Fundamentals 2003

Introduction

The presence of harmful organic compounds in water supplies and in the discharge of

wastewater from chemical industries, power plants, landfills, and agricultural sources is a

topic of global concern. Traditional water treatment processes include filtration and

flocculation, biological treatment, thermal and catalytic oxidation, and chemical treatment

using chlorine, potassium permanganate, ozone, hydrogen peroxide and high-energy

ultraviolet light, [1, 2]. All these water treatment processes, currently in use, have limitations

of their own and none is cost-effective:

1. Phase transfer methods remove unwanted organic pollutants from wastewater, but they

do not eliminate the pollutants entirely;

2. Cost of biological treatment is low, however, some of the toxic compounds present are

found to be lethal for microorganisms intended to degrade them, and there is a class of

non-biodegradable organic products noted as biorecalcitrant organic compounds;

3. While chemical treatments based on aqueous phase hydroxyl radical chemistry are

powerful to oxidize toxic organic compounds present in water, these processes either

use high-energy ultraviolet light or strong chemical oxidants of hazardous and

therefore, undesirable nature, [3].

Moreover, several intermediates, which are more hazardous, are formed in these

processes, and because of very low efficiencies, overall treatment cost becomes high if

destruction of intermediates and complete mineralization are to be achieved, especially for

treating dilute wastewater streams, [4].

Degradation or decomposition by photocatalysis is a novel method for the treatment of

air and water pollutants, [5]. Semiconductor photocatalysis with a primary focus on TiO2 is

widely used. Literature mentions that, photocatalytic processes on TiO2, under UV radiation,

can be efficiently applied for the degradation of non-biodegradable azo-dyes, [6, 7, 8]. Thus,

recently, TiO2 thin films have been reported as being successfully used for the photocatalytic

degradation of methyl orange and methylene blue, a typical pollutants in the textile industry, [9].

1. Legrini, O.; Oliveros, E.; Braun, A. M. Chem. Rev. 93 (1993) 671;2. Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. Rev. 12 (1993) 417;3. Roberts, D.; Malato, S. The Science of the Total Environment 291 (2002) 85;4. Ollis, D.F.; Pelizzetti, E.; Serpone, N. Photocatalysis: Fundamentals and applications;

Wiley: New York, 1989;

Powders and thin films of titania will photodegrade a wide range of organic and inorganic

chemicals in air and water. Other applications have included the elimination of microorganisms

such as bacteria, viruses, cancer cells and the reduction of trace heavy metals.

Photocatalysis requires large values of the specific surface; therefore TiO2 powder is

usually used as provided or as thin film. Several practical problems arising from the use of

powder are obvious during the photocatalytic process:

separation of the insoluble catalyst from the suspension is difficult,

the suspended particles tend to aggregate especially at high concentrations,

suspensions are difficult to apply to continuous flow systems.

The photocatalytic activity of TiO2 in UV spectral regions is highly dependent on the

preparation method [10]. There are a number of methods which can be used to obtain the TiO2

films, including: doctor blade [11], spray coating, dip coating [12], spin coating [13], chemical

vapour deposition [14]. Dip coating it is an attractive method to prepare a wide variety of

powders and thin film materials for various industrial applications (solar cell,

photodegradation, gas sensor)[15].

Modifications in the catalyst surface have also been investigated for prevention of

electron-hole recombination. The addition of platinum and other transition metals have been

successfully arrayed on the titanium dioxide surface. These metal additions have an optimum

at low weight percentages (less than 5%), above which the metal actually hinders the

photocatalytic ability.

1. Fundamentals

1.1 Wastewater; Industrial wastewater; Wastewaters from the textile industry

Our biosphere is under constant threat from continuing environmental pollution. Impact

on its atmosphere, hydrosphere and lithosphere by anthropogenic activities can not be

ignored. Man made activities on water by domestic, industrial, agriculture, shipping, radio-

active, aquaculture wastes; on air by industrial pollutants, mobile combustion, burning of

fuels, agricultural activities, ionization radiation, cosmic radiation, suspended particulate

matter; and on land by domestic wastes, industrial waste, agricultural chemicals and

fertilizers, acid rain, animal waste have negative influence over biotic and abiotic components

on different natural ecosystems. Some of the recent environmental issues include green house

effect, loss in bio-diversity, rising of sea level, abnormal climatic change and ozone layer

depletion etc..

The industrial wastewater is discharged with pre-treatment or neutralization either to

municipal sewers that flow into rivers or directly into rivers without pretreatment or

neutralization. The water quality of nearby river water is deteriorated by this direct discharge

of the raw effluent from the industry, which renders to reduce the aesthetic value as well as

the aquatic ecosystem hamper.

In recent years, different approaches have been discussed to tackle man made environmental

hazards. Clean technology, eco-mark and green chemistry are some of the most highlighted

practices in preventing and or reducing the adverse effect on our surroundings. Among many

engineering disciplines – Civil Engineering, Mechanical Engineering, Electrical Engineering

etc., Textile Engineering has a direct connection with environmental aspects to be explicitly

and abundantly considered.

Out of various activities in textile industry, chemical processing contributes about 70%

of pollution. It is well known that cotton mills consume large volume of water for various

processes such as sizing, desizing, scouring, bleaching, mercerization, dyeing, printing,

finishing and ultimately washing.

Due to the nature of various chemical processing of textiles, large volumes of waste water

with numerous pollutants are discharged. Since these streams of water affect the aquatic eco-

system in number of ways such as depleting the dissolved oxygen content or settlement of

suspended substances in anaerobic condition, a special attention needs to be paid

[16].

Large volume wastes from textile mill include wastewater from the preparation of the

substrate, rinsing and washing after dyeing operations and waste from batch dyeing

operations. These wastes are not only heavily contaminated but can put a burden on the

hydraulic load of the treatment system. More than 10 000 dyes are used in the textile industry

and 280 000 tonnes of textile dyes are discharged every year world wide. Textile dyeing and

finishing processes produce large quantities of wastewater that is highly coloured and

contains large concentration of organic matter, which is difficult to treat via classical methods.

Apart from the aesthetic problems created when coloured effluents reach the natural water

currents, dyes strongly absorb sunlight, thus impeding the photosynthetic activity of aquatic

plants and seriously threatening the whole ecosystem [17].

Sources and Causes of Generation of Textile Effluent

Textile industry involves wide range of raw materials, machineries and processes to

engineer the required shape and properties of the final product. Waste stream generated in this

industry is essentially based on water-based effluent generated in the various activities of wet

processing of textiles. The main cause of generation of this effluent is the use of huge volume

of water either in the actual chemical processing or during re-processing in preparatory,

dyeing, printing and finishing. In fact, in a practical estimate, it has been found that 45%

material in preparatory processing, 33% in dyeing and 22% are re-processed in finishing [18].

Table 1.1.1. Properties of Waste Water from Textile Chemical Processing

Property Standard Cotton Synthetic WoolpH 5.5-9 8-12 7-9 3-1BOD,mg/l, 5 days 30-350 150-750 150-200 5000-8000COD,mg/l, day 250 200-2400 400-650 10000-20000TDS, mg/l 2100 2100-7700 1060-1080 10000-13000

Effluents treatment plants are the most widely accepted approaches towards achieving

environmental safety. But, unfortunately, no single treatment methodology is suitable or

universally adoptable for any kind of effluent treatment. For instance, in the past, biological

treatment systems had been used extensively but they are not efficient for the colour removal

of the more resistant dyes. Therefore, the treatment of waste stream is done by various

methods, which include physical, chemical and biological treatment depending on pollution

17. J.M. Poyatos, M.M. Munios, M.C. Almecija, J.C. Torres, E. Hontoria, F. Osorio, Advanced

oxidation processes for wastewater treatment: State of the Art, Water Air Soil Pollut (2010)

205:187-204;18. C. N., Sivaramakrishnan, Colourage, LI, No. 9, 27-32, 2004;

load. The treatment processes may be categorized into preliminary, primary, secondary and

tertiary treatment process [1].

Various operations in each category are described below in Table 1.1.2

Table 1.1.2. Classification of waste water treatment process

Treatment OperationsPrimary Screening, Sedimentation, Equalization, Neutralisation

Mechanical Flocculation and Chemical coagulation

SecondaryAerated lagoon, Trickling filtration, Activated sludge processOxidation ditch and pond, Anaerobic digestion

Tertiary

Oxidation technique, Electrolytic precipitation and Foam fractionationMembrane technologies, Electrochemical processesIon exchange method, Photo- catalytic degradationAdsorption (Activated Carbon etc.), Thermal evaporation

The choice of the method for effluent treatment depends upon four factors: effluent quantity;

concentration in pollutants; quality conditions imposed for the treated water; and finance

available to the organization. The wastewater treatment process used to degrease the pollution

loads of industrial wastewater can be classified by many criteria, including the collection

disposal and treatment process of these waters. Irrespective of the processes that are used, the

industrial wastewater treatment has the following objectives: to remove the pollutants or the

substances that can be the further reduced with the final effect to obtain a treated effluent that

can be reintroduced in its natural circuit, recycled in technological processes or reused for

different purposes (aquifers loading, dual systems for water supply, irrigation); processing of

the sludge resulted from the industrial wastewater treatment.

The advanced treatment is used to increase the degree of purification and to eliminate the

priority pollutants that are partly removed by conventional treatment processes (colloids, non-

biodegradable organic compounds, inorganic toxic compounds, pathogen microorganisms).

Advanced treatment may be applied before or after the biological processes as requested by

the characteristics of the influent wastewater and the required degree of purification [19].

1.2 Advances wastewater treatment for industrial process

The treatment of spent dye wastewater effluent is a growing concern for the textile

industry because of aesthetic conditions, as well as ecotoxicological issues regarding colored

rinsing and process wastewater and the impact of that wastewater on the receiving streams. As

regulations become more stringent, the effectiveness and cost of treatment processes becomes

more significant. Conventional biological treatment can be ineffective for color removal, but

chemical oxidative processes seem to provide an opportunity for future use in industrial

wastewater [20]. The presence of organic dyes in textile wastewater- these dyes are synthetic

and non-biodegradable so, biological treatment of wastewater alone is usually not effective

waters may result in poor water quality [21].

Table 1.2.1 Advanced wastewater process

Process ReferencesAdsorbtion (activated carbon, silica, fly ash) [22]Ion exchange [23]

Membrane FiltrationReverse Osmosis [24]Ultrafiltration [25]

Ozonation [26]

EvaporationMultiple effect evaporation [27]Direct contact evaporation [28]

Crystallization [29]Specific Treatments [30]

However, none of these treatment methods is effective enough to produce water with

acceptable levels of the most persistent pollutants (e.g., phenols, pesticides, dyes, solvents,

household chemicals and drugs, etc.). A further treatment stage is often necessary to attain

this objective. This stage can entail the application of advanced oxidation processes (AOPs),

which are recommended when wastewater components have a high chemical stability and/or

low biodegradability [2].

1.2.1. Advanced oxidation process

In recent decades, very severe regulations have forced researchers to develop and

evolve novel technologies to accomplish higher mineralization rate with lower amount of

detectable contaminants. Different physical, chemical, and biological treatment processes

have been employed to treat various municipal and industrial wastewaters such as chemical

[31],[32], biological, food [33], pharmaceutical [34], [35], pulp and paper [36], dye processing and

textile [37], [38], [39], [40], and landfill leachate [41], effluents.

Advanced oxidation process (AOPs) are defined as near ambient temperature and

pressure water treatment processes which are based on the generation of hydroxyl radicals to

initiate oxidative destruction of organics. The hydroxyl radical is a powerful, non-selective

chemical oxidant (Table I.4.) which reacts typically a million to a billion times faster that

ozone and hydrogen peroxide resulting in reduced treatment costs and system size.

Table 1.2.1.1 Oxidizing potential for conventional oxidizing agents

Oxidizing agent Electrochemical oxidation potential (EOP), V

EOP relative to chlorine

Fluorine 3.06 2.25Hydroxyl radical 2.80 2.05Atomic oxygen 2.42 1.78Ozone 2.08 1.52Hydrogen peroxide 1.78 1.30Hypochlorite 1.49 1.10Chlorine 1.36 1.00Chlorine dioxide 1.27 0.93Oxygen molecular 1.23 0.90

AOPs used for the treatment of wastewater are based on:

Ozone (O3);

Hydrogen Peroxide (H2O2);

Ozone + Hydrogen Peroxide;

Fenton’s Reaction, Photo Fenton Process;

Photo- oxidation: UV+Ozone+H2O2, UV+H2O2

Photo- catalysis : UV+TiO2

Ozone

The half-life of ozone in industrial wastewater can be expected to vary from less than a

minute to up 30 min, depending on the types and ozone-reactivity of the pollutants as well as

upon pH. As the pH rises, the decomposition rate of ozone in water increases.

Ozone and hydrogen peroxide

At lower pH addition of hydrogen peroxide at O3/H2O2- ratio of 2:1 to ozonation processes

accelerates the decomposition of ozone resulting in the increased formation of hydroxyl

radicals. At a concentration of hydrogen peroxide above 10-7 M and a pH- value less than 12,

hydrogen peroxide reacts with ozone as the anion HO2- producing two hydroxyl radicals from

two ozone molecules.

Photo-oxidation

Completion of oxidation reactions, as well as oxidative destruction of compounds immune to

ozone or hydrogen peroxide oxidation alone, can be obtained by supplementing the reaction

with ultraviolet radiation. More importantly, UV radiation accelerates the decomposition of

ozone and hydrogen peroxide molecules. Although photochemical cleavage of H2O2 is

conceptionally the simplest method for hydroxyl radical production, the exceptionally low

molecular absorptivity of H2O2 at 254 nm limits the yield of hydroxylic radicals in the

solution.

Fenton’s Reaction, Photo Fenton process

Since hydrogen peroxide does not absorb significantly beyond 300 nm and absorbs only

weakly in the range of 200-300 nm, the UV/H2O2 process is often not suitable for the

treatment of polluted water with a high UV absorbance and/or a high background of total

organic carbon concentration. The oxidizing species in this reaction is again the hydroxylic

radical formed according to:

(1)

Photocatalysis

In photocatalysis, UV radiation is used to excite a solid-state metal catalyst creating a positive

and negative change (electro-hole, e- h+ pairs) on the catalyst’s surface. These positive and

negative charges promote redox reactions, e.g. oxidation of organics in the solution by the

ions or oxygen by the photogenerated negative charges. Titanium dioxide is the preferred

catalyst for photocatalysis due to its stability under various conditions, its high potential to

produce radicals and its easy availability and low price [42].

AOPs can often achieve oxidative destruction of compounds refractory to

conventional ozone or hydrogen peroxide oxidation. In addition, AOPs have the potential to

completely oxidize (mineralize) organic contaminants to carbon dioxide, water and mineral

salts. AOPs are suited for destroying dissolved organic contaminants such as halogenated

hydrocarbons, aromatic compounds, phenols, dyes and pesticides. Therefore, AOPs show

promise for destruction of hazardous organic compounds in water without generating

secondary pollution commonly associated with conventional treatment technologies [42].

1.2.2. Heterogeneous photocatalysis

Heterogeneous photocatalysis has been examined and explored extensively as a

potentially viable alternative technology to classical "best" technologies for both

environmental detoxification and for energy production [43].This techology employs

illuminated semiconductor particulate materials, TiO2, as photocatalysts to produce both

reducing and highly oxidizing species on the particle surface poised to unleash redox

processes in aqueous media, many of which would not otherwise be possible by normal

chemical means.

The term „photocatalyis” is composed of the combination of photochemistry and

catalysis what suggest that the light and catalyst are necessarily to drive or to accelerate a

chemical transformation [44]. The activation way of the catalyst differs slightly from classical

catalysis, because the thermal activation is replaced by photonic activation [45]. Nevertheless,

43. A. Salinaro, Fundamental of heterogenous photocatalysis, Quebec, Canada, 2001;

the term „photocatalysis”is still disputable among scentists [46]. Serpone and Emeline took an

effort to systematized the definitions and terminology related to the photocatalysis [47].

44. N.Serpone, E. Pelizzetti, Photocatalysis: fundamentals and applications, John Wiley and

Sons, Inc., New York, 2000; 5. Beydoun, D., Amal, R., Low, G., McEvoy, S., J. Nanopart. Res. 1 (1999) 439 ;6. Guettaı, N., Ait Amar, H., Desalination 185 (2005) 427 ;7. Wu, J.M., Zhang, T.W., J. Photochem. Photobiol., A 162 (2004) 171;8. Saquib, M., Muneer, M., Desalination 155 (2003) 255;9. Kontos, A.I., Arabatzis, I.M., Tsoukleris, D.S., Kontos, A.G., Bernard, M.C., Petrakis, D.E.,

Falaras, P., Catal. Today 101 (2005) 275;10. Coronado, J.M., Maira, A.J., Conesa, J.C., Yeung, K.L., Augugliaro, V., Soria, J.,

Langmuir 17 (2001) 5368;11. Arabatzis, I.M., Antonaraki, S., T. Stergiopoulos, A. Hiskia, E. Papaconstantinou, M.C.

Bernard, P. Falaras, J. Photochem. Photobiol., A 149 (2002) 237;

12. Lei Ge. Mingxia Xu, Haibo Fang, Fabrication, characterization and

photocatalytic activities of TiO2 thin films from autoclaved-sol, Thin Solid Films 515 (2007)

3414–3420;13. Leen, S.C., Lee, J.H., Oh, T.S., Kim, Y.H., Sol. Energy Mater. Sol. Cells 75 (2003) 481;14. Seifried, S., Winterer, M., Hahn, H., Chem. Vap. Deposition 6 (2000) 239;15. Okuya, M., Nakade, K., Kaneko, S., Sol. Energy Mater. Sol. Cells, 70 (2002) 425;16. Subrata Das,Textile effluent treatment – A solution to the environmental pollution, 2004;19. C. Draghici, V. Oros, J. Pretty, Waste management, Ed. Academiei Romane, Bucuresti,

2003;20. J.C. Edwards, investigation of color removal by chemical oxidation for three reactive

textile dyes and spent textile wastewater, Blackburg, Virginia, 2000;21. R.C. Meena, Ram Babu Pachwarya, Vijay Kumar Meena and Shakuntla Arya, Degradation

of Textile Dyes Ponceau-S and Sudan IV Using Recently Developed Photocatalyst,

Immobilized Resin Dowex-11, American Journal of Environmental Sciences 5 (3): 444-450,

2009, ISSN 1553-345X;

22. Maria Visa, Absorbant materials with controlled surface properties, based on solid wastes,

for advanced wastewater treatment, Ph.D Thesis, Brasov, 2008;23. Roberto Juan, Susana Hernández, José M. Andrés, Carmen Ruiz, Ion exchange uptake of

ammonium in wastewater from a sewage treatment plant by zeolitic materials from fly ash,

Journal of Hazardous Materials, Volume 161, Issues 2-3, Pages 781-786, 30 January 2009,;24. S.S. Madaeni, M.R. Eslamifard, Recycle unit wastewater treatment in petrochemical

complex using reverse osmosis process , Journal of Hazardous Materials, Volume 174, Issues

1-3, Pages 404-409,15 February 2010;25. Juan Arévalo, Gloria Garralón, Fidel Plaza, Begoña Moreno, Jorge Pérez, Miguel Ángel

Gómez ,Wastewater reuse after treatment by tertiary ultrafiltration and a membrane bioreactor

(MBR): a comparative study, Desalination, Volume 243, Issues 1-3, Pages 32-41 July 2009;26. Kadir Turhan, Zuhal Turgut, Decolorization of direct dye in textile wastewater by

ozonization in a semi-batch bubble column reactor, Desalination, Volume 242, Issues 1-3,

Pages 256-263, June 2009;27. G. Libralato, A. Volpi Ghirardini, F. Avezzù, Evaporation and air-stripping to assess and

reduce ethanolamines toxicity in oily wastewater, Journal of Hazardous Materials, Volume

153, Issue 3, Pages 928-936, 30 May 2008;28. Gautham Parthasarathy, Russell F. Dunn, Graphical strategies for design of evaporation

crystallization networks for environmental wastewater , Advances in Environmental

Research,Volume 8, Issue 2, Pages 247-265, January 2004; 29. Stephan Tait, William P. Clarke, Jurg Keller, Damien J. Batstone , Removal of sulfate from

high-strength wastewater by crystallisation   Water Research, Volume 43, Issue 3,Pages 762-

772, February 2009;30. I. Oller, S. Malato, J.A. Sánchez-Pérez, Combination of Advanced Oxidation Processes

and biological treatments for wastewater decontamination—A review, Science of The Total

Environment, Available online 16 October 2010;

Fig. 1.2.2.1. The definition and the schema of catalysed photolysis (left) and photogenerated catalysis (right)

They classified the catalytic reactions, which are driven by interaction with a light as:

a) Catalysed photolysis also named in the literature as “catalysed photoreaction” or “a

photosensitization”. In this process, a photon is absorbed by dye molecule (ec. 2), which

transfers an electron into the conduction band of the semiconductor (ec.3) . the catalyst in this

case acts as an electron-transfer mediator (ec3, ec.4) and the oxygen as an electron acceptor

(ec.4, 5, 6, 7) leading to efficient separation of the injected electron and the radical cation

Fig.1.2.2.1. [46], [47], [48]. The dye is decomposed in the following steps:

31. P.R. Gogate and A.B. Pandit. “A review of imperative technologies for wastewater

treatment II: Hybrid methods”. Advances in Environmental Research, 8 (3-4): 553-597, 2004;32. P.R. Gogate and A.B. Pandit. “A review of imperative technologies for wastewater

treatment I: Oxidation technologies at ambient conditions”. Advances in Environmental

Research, 8 (3-4): 501-551, 2004;33. P. Paraskeva and E. Diamadopoulos. “Technologies for olive mill wastewater (OMW)

treatment: A review”. J. Chem. Technol. Biot., 81 (9):1475-1485, 2006;34. S. Esplugas, D.M. Bila, L.G.T. Krause, and M. Dezotti “Ozonation and advanced oxidation

technologies to remove endocrine disrupting chemicals (EDCs) and pharmaceuticals and

personal care products (PPCPs) in water effluents”. J. Hazard. Mater., 149 (3): 631-642, 2007;35. M.B. Johnson and M. Mehrvar. “Aqueous metronidazole degradation by UV/H2O2 process

in singleand multi-lamp tubular photoreactors: Kinetics and reactor design”. Ind. Eng. Chem.

Res., 47 (17): 6525- 6537, 2008;36 . H.K. Moo-Young. “Pulp and paper effluent management”. Water Environ. Res., 79 (10):

1733-1741, 2007;37. G. Crini. “Non-conventional low-cost adsorbents for dye removal: A review”. Bioresour.

Technol., 97 (9): 1061-1085, 2006;38. P.C. Vandevivere, R. Bianchi and W. Verstraete. “Treatment and reuse of wastewater from

the textile wet-processing industry: Review of emerging technologies”. J. Chem. Technol.

Biot., 72 (4): 289-302, 1998;39. T. Aye, W.A. Anderson, and M. Mehrvar. “Photocatalytic treatment of cibacron brilliant

yellow 3G-P (reactive yellow 2 textile dye)”. J. Environ. Sci. Heal. A, 38 (9): 1903-1914,

2003;40. T. Aye, M. Mehrvar, and W.A. Anderson. “Effects of photocatalysis on the

biodegradability of Cibacron Brilliant Yellow 3G-P (Reactive Yellow 2)”. J. Environ. Sci.

Heal. A, 39 (1): 113-126, 2004;

(2)

(3)

(4)

(5)

(6)

(7)

(8)

b) Photogenerated catalysis is also called “a sensitized photoreaction”. Here, the

illuminated semiconductor absorbs a photon with energy equal or larger than its band gap

energy (hʋ ≥Eg ). This leads to the creation of the charge caries: an electron,which is excited

into the conduction band and the hole, which remains in the valence band (ec. 9).

Simultaneously, a spontaneous adsorbtion of the pollutant molecules on the surface of the

catalyst occurs. Whit respect to redox potential or energy level an electron transfer proceeds

towards acceptor molecules (ec. 4-7), whereas photo-holes are transferred to donors

molecules (ec. 10,11). Each formed ion and radical react through the intermediate reaction

into direction of the full decomposition of the pollutant molecules ( Fig. right) [46], [47], [48].

41. S. Renou, J.G. Givaudan, S. Poulain, F. Dirassouyan, and P. Moulin. “Landfill leachate

treatment: Review and opportunity”. J. Hazard. Mater., 150 (3): 468-493, 200842. A. Vogelpohl, S.M. Kim, Advanced oxidation processes (AOPs) in wastewater treatment,

J. Ind.Eng.Chem., Vol. 10, No.1.2002,33-40; 45. J.M. Herrmann, Heterogenous photocatalysis: fundamental and applications to the removal

of various types of aqueous pollutant, Catalysis Today, 115-129, 2000; 46. A. Mills, S. Le Hunte, An overview of semiconductor photocatalysis, Journal of

Photochemistry and Photobiology A: Chemistry, 108, 1999, 1-35;47. N. Serpone, A.V. Emeline, Suggested terms and definitions in photocatalysis and

radiocatalysis, International Journal of Photoenergy, 4, 2002, 91-233;48. J. Zhao, T. Wu, K. Wu,K. Oikawa, H.Hidaka, N. Serpone, Photoassisted degradation of

dye pollutants, Environ. Sci.Technol., 32, 2000, 2394-2400;

Intermediates

(9)

(10)

(11)

(12)

(13)

(14)

(15)

The catalysed photolysis is mostly related to dyes photodecomposition under the solar light

irradiation [48], [49], [50]. The so-called photogenerated catalysis is applied not only for the

decomposition of dyes but also for mineralization of many groups of organic pollutants (Tab.

1.2.2.1).

Table 1.2.2.1. General group classification of organic pollutants decomposed by

photocatalysis [22].

GROUP OF ORGANICS EXAMPLESAlkanes isobutene, pentane, heptanes, cyclohexane, paraffinsAliphatic carboxylic acids formic,ethanoic, propanoic, oxalic, butyric, malic acidsAlkenes propene, cyclohexeneAromatics benzene, naphtalenePhenolic compounds Phenol, hydroquinone, catechol, methylcatechol, resorcinolHalophenols 2-,3-,4-chlorophenol, pentachlorophenol, 4- fluorophenol

Surfactants sodium dodecylsulphate, polyethylene glycol, sodium dodecylbenzene sulphonate, trimethyl phosphate,

Herbicides atrazine, prometron, propetryne, bentazonPesticides DDT, parathion, lindane, tetrachlorvinphos, phenitrothionDyes methylene blue, rhodamine B, methyl orange, fluorescein

In this type of process, the semiconductor plays the main role and its photocatalytic activity,

which can be modified during the production step. In the photogenerated catalysis the

efficiency of the process can be improved by enhancement of the photoactivity of the

photocatalyst. The reactive species in n-type of semiconductors are photogenerated holes.

These kind of materials are most often applied as a photocatalyst thanks to their stability

against photo-corrosion (ec. 10-12). Oxygen performs in this case as an electron scavenger,

stabilize the primary photo-oxidation reactions and increase the oxidation yield (ec. 5-7)

Intermediates

therefore presence of oxygen is necessarily to enhanced the efficiency of the photooxidation

process [51]. The role of the holes in the photocatalytic process can be explained in two ways:

The holes react over indirect oxidation via a surface-bond hydroxyl radical mechanism

(ec. 16) [52], [53];

The holes react over direct oxidation via the valence band holes [54].

So far, first proposed interpretation has been supported by experimental data like ESR studies

by Hoffman [55] and Linsebigler et al. [56] who confirmed the existence of hydroxyl and

hydroperoxy radicals in aqueous solution of illuminated TiO2. Another fact, which confirmes

this statement, is the presence of intermediates of typical hydroxylated structure during

photodecomposition of halogenated aromatics in presence of TiO2 [55], [56], [57].

From the assumption that hydroxyl radicals OH- are the main oxidative species in the

photocatalytic process three kinetic mechanisms of the oxidation of the organic pollutants can

be proposed:

51. H. Gerischer, A. Heller, The role of oxigen in photooxidation of organic molecules on

semiconductor particles, The Journal of Physical Chemistry, 95, 2000, 5261-5267; 52. G. Mills, M.R. Hoffman, Photocatalytic degradation of pentachlorophenol on titanium

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Technol., 27, 2001, 1681-1689;53. C. S. Turchi, D. F. Ollis, Photocatalytic degradation of organic water contaminants:

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reflectance flash photolysis in aqueous suspension of TiO2 powder, Langmuir, 6,2000, 178-

192; 55. M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental application of

seconductor photocatalysis, Chemival Reviews, 95, 1999, 69-96; 56. A.L. Linsebigler, G. Lu, J. T. Y. Jr, Photocatalysis on titanium dioxide surface:principles,

mechanisms and selected results, Chem Rev., 95, 1998, 735-758;57. H. D. Lasa, B. Serrano, M.Salaices, Photocatalytic reaction engineering, Springer, New

York, USA, 2004, p. 1-15; 49. G. Liu, T. Wu, J. Zhao, Irreversible degradation of alizarin red under visible light radiation

in air-equilibrated aqueous titanium dioxide dispertions, Environ. Sci. Technol., 2001, 2081-

2087;50. J. Yang, C. Chen, H. Ji, Mechanism of titanium dioxide- assisted photocatalytic

degradation of dyes under visible radiation, J, Phys. Chem.B,109, 2005, 21900-21907;

Langmuir –Hinshelwood mechanism – a reaction between the •OH radicals at the surface

of the catalyst and the adsorbed pollutant molecule (P)

(16)

Eley – Rideal mechanism – a reaction between the •OH at the catalyst’s surface and the

pollutant molecule in the solution

(17)

A reaction occurring between the •OH in the solution and the pollutant molecule in the

solution.

All three mechanisms can be expressed by the general kinetic equation:

(18)

where •OH and pollutant molecule (P) of concentration C pollutant are adsorbed on the surface of

the catalyst or are in the solution according to each mechanism [42].

1.3 . Photocatalytic materials

An ideal photocatalyst for photocatalytic oxidation is characterized by the following

attributes [58]: photo-stability, chemically and biologically inert nature, availibility and low

cost. Many chalcogenide semiconductors such as TiO2, ZnO, ZrO2, CdS, Fe2O3 and WO3 have

been examined and used as photocatalysts for the degradation of organic contaminants [59].

The minimum band gap energy required for photon to cause photogeneration of charge

carriers over TiO2 semiconductor (anatase form) is 3.2 eV corresponding to wavelength of 38

8nm [60]. Actually with TiO2, photoactivation takes place in the range 300–388 nm. The

photoinduced transfer of electrons that take place with adsorbed species over semiconductor

photocatalyst depends on the band-edge position of the semiconductor and the redox

potentials of the adsorbates [61]. In spite of the constant vigorous research activities over two

decades in search for an ideal photocatalyst, titania in its anatase modification has remained a

benchmark against which any emerging material candidate will be measured [62]. The anatase

form of titania is reported to give the best combination of photoactivity and photostability

[63].Zhang and Maggard [64], reported the preparation of hydrated form of amorphous titania

with wider band energy gap than anatase and significant photocatalytic activity.

The schematic diagram of band positions for various semiconductors is shown in

Figure 1.3.1.

Titanium

dioxide is one of the most widely applied metal oxide thanks to its unique properties. Due to

its high refractive index it is used as a pigment in the paint industry [65].

Among all properties of titanium dioxide, in the recent years the photocatalytic property is the

most investigated for various applications: disinfection of the operating hospitals rooms, self-

cleaning surfaces Table 1.3.1. Among them, the photodegradation of the organic pollutants

(dyes) in aqueous environment is of the main interest of this work [66].

65. J. Winkler, Titanium dioxide, Vincentz, Hannover, 2003, p. 3037;58 . O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33.59. I.K. Konstantinou, V.A. Sakkas, T.A. Albanis,Water Res. 36 (2002) 2733.60. J. Perkowski, S. Bzdon,A. Bulska,W.K. Jo´ z´wiak, Polish J.Environ. Stud. 15(2003)

457.61. A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1;62. K. Rajeshwar, C.R. Chenthamarakshan, S. Goeringer, M. Djukic, Pure Appl. Chem. 73

(2001) 1849;63. W.A. Zeltner, D.T. Tompkin, Ashrae Transactions, vol. III, American Society of Heating

and Air-Conditioning Engineers Inc., 2005, part 2, p. 532;64. Z. Zhang, P.A. Maggard, J. Photochem. Photobiol. A 186 (2007) 8. 66. U. Diebold, The surface science of titanium dioxide, Surface Science Reports, 48, 2003,

53-229;

Fig.1.3.1. The conduction and valence band positions of selected metal oxide semiconductors at pH 0. The left hand scale represents the internal energies to the vacuum level. The right

hand scale is the normal hydrogen electrode scale which allows predictions based on reduction and oxidation.

Table 1.3.1. Selected application of titanium dioxide in respect to its photoelectrochemical

properties [67], [68].

PROPERTY CATEGORY APPLICATION

Photocatalytic water purification

Drinking waterRiver water, ground water, lakes and water storage tanks

Others Fish feeding tanks, drain age water and industrial wastewater

Photoelectrochemical water decomposition

Hydrogen production

Fuel for automobiles sector, fuel for solid oxide fuel cell, fuel for energetic sector, for pharmaceutical and food industry

Self-cleaning

Materials for residential and office buildings

Exterior tiles, kitchen and bathroom components, interior furnishing, plastic surfaces, aluminium siding, building stone and curtains, paper window blinds

Materials for roads

Tunnel hall, noise barrier, traffic signs and reflectors

Air cleaning

Indoor air cleaners

Room air cleaner, photocatalyst-equipped air conditioners and interior air cleaner for factories

Outdoor air purifiers

Concrete for highway, roadways and footpaths, tunnel walls, noise barriers and building walls

Self-sterilizing HospitalTiles to cover the floor and walls of operating rooms, silicone rubber for medical catheters and hospital garments and uniforms

Crystallographic structure and polymorphic forms

Titanium dioxide occurs in the nature in three polymorphic forms: brookite, anatase, and

rutile where the two last are commonly used in photocatalysts. Anatase and brookite as

metastable phases transform to rutile in the range 973K- 1173K. The anatase to rutile

transformation temperature depends on purity, type of impurities, particle shape/size,

atmosphere and reaction conditions.

Table 1.3.2. Basic crystallographic and physical properties of antase and rutile [69], [70].

Property Anatase Rutile Crystallographic structure Tetragonal Tetragonal Space group I41/amd P42/mmm

Lattice parameters [nm]a= 0.3784b= 0.9515

a= 0.4594c= 0.2959

Volume of the unit cell/molecule [10-3nm3] 34.172 31.216

69. M. Schiavello, Heterogenius photocatalysis, John Wiley& Sons Ltd, 1997, p. 33-34; 70. K. Zakrzewska, Titanium dioxide thin film for gas sensors and photonic application, AGH,

Krakow, 2003, p. 14;

Density [g/cm3], T=298K 3.894 4.270Band gap energy (Eg) [eV] 3.26 3.05Electron effective mass (m*) 1m0 20m0

Hall mobility of electron [cm2/Vs], T=298K 4 0.1

Basic crystallographic properties of two polymorphic forms of titanium dioxide are listed in

the Table 1.3.2. [69].

Titania photocatalyst can be used either as free-standing particulate or as coating on a

substrate. Most experiments utilized finely powdered TiO2 particles suspended in

contaminated water, which provides large surface area and makes recovery easy after

treatment [71]. Larger particulates may prove useful even in the case of gaseous organic

contaminants but are rather commercially unavailable and may be costly [72].A reduction of

60–70% reduction in performance is reported in aqueous systems for immobilized TiO2 as

compared to the unsupported catalyst [73].

Many kinds of support have been explored for TiO2 photocatalyst which include soda

lime glass [74], aluminium [75], ceramic tiles [76] and coated glass [77]. Since coatings are very

thin, the actual active surface area of the photoreactor compared to the overall volume is low.

Despite aforementioned drawbacks,more coated photocatalysts and immobilisation techniques

are still investigated. In many of these cases TiO2 coated on support assumed more efficiency

in organic compound removal than uncoated TiO2 [78].

1.4 . Techniques for obtaining photocatalytic materials

There are different routes that can be used to synthesis anatase and rutile. It has been

said that the precursor and the method of preparation affect the physico-chemical properties of

the synthesised particle. The routes of synthesis TiO2 used like photocatalyst include:

chemical vapor deposition [79], e-beam evaporation [80], magnetron sputtering discharge [81],

spray vapor deposition [82], [83], hydrothermal synthesis [84], sol–gel method [85], doctor blade

[86].

Table 1.4.1. Routes of syntesis of titanium oxide

Preparation methods

Precursors References

Hydrothermalmethod

The synthesis of mesoporous TiO2 was carried out with the following procedure. Four grams (0.69 mmol) of the triblock copolymer was dissolved in 100 mL of distilled water at 40°C. After the surfactant had been dissolved sufficiently, 1.5 g (15.3 mmol) of sulfuric acid was added. Titanium (IV) isopropoxide (11.76 g, 41.4 mmol) was mixed with 2,4- pentanedione (4.14 g, 41.4 mmol) in a

[87]

separate beaker and dropped slowly into the surfactant solution with vigorous stirring. After addition, the reaction was carried out at 55°C for 2 h without stirring. At first, there was no precipitation, but after several minutes, a light yellow powder was obtained.

Sol-gel synthesis

The TiO2 sol was synthesized by acid catalyzed sol–gel formation method using 30 ml of 1 M HNO3 and 7.4 ml of titanium tetra-isopropoxide following the hydrolysis reaction. Ti(iso-OC3H7)4+4HO Ti(OH)4+4 C3H7OHTitanium tetra-isopropoxide was added gradually to the aqueous solution of HNO3 solution under continuous stirring for 1.5–2 h to produce a transparent sol containing 2g of TiO2

[88]

Chemical vapor

deposition

Photocatalytic TiO2 thin film depositions were carried out in a vertical low-pressure CVD reactor. CVD reactor was home-built cold-wall type. Sources for titanium and oxygen were tetraisopropoxide and O2 gas, respectively. Deposition temperature was varied from 287°C to 362°C, and the processing pressure was kept at 1 Torr.

[89]

Spray pyrolysis

deposition

The precursor solution contained titanium(IV) isopropoxide TTIP, acetylacetone (AcAc) and ethanol with TTIP concentration of 6 vol.% at TTIP:AcAc molar ratio of 1:2. The films were deposited at 300–500 -C

[90]

Among the different methods (Table I.8.) for the preparation of thin films titanium dioxide for

photocatalytic process, sol-gel method has many advantages, particularly the possibility of

producing large surfaces [91].

I.4.1. Sol-Gel Methods

The sol-gel process is a technique widely employed recently in the fields of materials

science and ceramic engineering. Such methods are utilized primarily for the fabrication of

materials (typically a metal oxide) starting from a chemical solution which acts as the

precursor for an integrated network (or gel) of either discrete particles or network polymers.

Typical precursors are metal alkoxides and metal chlorides, which undergo various forms of

hydrolysis and polycondensation reactions, rangind from 1 nm to 1 micrometer.

The formation of a metal oxide involves connecting the metal centers with either oxo

(M–O–M) or hydroxo (M–OH–M) bridges, and generating metal-oxo or metal-hydroxo

polymers in solution.

Thus, the sol evolves towards the formation of a gel-like diphasic system containing

both liquid and solid phases whose morphologies range from discrete particles to continuous

polymer networks (Figure I.3.). Removal of the remaining liquid (solvent) phase requires a

drying process, which is typically accompanied by a significant amount of shrinkage and

densification. The rate at which the solvent can be removed is ultimately determined by the

distribution of porosity in the gel. The ultimate microstructure of the final component will be

strongly influenced by changes imposed upon the structural template during this phase of

processing. Afterwards, a thermal treatment, or a firing process, is also necessary in order to

favor further poly-condensation and to enhance the mechanical properties and structural

stability via final sintering, densification, and grain growth.

Sol-gel derived materials have diverse applications in optics, electronics, energy,

space, bio-sensors, medicine (e.g., controlled drug release), and in reactive material and

separation (e.g., chromatography) technology [92], [93]. Sol-gel technology has found

increasing applications in the development of new materials for catalysis[94], [95], chemical

sensors[96], [97], membranes[98], fibers[99], optical gain media[100], photochromic and non

linear applications [101], [102], [103], and in solid state electrochemical devices [104]. The

technology is utilized in a diverse range of scientific and engineering fields, such as the

ceramic industry [93], nuclear field industry [93], and electronics industry [105]. The inherent

advantages of the sol-gel process are summarized as follows [106]: better homogeneity and

purity from raw materials; lower temperature of preparation;effective control of particle size,

shape and properties; creation of special products as films and the possibility of designing the

material structure and property through the proper selection of sol-gel precursor and other

building blocks.

Fundamental chemical reactions in the sol-gel process

The hydrolysis and the polycondensation of titanium alkoxides proceed according to the

following scheme [107]:

(19)

Fig. 1.4.1.1. Sol-Gel process

(20)

(21)

Hydrolysis and condensation can be exothermic and violent, particularly in the case of

transition metal alkoxides and usually they lead to undesirable routes. Therefore stabilizing

agents can be added into the sols (acetic acid, ethyl acetoacetate…), which prevent the

process of precipitation by decreasing the rate of the hydrolysis and condensation reactions. In

such case stable sols (colloidal solutions) are obtained. Condensation reactions can continue

to build larger and larger metal–containing molecules by the process of polymerization. When

cross-linked polymers with an average size of several nanometers are formed, the sol is

obtained. The final result of condensation/polymerization reactions is a gel, consisted of a

three dimensional titania network that extends throughout the solution.

(22)

Methods to obtain TiO2 sol-gel

Titanium dioxide formed by sol-gel method with applications in wastewater

photocatalytic processes, can be obtained, according to the literature [84], using various

methods, various precursors, obtaining time and temperatures. Materials used in sol-gel

methods: TTIP – titanium tetraisopropoxide, TiCl4 – titanium tetrachloride, CHD – 1,4

cyclohexane diol, EtOH – absolute ethanol, MeOH – methanol, TEA – tetraethylammonium,

NH4OH – ammonium hydroxide, C6H14 – hexane.

Fig. 1.4.1.2. Four methods to obtain titanium dioxide by sol-gel technique

A gel forms because of the condensation of hydrolyzed species into a three-dimensional

polymeric network. Any factor that affects either or both of these reactions is likely to impact

the properties of the gel. These factors, generally referred to as sol-gel parameters, includes

type of precursor, type of solvent, water content, acid or base content, precursor

concentration, and temperature [108]. These parameters affect the structure of the initial gel

and, in turn, the properties of the material at all subsequent processing steps. After gelation,

the wet gel can be optionally aged in its mother liquor, or in another solvent, and washed. The

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parameter. A gel is not static during aging but can continue to undergo hydrolysis and

condensation[109]. Furthermore, syntesis, which is the expulsion of solvent due to gel

shrinkage, and coarsening, which is the dissolution and reprecipitation of particles, can occur.

These phenomena can affect both the chemical and structural properties of the gel after its

initial formation. Then it must be dried to remove the solvent.

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Table 1.4.1.1.Important parameters in the various steps of sol-gel proces [110], [111], [112],

[113].

Step Purpose Important ParametersSolution

chemistryTo form gel

Type of precursor; Type of solvent;Water content; Precursor concentration; Temperature; pH

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A934, p. 51–57;100. Gvishi, R. Narang, U. Ruland, G. Kumar, D.N. Prasad, P.N. Novel, organically doped, sol-

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Journal Mater.Chem. 2001, 1, p. 903–913;103. Levy, D. Recent applications of photochromic sol–gel materials. Mol. Crys. Liq. Crys. A

2007, 297, p. 31–39;104. Dunn, B.; Farrington, G.C.; Katz, B. Sol-gel approaches for solid electrolytes and

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sol-gel monoliths for reversed phase capillary electrochromatography. J. Sep. Sci. 2002, 25, p.

3–9;106. MacKenzie, J.D. Sol-gel research – Achievements since 1981 and prospects for the

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Am. Ceram. Soc., 85, 2002, 346-354;

Aging To allow a gel to undergo changes in properties

Time; Temperature; Composition of the pore liquid; Aging environment;

DryingTo remove solvent for a gel

Drying method (evaporative, supercritical, and freeze drying);

Temperature and heating rate; Time Pressure and pressurization rate;

CalcinationTo change the physical / chemical properties of the solid

Temperature Heating rate;Time; Gaseous environment (inert, reactive gases);

In most of the experiments concerning the TiO2 photocatalysis, the photocatalyst was

used in the powder form and the recovery of powder was difficult to execute. Thus,

immobilization of the TiO2 semiconductor particles is necessary for industrial applications.

The methods used for synthesis of titanium oxide powder include alkali precipitation,

thermal decomposition, hydrothermal synthesis, sol-gel [114], [115], [116] and other routes.

Among all synthetic procedures, preparation of TiO2 by a sol–gel route remains one of the

most attractive due to the possibility of preparing powders or thin films. For the preparation

of thin films with sol–gel powder the dipping and doctor blade techniques could be used.

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