Band gap energies of various semiconductors at relevant wavelengths 
A photocatalyst is defined as a substance which is activated by adsorbing a photon and is capable of accelerating a reaction without being consumed . These substances are invariably semiconductors. Semiconducting oxide photocatalysts have been increasingly focused in recent years due to their potential applications in solar energy conversion and environmental purification. Semiconductor heterogeneous photocatalysis has enormous potential to treat organic contaminants in water and air. This process is known as advanced oxidation process (AOP) and is suitable for the oxidation of a wide range of organic compounds. Among AOPs, heterogeneous photocatalysis have been proven to be of interest due to its efficiency in degrading recalcitrant organic compounds. Developed in the 1970s, heterogeneous photocatalytic oxidation has been given considerable attention and in the past two decades numerous studies have been carried out on the application of heterogeneous photocatalytic oxidation process with a view to decompose and mineralize recalcitrant organic compounds. It involves the acceleration of photoreaction in the presence of a semiconductor catalyst . Several semiconductors (TiO2, ZnO, Fe2O3, CdS, ZnS) can act as photocatalysts but TiO2 has been most commonly studied due to its ability to break down organic pollutants and even achieve complete mineralization. Photocatalytic and hydrophilic properties of TiO2 makes it close to an ideal catalyst due to its high reactivity, reduced toxicity, chemical stability and lower costs . Fujishima and Honda  pioneered the concept of titania photocatalysis (also known as “Honda-Fujishima effect”). Their work showed the possibility of water splitting in a photoelectrochemical cell containing an inert cathode and rutile titania anode. The applications of titania photoelectrolysis has since been greatly focused in environmental applications including water and wastewater treatment. This chapter provides insight into the fundamentals of the TiO2 photocatalysis, discusses the effect of variables affecting the performance of degradation of organic pollutants in water with a view to current state of knowledge and future needs.
2. Mechanism and fundamentals of photocatalytic reactions
Heterogeneous photocatalysis using UV/TiO2 is one of the most common photocatalytic process and is based on adsorption of photons with energy higher than 3.2 eV (wavelengths lower than ~390 nm) resulting in initiating excitation related to charge separation event (gap band) . Generation of excited high-energy states of electron and hole pairs occurs when wide bandgap semiconductors are irradiated higher than their bandgap energy. It results in the promotion of an electron in the conductive band (eCB−) and formation of a positive hole in the valence band (hVB+)  (Eq. 1). The hVB+ and eCB− are powerful oxidizing and reducing agents, respectively. The hVB+ reacts with organic compounds resulting in their oxidation producing CO2 and H2O as end products (Eq. 2). The hVB+ can also oxidize organic compounds by reacting with water to generate •OH (Eq. 3). Hydroxyl radical (•OH) produced by has the second highest oxidation potential (2.80 V), which is only slightly lower than the strongest oxidant – fluorine. Due to its electrophilic nature (electron preferring), the •OH can non-selectively oxidize almost all electron rich organic molecules, eventually converting them to CO2 and water (Eq. 4).
where R represents the organic compound.
The conductive band can react with O2 forming an anion radical superoxide as shown in Eq. 5. Further reaction can lead to the formation of hydrogen peroxide which lead to the formation of •OH . The mechanism of the electron hole-pair formation when the TiO2 is irradiatied is given in Figure 1 .
The presence of dissolved oxygen is extremely important during photocatalytic degradation as it can make the recombination process on TiO2 (eCB−/hVB+) difficult which results in maintaining the electroneutrality of the TiO2 particles . In other words, it is important for effective photocatalytic degradation of organic pollutants that the reduction process of oxygen and the oxidation of pollutants proceed simultaneously to avoid the accumulation of electron in the conduction band and thus reduce the rate of recombination of eCB− and hVB+ [8, 9].
3. Types of photocatalysts and their characteristics
A number of solids can be referred to as photocatalysts and as mentioned earlier, metal oxide semiconductors are considered to be the most suitable photocatalysts due to their photocorrosion resistance and wide band gap energies . Table 1 provides the band gap energies at corresponding wavelength for well known semiconductors. TiO2 stands out as the most effective photocatalyst and has been extensively used in water and wastewater treatment studies because it is cost effective, thermally stable, non-toxic, chemically and biologically inert and is capable of promoting oxidation of organic compounds . The photocatalytic activity of TiO2 is dependent on surface and structural properties which include crystal composition, surface area, particle size distribution, porosity and band gap energy . TiO2 is also known as titania, titanic oxide, titanium white, titanic anhydride, or titanic acid anhydride. It is prepared using ilmenite and rutile in crystalline forms called anatase and rutile. The anatase form is achieved by processing of titanium sulphate, which is achieved when ilmenite is treated with sulphuric acid. Rutile crystalline form is obtained when raw rutile is chlorinated and the resulting titanium tetrachloride is subjected to vapor phase oxidation . When photon energy (hv) of higher than or equal to the bandgap energy of TiO2 is illuminated onto its surface, typically 3.2 eV (anatase) or 3.0 eV (rutile), the lone electron is photoexcited to the empty conduction band in femtoseconds . Degussa P25 which is the most widely used form of TiO2 is composed of 75% anatase and 25% rutile and has a specific BET surface area of 50 m2/g. The high effectiveness of D25 is related to the inhibition of recombination process on TiO2 (eCB−/hVB+) due to the smaller band gap of rutile that absorbs photons and generates electron-hole pairs and the electron transfer from the rutile conductive band to the electron traps occurs in the anatase phase .
4. Radiation sources for photocatalysis
Both artificial UV lamps and sunlight can be used as the radiation source for photocatalytic process. Artificial UV lamps containing mercury are the most commonly used source of UV irradiation. These can be divided into low pressure mercury lamp, medium pressure mercury lamp and high pressure mercury lamp. Sunlight has also been used in the photocatalytic process as nearly 4-5% of the sunlight that reaches the earth’s surface is in the 300-400 nm near UV light range. Furthermore solar energy has limitations due to the graphical variations when compared with the artificial UV lamps. However ongoing interests and developments in harnessing solar energy are expected to increase its use in photocatalytic degradation applications.
5. Photocatalytic reactors
Photocatalytic reactors can be classified based on the deployed state of the photocatalyst, i.e., suspended or attached. Photocatalytic reactors can use either UV or solar radiation. Solar photocatalytic reactors have been of great interest for the photoxodation of organic contaminants in water. Such kind of reactors can be divided into concentrating or non-concentrating reactors . Both the reactor types extend certain advantages and disadvantages. For example, non-concentrating reactors have negligible optical losses and therefore can use direct and diffuse sun irradiation but are larger in size compared with the concentrating reactors and have high frictional pressure losses . However, the use of solar radiated photoreactors is limited due to the intrinsic nature of the TiO2 particles. Following section provides details on the type of reactors used in various studies for the degradation of organic pollutants in water.
5.1. Slurry reactors
Until recently, TiO2 slurry reactors are most commonly type used in water treatment. These show largest photocatalytic activity compared with the immobilized photocatalyst and provide a high total surface area of photocatalyst per unit volume which is one of the most important factor configuring a photocatalytic reactor . However, these reactors require separation of the sub-micron TiO2 particles from the treated water which complicates the treatment process. Several techniques were proposed to achieve post-treatment separation such as the use of settling tanks (overnight particle settling) or external cross-flow filtration system . However the use of filtrations systems increases the cost of the treatment process.
5.2. Immobilized TiO2 reactors
Photocatalytic reactors with immobilized TiO2 are those in which catalyst is fixed to support via physical surface forces or chemical bonds. These reactors extend the benefit of not requiring catalyst recovery and permit the continuous use of the photocatalyst . Hybrid photocatalytic membrane reactors have been developed to achieve the purpose of downstream separation of photocatalyst. The photocatalytic membrane reactors can be generalized in two categories (1) irradiation of the membrane module and (2) irradiation of feed tank containing photocatalyst in suspension . Various membranes such as microfiltration, ultrafiltration, and nanofiltration membranes may be used for this purpose depending on the requirements of the treated water quality . Photocatalytic membrane reactors have been successfully used for the degradation of tricholoroethylene and 4-nitrophenol [18, 19]. However, these reactors possess drawbacks such as low surface area to volume ratios, catalyst fouling and significant pressure drop . Another problem associated with the membrane photocatalytic reactors is the diffusion of organic compounds to the catalyst surface which is slow particularly when the organic compounds concentration is low . One possible solution to the slow diffusion is using pores of nano size to enable photocatalyst to perform selective permeation and to produce an oxidized permeate stream .
It can be observed that the photocatalytic reactors can be either slurry or immobilized systems and each possess certain advantages and disadvantages related to their design and efficiency. Further research on the design and energy efficiency of photocatalytic reactors could make photocatalytic degradation process more feasible for future applications in water treatment. Membrane photoreactors appear to be a promising alternative to conventional photoreactors and more research in this area can assist overcome some of the problems faced with the use of conventional reactors.
6. Factors affecting the degradation performance
6.1. Catalyst loading
The amount of TiO2 being directly proportional to the overall photocatalytic reaction rate, the concentration of the TiO2 particles affects the overall photocatalysis reaction rate in a true heterogeneous catalytic regime . However, when the amount of TiO2 is above certain level (saturation stage), the light photon adsorption co-efficient decreases radially and the excess photocatalyst can create a light screening effect that leads to the reduction in the surface area exposed to irradiation and thus reduces the photocatalytic efficiency of the process . A number of studies have reported the effect of TiO2 loadings on the treatment efficiency of the photocatalytic reactor [2, 22-24]. Although a direct comparison between these studies is difficult to be made due to the differences in the working geometry, radiation fluxes and wavelengths used, it was evident that the optimum dosages of photocatalyst loading were dependent on the dimension of the reactor. The importance of the determination of the reactor diameter has been emphasized to achieve effective photon absorption . The optimum dosage of TiO2 used by various authors either alone or in combination with other catalysts is given in Table 2.
6.2. pH of the solution
The effect of pH on the photocatalytic reaction has been extensively studied [26, 27] due to the fact that photocatalytic water treatment is highly dependent on the pH as it affects the charge on the catalyst particles, size of aggregates and the position of conductance and valance bands . Furthermore the surface of the TiO2 can be protonated or deprotonated under acidic or alkaline conditions , respectively according to the reaction given below.
The point of zero discharge for P25 Degussa, the most commonly used form of TiO2 is 6.9 . Therefore the surface of the TiO2 is positively charged under acidic conditions and negatively charged under alkaline conditions. The maximum oxidizing capacity of the titania is at lower pH however the reaction rate is known to decrease at low pH due to excess H+ . The selection of pH is thus need to be appropriate in order to achieve maximum degradation efficiency.
|Direct red 23||Ag-TiO2||3|||
6.3. Size and structure of the photocatalyst
Surface morphology such as particle size and agglomerate size, is an important factor to be considered in photocatalytic degradation process because there is a direct relationship between organic compounds and surface coverage of the photocatalyst . The number of photon striking the photocatalyst controls the rate of reaction which signifies that the reaction takes place only in the absorbed phase of the photocatalyst [2, 31]. A number of different forms of TiO2 have been synthesized to achieve the desired characteristics of the photocatalyst . Some of the examples include UV100, PC500 and TTP. For the degradation of various organic compound such as pesticides and dyes, the efficacy of these photocatalysts has generally been reported in the order of Degussa P25 > UV100 > PC500 >TTP [33-36].
6.4. Reaction temperature
An increase in reaction temperature generally results in increased photocatalytic activity however reaction temperature >80oC promotes the recombination of charge carriers and disfavor the adsorption of organic compounds on the titania surface . A reaction temperature below 80oC favours the adsorption whereas further reduction of reaction temperature to 0oC results in an increase in the apparent activation energy . Therefore temperature range between 20-80oC has been regard as the desired temperature for effective photomineralization of organic content.
6.5. Concentration and nature of pollutants
The rate of photocatalytic degradation of certain pollutant depends on its nature, concentration and other existing compounds in water matrix. A number of studies have reported the dependency of the TiO2 reaction rate on the concentration of contaminants in water . High concentration of pollutants in water saturates the TiO2 surface and hence reduces the photonic efficiency and deactivation of the photocatalyst . In addition to the concentration of pollutants, the chemical structure of the target compound also influences the degradation performance of the photocatalytic reactor. For example, 4-chlorophenol requires prolonged irradiation time due to its transformation to intermediates compared with oxalic acid that transforms directly to carbon dioxide and water, i.e., complete mineralization . Furthermore if the nature of the target water contaminants is such that they adhere effectively to the photocatalyst surface the process would be more effective in removing such compounds from the solution. Therefore the photocatalytic degradation of aromatics is highly dependent on the substituent group . The organic substrates with electron withdrawing nature (benzoic acid, nitrobenzene) strongly adhere to the photocatalyst and therefore are more susceptible to direct oxidation compared with the electron donating groups .
6.6. Inorganic ions
Various inorganic ions such as magnesium, iron, zinc, copper, bicarbonate, phosphate, nitrate, sulfate and chloride present in wastewater can affect the photocatalytic degradation rate of the organic pollutants because they can be adsorbed onto the surface of TiO2 [41-43]. Photocatalytic deactivation has been reported whether photocatalyst is used in slurry or fixed-bed configuration which is related to the strong inhibition from the inorganic ions on the surface of the TiO2 . A number of studies have been conducted on the effect of inorganic ions (anions and cations) on TiO2 photocatalytic degradation [30, 45-51]. Some of the cations such as copper, iorn and phosphate have been reported to decrease the photodegradation efficiency if they are present at certain concentrations whereas calcium, magnesium and zinc have little effect on the photodegradation of organic compounds which is associated to the fact that these cations have are at their maximum oxidation states that results in their inability to have any inhibitory effect on the degradation process .
The inorganic anions such as nitrate, chlorides, carbonates and sulphates are also known to inhibit the surface activity of the photocatalyst. The presence of salts diminishes the colloidal stability, increases mass transfer and reduces the surface contact between the pollutant and the photocatalyst . Other than fouling of the TiO2 surface certain anions such as chlorides, carbonates, phosphate and sulphates also scavenge both the hole and the hydroxyl radicals . The mechanism of hole and radical scavenging by chloride has been proposed by Matthews and McEnvoy  as follows.
The inhibitory effect of chloride ions occurs through preferential adsorption displacement mechanism which results in reducing the number of OH- available on the photocatalyst surface .
The fouling of photocatalytic surface can be reduced by pre-treatment of water such as with ion exchange resins which have been reported to reduce the fouling and so the cost of treatment (Burns et al., 1999). Similarly the fouling induced by sulphates and phosphates has been reported to be displaced by NaOH, KOH and NaHCO3 . However, most of studies conducted on the effect of inorganic ions are based on the model compounds and therefore do not necessarily represent their effect in real water matrix where several ions exist. More work concentrating on the effect of complex mixtures of inorganic ions is thus required.
Photocatalytic degradation of organic pollutants is promising technology due to its advantage of degradation on pollutants instead of their transformation under ambient conditions. The process is capable of removing a wide range of organic pollutants such as pesticides, herbicides, and micropollutants such as endocrine disrupting compounds. Although significant amount of research has been conducted on TiO2 photocatalysis at laboratory scale, its application on industrial scale requires certain limitations to be addressed. However the application of this treatment is constrained by several factors such as wide band gap (3.2eV), lack and inability of efficient and cost-effective catalyst for high photon-efficiency to utilize wider solar spectra. The effect of variables is required to be further studied in real water matrix to achieve representative results. The results achieved can be used to optimize the process and design appropriate reactor for potential large scale applications. The use of solar radiation has to be improved by virtue of the design of the photoreactor in order to reduce the cost of treatment. Further research to investigate the degradation of the real water constituents is required to better comprehend the process applications.
Fox M., Photocatalytic Oxidation of Organic Substances. In: Kluwer (ed.) Photocatalysis and Environment: Trends and Applications. New York Academic Publishers: 1988. p. 445–467.
Gaya U.I., Abdullah A.H. Heterogeneous Photocatalytic Degradation of Organic Contaminants over Titanium Dioxide: A Review of Fundamentals, Progress and Problems. Journal of Photochemistry and Photobiology C Photochemistry Reviews 2008 (9) 1-12.
Fujishima A., Rao T.N., Tryk D.A. Titanium Dioxide Photocatalysis. Journal of Photochemistry and Photobiology C Photochemistry Reviews 2000 (1) 1-21.
Fujishima A., Honda K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972 (238) 37-38.
Boroski M., Rodrigues A.C., Garcia J.C., Sampaio L.S., Nozaki J., Hioka N. Combined Electrocoagulation and TiO2 Photoassisted Treatment Applied to Wastewater Effluents from Pharmaceutical and Cosmetic Industries. Journal of Hazardous Materials 2009 (162) 448–454.
Pirkanniemi K., Sillanpaa M. Heterogeneous Water Phase Catalysis as an Environmental Application: A Review. Chemosphere 2000 (48) 1047–1060.
Chong M.N., Jin B., Chow C.W.K., Saint C. Recent Developments in Photocatalytic Water Treatment Technology: A Review. Water Resources 2010 (44) 2997-3027.
Hoffmann M.R., Martin S.T., Choi W., Bahnemann D.W. Environmental Applications of Semiconductor Photocatalysis. Chemical Reviews 1995 (95) 69-96.
Herrmann J.M. Heterogeneous Photocatalysis: Fundamentals and Applications to the Removal of Various Types of Aqueous Pollutants. Catalysis Today 53 (1999) 115-129.
Fox M.A., Dulay M.T. Heterogeneous Photocatalysis. Chemical Reviews 1993 (93) 341–357.
Mandelbaum P., Regazzoni A., Belsa M., Bilme S. Photo-electron-oxidation of Alcohol on Titanium Dioxide Thin Film Electrodes. Journal of Physics and Chemistry B. 1999 (103) 5505-5511.
Ahmed S., Rasul M.G., Brown R., Hashib M.A. Influence of Parameters on the Heterogeneous Photocatalytic Degradation of Pesticides and Phenolic Contaminants in Wastewater: A Short Review. Journal of Environmental Management 2011 (92) 311-330.
Hawley G. The Condensed Chemical Dictionary. 8th Ed. (revised). Litton Educational Publishing Incorporation, 1971.
Hurun D.C., Agrios A.G., Gray K.A., Rajh T., Thurnaur M.C. Explaining the Enhanced Photocatalytic Activity of Degussa P 25 Mixed-phase TiO2 using EPR. The Journal Physics and Chemistry B 2003 (107) 4545-4549.
Rajeshwar, K., Ibanez, J. Environmental Electrochemistry, Fundamentals and Fundaments in Pollution Abatement. Acadmic Press, San Diego 1997.
de Lasa H., Serrano B., Salaices M. Photocatalytic Reaction Engineering. Springer Science: USA 2005.
Molinari R., Palmisano L., Drioli E., Schiavello M. Studies on Various Reactor Configurations for Coupling Photocatalysis and Membrane Processes in Water Purification. Journal of Membrane Science 2002 (206) 399–415.
Artale M.A., Augugliaro V., Drioli E., Golemme G., Grande C., Loddo V., Molinari R., Palmisano L., Schiavello M. Preparation and Characterisation of Membranes with Entrapped TiO2 and Preliminary Photocatalytic Tests. Annali di Chimica 2000 (91) 127–136.
Tsuru T.,Toyosada T., Yoshioka T., Asaeda M. Photocatalytic Membrane Reactor using Porous Titanium Dioxide Membranes. Journal of Chemical Engineering Japan 2003 (36) 1063–1069.
Augugliaro V., Litter M., Palmisano L., Soria J. The Combination of Heterogeneous Photocatalysis with Chemical and Physical Operations: A tool for Improving the Photoprocess Performance. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2006 (7) 127–144
Herz R.K. Intrinsic Kinetics of First-order Reactions in Photocatalytic Membranes and Layers. Chemical Engineering Japan 2004 (99) 237–245.
Chin S.S., Chiang K., Fane A.G. The Stability of Polymeric Membranes in TiO2 Photocatalysis process. Journal of Membrane Science 2006 (275) 202-211.
Daneshvar N., Salari D., Niaei A., Khataee A. R. Photocatalytic Degradation of the Herbicide Erioglaucine in the Presence of nanosized titanium dioxide: Comparison and Modeling of Reaction Kinetics. Journal of Environmental Science and Health Part B: Pesticides, Food Contaminants and Agricultural Wastes. 2006 (41) 1273-1290.
Chong M.N., Jin B., Zhu H.Y., Chow C.W.K., Saint C. Application of H-titanate Nano Fibers for Degradation of Congo Red in an Annular Slurry Photoreactor. Chemical Engineering Journal 2009 (150) 49-54.
Malato S., Fernández-Ibánez P., Maldonado M.I., Blanco J., Gernjak W. Decontamination and Disinfection of Water by Solar Photocatalysis: Recent Overview and Trends. Catalysis Today 2009 (147) 1-59.
Mrowetz M., Selli E. Photocatalytic Degradation of Formic and Benzoic Acids and Hydrogen Peroxide Evolution in TiO2 and ZnO Water Suspensions. Journal of Photochemistry and Photobiology A: Chemistry 2006 (180) 15-22.
Wang W-Y., Ku Y. Effect of solution pH on the adsorption and photocatalytic reaction behaviors of dyes using TiO2 and Nafion-coated TiO2. Colloids and Surfaces A Physicochemical and Engineering Aspects. 2007 (302) 261-268.
Kosmulski M. pH-dependent Surface Charging and Points of Zero Charge: III. Update. Journal of Colloid Interface Science 2006 (298) 730-741.
Sun J., Wang S., Sun J., Sun R., Sun S., Qiao, L. Photocatalytic Degradation and Kinetics of Orange G using Nano-sized Sn(IV)/TiO2/AC Photocatalyst. Journal of Molecular Catalysis A: Chemical 2006 (260) 241-246.
Guillard C., Lachheb H., Houas A., Elaloui E., Hermann J-M. Influence of Chemical Structure of Dyes, of pH and of Inorganic Salts on their Photocatalytic Degradation by TiO2 Comparison of the Efficiency of Powder and Supported TiO2. Journal of Photochemistry and Photobiology A: Chemical 2003 (158) 27-36.
Kogo K., Yoneyama H., Tamura H. Photocatalytic oxidation of cyanide on platinized titanium dioxide. The Journal of Physical Chemistry1980 (84) 1705–1710.
Gao Y., Liu H. Preparation and Catalytic Property Study of a Novel kind of Suspended Photocatalyst of TiO2-activated Carbon Immobilized on Silicone Rubber Film. Materials Chemistry and Physics 2005 (92) 604-608.
Muneer M., Qamar M., Saquib M., Bahnemann D. Heterogeneous Photocatalysed Reaction of three Selected Pesticide Derivatives, Propham, Propachlorand Tebuthiuron in Aqueous Suspensions of Titanium Dioxide. Chemosphere 2005 (61) 457-468.
Qamar M., Muneer M., Bahnemann D. Heterogeneous Photocatalysed Degradation of two Selected Pesticide Derivatives, Triclopyr and Daminozid Inaqueous Suspensions of Titanium Dioxide. Journal of Environmental Management 2006 (80) 99-106.
Tariq M.A., Faisal M. Muneer M. Semiconductor-mediated Photocatalysed Degradation of two selected Azo Dye Derivatives, Amaranth and Bismarck Brown in Aqueous Suspension. Journal of Hazardous Materials B 2005 (127) 172–179
Tariq M.A., Faisal M., Saquib M., Muneer M. Heterogeneous photocatalytic degradation of ananthraquinone and a triphenylmethane dye derivative in aqueous suspensions of semiconductor. Dyes and Pigments 2008 (76) 358-365.
Chong M.N., Lei S., Jin B., Saint C., Chow C.W.K. Optimisation of an Annular Photoreactor Process for Degradation of Congo Red using a newly Synthesized Titaniaim Pregnated Kaolinite Nano-photocatalyst. Separation and Purification Technology 2009 (67) 355-363.
Saquib M., Muneer M. TiO2-mediated Photocatalytic Degradation of a Triphenyl Methane Dye (Gentian Violet), in Aqueous Suspensions. Dyes and Pigments 2003 (56) 37-49.
Bahnemann D. Photocatalytic Water Treatment: Solar Energy Applications. Solar Energy 2004 (77) 445-459.
Bhatkhnade D.S., Kamble S.P., Sawant S.B., Pangarkar V.G. Photocatalytic and photochemical degradation ofnitrobenzene using artificial ultraviolet light. Chemical Engineering Journal 2004 (102) 283-290.
Abdullah M., Low G., Mathews R.W. Effects of common inorganic ions on rates of photocatalytic oxidation of organic carbon over illuminated titanium dioxide. Journal of Physical Chemistry 1990 (94) 6820.
Lin C., Lin K. Photocatalytic Oxidation of Toxic Organohalides withTiO2/UV: The effects of Humic Substances and Organic Mixtures. Chemosphere 2007 (66) 1872–1877.
Parent Y., Blake D., Magrini-Bair K., Lyons C., Turchi C., Watt A., Wolfrum E., Praire M. 1996. Solar Photocatalytic Process for the Purification of Water: State of Development and Barriers to Commercialization. Solar Energy 1996 (56) 429–437.
Crittenden J.C., Zhang Y., Hand D.W., Perram D.L., Marchand E.G. Solar detoxification of fuel-contaminated groundwater using fixed-bed photocatalysts. Water Environment Research 1996 (68) 270-278.
Habibi M.H., Hassanzadeh A., Mahdavi S. The effect of operational parameters on the photocatalytic degradation of three textile azo dyes in aqueous TiO2 suspensions. Journal of Photochemistry and Photobiology A: Chemical 172 (2005) 89-96.
Leng W., Liu H., Cheng S., Zhang J., Cao C. Kinetics ofphotocatalytic degradation of aniline in water over TiO2 supported on porous nickel. Journal of Photochemistry and Photobiology A: Chem. 2000 (131) 125-132.
Özkan A., Özkan M.H., Gürkan R., Akçay M., Sökmen M. Photocatalytic Degradation of a Textile Azo Dye, Sirius Gelb GC on TiO2 or AgeTiO2 Particles in the Absence and Presence of UV Irradiation: The Effects of Some Inorganic Anions on the Photocatalysis. Journal of Photochemistry and Photobiology A: Chemical 2004 (163) 29-35.
Riga A., Soutsas K., Ntampegliotis K., Karayannis V., Papapolymerou G. Effect of System Parameters and of Inorganic Salts on the Decolorization and Degradation of Procion H-exldyes. Comparison of H2O2/UV, Fenton, UV/ Fenton, TiO2/UV and TiO2/UV/H2O2 Processes. Desalination 2007 (211) 72-86.
Rincón A.G., Pulgarin C. Effect of pH, Inorganic Ions, Organic Matter and H2O2 on E. coli K12 Photocatalytic Inactivation by TiO2-implications in Solar Water Disinfection. Applied Catalysis B: Environmental 2004 (51) 283-302.
Schmelling D.C., Gray K.A., Vamat P.V. The Influence of Solution Matrix on the Photocatalytic Degradation of TNT in TiO2 slurries. Water Resources 1997 (31) 1439-1447
Wang K., Zhang J., Lou L., Yang S., Chen Y. UV or Visible Light Induced Photodegradation of AO7 on TiO2 Particles: The Influence of Inorganic Anions. Journal of Photochemistry Photobiology A: Chemical 165 (2004) 201-207.
Diebold U. The Surface Science of Titanium Dioxide. Surface Science Reports 2003 (48) 53-229.
Matthews R.W., McEnvoy S.R. Photocatalytic Degradation of Phenol in the Presence of near-UV Illuminated Titanium Dioxide. Journal of Photochemistry and Photobiology A: Chemical 1992 (64) 231.
Burns, R., Crittenden, J.C., Hand, D.W., Sutter, L.L., Salman, S.R., 1999. Effect of inorganic ions in heterogeneous photocatalysis. J. Environ. Eng. 125, 77-85.