concentration of electrolyte 10g/l Na2SO3+ 7.5 g/l CuSO4+50 g/l
H2SO4, current density 200 А/m2, electrolysis duration 1 hour.
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-sulfur contain composition electrodes’ anodic oxidation in Na2CO3 and NaCI
solution were studied comprehensively. At anodic side formed sulfate ions current
output were 135% and 94%.
-for the first time shown that chemical current source can be made by using the
oxidation reaction of composite conductive sulfur-sulfur graphite electrode. Novelty
of introduced method was protected by innovation patent of RK (№ 31177). This
specified galvanic pair was determined that the value EMF is 1050mV and SCC value
equal to 40 mA. The galvanic pair of "sulfur and lead dioxide" in the sulfuric acid
solution can be used as a chemical power source in laboratories.
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