Chem Oxidation

7/29/2019 Chem Oxidation

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The use of oxidizing agents without the need of

microorganisms for the reactions to proceed

oxidizing agents : O3, H2O2, Cl2 or HOCl or O2 etc

catalysts : pH , transition metals, light , ..etc

CHEMICAL OXIDATION


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Types of oxidation Processes

1. Conventional oxidation processes

- Use common oxidizing agents such as ozone,

chlorine without production of highly reactive species

2. Oxidation processes carried out at high

temperature/ high pressure

- Produce highly reactive species, hydroxyl radicals

(HO)


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3. Advanced oxidation processes (AOPs)

- Produce highly reactive species, hydroxyl radicals

(HO ) using oxidizing agents and catalysts

Oxidation rate observed

HO > O3 > H2O2 > HOCl > ClO2 > KMnO4 > Cl2 > O2


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Application

o For nonbiodegradable , toxic organic compounds

o For compounds that inhibit microbial growth

o Effective for destruction of many inorganic compounds

o Elimination of odorous compounds e.g. H2S


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Extent of degradation

1. Primary degradation

2. Acceptable degradation

3. Ultimate degradation (mineralization)

4. Unacceptable degradation (fusing)

- a destruction change in the parent compounds

- a structural change in the parent compounds to the extent

that toxicity is reduced

- conversation of organic carbon to inorganic carbon CO2

- a structural change in the parent compounds resulting inan increase in toxicity


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Stoichiometry

Stoichiometric relationship between an oxidant and the

compoundsto be treated To estimate the required oxidant dosages for the

treatment

General approach

Express the half reaction for each oxidant in terms

of free reactive oxygen OExp. Oxygen , O2 2O


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For convenient :

1. Use the electrochemical half-reaction (reduction reaction)

of the oxidant

2. Balance e- with half reaction of water (that gives O in this

approach)H2O O + 2 H

+ + 2 e- ( O = O2)

Exp. Ozone O3 + 2H+ + 2e- O2 + H2O

H2O O + 2 H+ + 2 e- (O = O2)

O3 O + O2


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For ultimate conversion

CaHbOc + d O

a CO2 + (b/2) H2O

Balance equation using free reactive oxygen produced from

an oxidant

moles oxidant required for conversion of a mole of pollutant

Ex. The ultimate conversion of phenol using O3

C6H5OH + 14 O3 6 CO2 + 14 O2 + 3 H2O


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Note : Wastewater contains a wild variety of compounds

Exp. For O3

O3 stoichiometric demand = (2/1)(48/32) COD

= 3 COD

Adapt free reactive oxygen approach to the surrogate COD

Oxidant demand = (2/n)(MW/32) COD

COD = Chemical oxygen demand (mg O/L)

MW = Molecular weight of oxidantn = mole O/mole oxidant


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Exp. Estimate stoichiometric dosage of HClO needed

to treat an effluent having COD = 288 mg/L

HOCl + H+ + 2 e- Cl- + H2O

H2O O + 2 H+ + 2 e- (O = O2)

1. Mole of O per mole of HOCl

HOCl Cl- + O + H+

2. Oxidant demand

Oxidant demand = (2/n)(MW/32) COD

= (2/1) (52/32) (288)

= 936 mg/L


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the stoichiometric requirement -- yield of free reactive oxygen

( e.g. Moles of O per kg oxidant)

Establish a Ranking of oxidants based on

- cost

- effectiveness of oxidant

Choose a suitable oxidant

How to choose an oxidant for treatment


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Applicability

Apart from O2, most oxidants are expensive and not

competitive with biological treatment for high strength,

large-volume wastewater.

Coupling chemical oxidation and biological treatment

Chemical oxidation processes are designed for toxic,

inhibitory and refractory compounds

- to reduce toxicity

- to increase biodegradability of the parents compounds

at dosages far less than required for ultimate degradation


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Oxidants

O2 (in wet air oxidation)

Ozone (O3)

Hydrogen peroxide (H2O2)

Fentons reagent (Fe(II)/ H2O2)

H2O2/UV

O3/H2O2Advanced Oxidation Processes (AOPs)


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Ozone :

* powerful oxidant* unstable gas which decomposes to O2 at

normal temperature

* decomposition is accelerated by contact with

solid surfaces, chemical substances and by heat

* generated by electric discharge of air, O2


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The corona discharge device can be fabricated and

configured in many different ways.

The primary feature is to generate a corona between

two electrode surfaces

air or oxygen pass between these electrodes

high-energy electrons bombard gas molecules

gas molecules are ionized forming a light emitting

gaseous plasma referred to as a corona


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Reaction of ozone

O3 (aq) oxidized MAdded to water

O3 (g)

O3 (g)

M

OH

R

MS

OH -

Direct reaction

Indirect reaction

M : organic / inorganic molecule

S : scavenger : by-products


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Direct reactions of ozone

Ozone can be electrophilic and nucleophilic

I . Reaction with organic compounds

Examples :

- oxidation of alcohols to aldehydes, organic acids

- substitution of an oxygen atom onto an aromatic ring

- clevage of carbon double bonds

II. Reactions with inorganic compounds

a. Oxidation of ammonia

4 O3 + NH3 NO3- + 4 O2 + H3O

+


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b. Oxidation of iron and manganese

Fe 2+ (aq) 2 Fe3+

(aq) 2Fe(OH)3 (s)

O3 O2

Mn2+ (aq) Mn4+

(aq) MnO2 (s)

O3 O2

c. Oxidation of nitrite

NO2- + O3 NO3

- + O2

Indirect reactions of ozone

Free radicals reactions

Radicals produced will react with compounds in water


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Phenol and aromatic hydrocarbon destruction

Color removal

Drinking water purification

Water bottling plants

Swimming pools

Laundry recycling

Applications


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Hydrogen peroxide

- Colorless liquid at room temperature

- decomposes easily to give O2

- the decomposition is slow in dilute solutionor in pure solution well conserved in dark

Reactivity of H2O2 : 2 types

Direct or molecular reactivity

indirect or radical reactivity


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As an oxidant , H2O2 can react with number of organic

and inorganic pollutants. Examples are shown below.

a) Treatment of sulfide , H2S

rapid oxidation : H2S + H2O2 S + 2H2O

b) Treatment of cyanides

H2O2 + CN- + H+ CNO- + H2O

c) Purification of iron and manganes containing groundwaterH2O2 + 2Fe

2+ + 2H+ 2Fe3+ + 2H2O

Fe(OH)3 (S)

I. Direct reaction


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Limit : - oxidation by H2

O2

alone is not effective for high

concentration of certain contaminants

e.g. Highly chlorinated compounds

: - Low rate of reaction at reasonable [H2O2]

II. Direct reaction : Reaction of hydroxyl radicals formed in

H2O2 decomposition


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Advanced Oxidation Processes (AOPs)

The advanced oxidation process can be used to

decompose many hazardous chemical compounds

to acceptable levels.

The term advanced oxidation processes refers

specifically to processes in which oxidation of organic

contaminants occurs primarily through reactions withhydroxyl radicals.

Source: http://www.spartanwatertreatment.com/advanced-oxidation-processes.html


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The most widely applied AOPs are:

Peroxide/ultraviolet light (H2O2/UV),

Ozone/ultraviolet light (O3/UV),

Hydrogen peroxide/ozone (H2O2/O3)

Hydrogen peroxide/ozone/ultraviolet (H2O2/O3/UV)


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H2O2/UV process

- Irradiation of UV light ( < 365 nm) can break H-O bond

chain reactions

Initiation : H2O2 + h 2 OHx = 18.6 L mol-1 cm-1

Propagation : H2O2 + 2 OHx H2O + HOOx

Termination : HOOx + HOOx H2O2 + O2


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O3

/UV process

O3 decomposes to produce radicals at high pH values(photolysis).

Initiation : O3 + OH- OH2

+ O2 -

OH2 O2

- + H+

Propagation : O3 + O2 - 2O2 + OH

O3 + OH OH2

+ O2

Termination : OH2 + OH O2 + H2O

h

Source: http://www.spartanwatertreatment.com/advanced-oxidation-UV-Ozone.html

Radicals produced will react with compounds in water


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Chlorine

Use in water and wastewater treatment e.g. Color removal

At room temperature, Cl2 can dissolved in water giving

HClO.

Cl2 + H2O HClO + Cl- + H+

HOCl + H2O ClO- + H3O+

ka = 1.6-3.2 x 10-8

Hypochlorous acid hypochlorite ion


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Nature of species in water depends on pH

(HClO, ClO-, H2ClO+, Cl2O)

Cl2 and its derivatives can react with organic

Matters in water by :

- oxidation reaction

- addition / substitution reaction

==> results in formation of organochlorinated compounds

==> reduce the use of chlorine


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Applications of chlorine

Treatment of CN-

- change CN- to CNO- and finally to N2

Reactions with NH3, NH4+

- result in formation of chloroamines (NH2Cl, NHCl2, NCl3)

- possible reactions leading to disappearance of

chloroamines :

4 NH2Cl + 3Cl2 + H2O N2 + N2O + 10HCl

2 NH2Cl + HClO N2 + H2O + 3HCl

NH2Cl + NHCl N2 + 3HCl


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Overall reactions for total degradation of NH3

2 NH3 + 3 HOCl N2 + 3H2O + 3HCl

2 NH3 + 3 Cl2 N2 + 6H + 6Cl-

Molar ratio Cl2 : NH3 = 3 : 2Mass ratio Cl2 : NH3 = 7.6 : 1

CHLORINE DEMAND

amount of Cl2

used in reactions

= Cl2 added to water - total available residual

chlorine remaining at the end of the specific time (Cl2, HClO,

ClO-)


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CHLORINE DEMAND

C

hlorine

residua

ls

Cl2 added (mg/L)

Zero demand residual

A

B

D

Chlorine demand


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Point A : low chlorine residual

= > consumption of chlorine by reducing compounds

e.g. Fe

2+

, Mn

2+

, H2S and organic matters

Above point A : Formation of chlorinated org compounds

e.g. chloroamines

Point B : All compounds has been reacted

B D : some chlorinated org compounds are oxidized

(then chlorine residual is reduced to Cl-)


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Point D : Breakpoint (most of chlorinated organic

Compounds are oxidized)

Beyond D : - presence of some resistant chlorinated

compounds

- all added chlorine residual is free

available chlorine (Cl2, HClO, ClO-)


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Chemicals

Hypochlorites (salt of HClO)

- NaOCl, Ca(OCl)2

give OCl-

- use in small installation e.g. Swimming pools

Liquid chlorine- used in water treatment plant e.g. In US

ClO2 - prepared by following reactions

2 NaClO2 + Cl2 2ClO2 + 2NaCl

2 NaClO2 + 4HCl 4ClO2 + 5NaCl + 2H2O


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CHOICE OF USAGE

- Sterilization of water

ClO2 is superior to chlorine in destruction of spores,

bacterias, virus and other pathogen organisms

- Industrial water treatment

- chlorine are more reactive than ClO2 and will react with

most organic compounds (ClO2 does not react with NH3 or

NH4+

)


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http://www.iwawaterwiki.org/xwiki/bin/view/Articles/CHEMICALOXIDATIO

NAPPLICATIONSFORINDUSTRIALWASTEWATERS

http://www.lenntech.com/products/chemicals/water-treatment-chemicals.htm

http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399252&section=1.4.7

References

( 6 Feb 2011)

Chem Oxidation

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