Photocatalytic Degradation of Organic Pollutants: Mechanisms and Kinetics

Ten years after coming into force of the Stockholm Convention on Persistent Organic Pollutants (POPs), a wide range of organic chemicals (industrial formulations, plant protection products, pharmaceuticals and personal care products, etc.) still poses the highest priority environmental hazard. The broadening of knowledge of organic pollutants (OPs) environmental fate and effects, as well as the decontamination techniques, is accompanied by an increase in significance of certain pollution sources (e.g. sewage sludge and dredged sediments application, textile industry), associated with a potential generation of new dangers for humans and natural ecosystems. The present book addresses these aspects, especially in the light of Organic Pollutants risk assessment as well as the practical application of novel analytical methods and techniques for removing OPs from the environment. Providing analytical and environmental update, this contribution can be particularly valuable for engineers and environmental scientists.


www.intechopen.com
If charge separation is maintained, the electron and hole may migrate to the catalyst surface where they participate in redox reactions with sorbed species. Specially, h + vb may react with surface-bound H 2 O or OH to produce the hydroxyl radical and e cb is picked up by oxygen to generate superoxide radical anion (O 2 ), as indicated in the following equations 1-3; -absorption of efficient photons by titania (h ≥ E bg = 3.2 ev) It has been suggested that the hydroxyl radical ( • OH) and superoxide radical anions (O 2 •-) are the primary oxidizing species in the photocatalytic oxidation processes. These oxidative reactions would results in the degradation of the pollutants as shown in the following equations 4-5; -oxidation of the organic pollutants via successive attack by OH radicals or by direct reaction with holes R + h + → R •+ → degradation products (5) For oxidation reactions to occur, the VB must have a higher oxidation potential than the material under consideration. The redox potential of the VB and the CB for different semiconductors varies between +4.0 and -1.5 volts versus Normal Hydrogen Electrode (NHE) respectively. The VB and CB energies of the TiO 2 are estimated to be +3.1 and -0.1 volts, respectively, which means that its band gap energy is 3.2 eV and therefore absorbs in the near UV light ( <387 nm). Many organic compounds have a potential above that of the TiO 2 valence band and therefore can be oxidized. In contrast, fewer organic compounds can be reduced since a smaller number of them have a potential below that of the TiO 2 conduction band.

Use of semiconductor (TiO 2 ) in various fields
Due to non-toxic, easily available, inexpensive, biologically and chemically inert and stable to photo and chemical corrosion, TiO 2 is used in various fields as shown in Figure 2. www.intechopen.com The process of photocatalysis is also widely being contributed to various sub-discipline of Chemistry as shown in Figure 3.

Our research focus
We have studied the photocatalysed degradation of a large variety of organic pollutants under different reaction conditions to determine the detailed degradation kinetics and product identification in few selected systems for better mechanistic understanding. The different class of organic pollutants studied by our research group are shown below in chart 1. The following text describes the results of the photocatalysed degradation of different pollutants under variety of conditions for degradation kinetics and product identification.

Procedure for conducting the degradation experiments
Stock solutions of the pollutants containing the desired concentration were prepared in double distilled water. An immersion well photochemical reactor made of Pyrex glass equipped with a magnetic stirring bar, water circulating jacket and an opening for supply of molecular oxygen was used. A simplified diagram of the reactor system is shown in Figure 4. For irradiation experiment an aqueous solution of the pollutants with desired concentration is taken into the photoreactor and required amount of photocatalyst was added. Then the solution was stirred and bubbled with molecular / atmospheric oxygen for at least 15 minutes in the dark to allow equilibration of the system so that the loss of compound due to adsorption can be taken into account. The zero time reading was obtained from blank solution kept in the dark in the presence of TiO 2 and oxygen but otherwise treated similarly to the irradiated solution. The suspensions were continuously purged with molecular / atmospheric oxygen throughout each experiment. Irradiations were carried out using medium pressure mercury lamp. The light intensity was measured using UV-light intensity detector. IR-radiation and short-wavelength UV-radiation were eliminated by water circulating Pyrex glass jacket. Samples (10 mL) were collected before and at regular intervals during irradiation and analyzed after centrifugation.
The sunlight experiments were carried out between 9:00 A.M to 2:30 P.M. on a sunny day. The light intensity was measured using UV-light intensity detector (Lutron UV-340), which was found to be in the range of 0.370 to 0.480 mW/cm 2 . Reactions were carried out in the same reaction vessel as described above. Aqueous solution of desired concentration of the model compound containing required amount of photocatalyst was taken and stirred for 15 min. in the dark in presence of oxygen for equilibration. The solution was then placed on flat www.intechopen.com platform under sunlight with continuous stirring and purging of molecular oxygen. Samples (10 mL) were collected before and at regular intervals during the illumination and analyzed after centrifugation.

Photomineralization of organic pollutants
The photomineralization of the pesticide was measured using Total Organic Carbon analyzer (Shimadzu TOC 5000 A). The main principle of TOC analyzer involves the use of carrier gas (oxygen), which is flow-regulated (150 ml / min) and allows to flow through the total carbon (TC) combustion tube, which is packed with catalyst, and kept at 680 0 C. When the sample enters the TC combustion tube, TC in the sample is oxidized to carbon dioxide. The carrier gas containing the combustion products from the TC combustion tube flows through the inorganic carbon (IC) reaction vessel, dehumidifier, halogen scrubber and finally reaches the sample cell of the nondispersive infrared (NDIR) detector which measures the carbon dioxide content. The output signal (analog) of the NDIR detector is displayed as peaks. The peak areas are measured and processed by the data processing unit.
Since the peak areas are proportional to the total carbon concentration, the total carbon in a sample may be easily determined from the calibration curve prepared using standard solution of known carbon content. Total carbon is the sum of TOC (Total Organic Carbon) and IC (Inorganic Carbon).

Photodegradation of the pesticide
The photodegradation of the pesticide was measured using UV-Vis spectrophotometry or HPLC analysis techniques.

Characterization of intermediate products
Intermediate product formed during the photooxidation process was characterized by monitoring the reaction as a function of time using GC/MS analysis technique. For GC/MS analysis a Shimadzu Gas Chromatograph and Mass Spectrometer (GCMS-QP 5050) equipped with a 25m CP SIL 19 CB (d=0.25mm) capillary column, operating temperature programmed (220 0 C for 40 min at the rate of 10 0 C min -1 ) in splitless mode injection volume (1.0 L) with helium as a carrier gas was used.

In the presence of TiO 2
Irradiation of an aqueous suspension of desired organic pollutants in the presence of TiO 2 lead to decrease in absorption intensity and depletion in TOC content as a function of time.
As a representative example Figure 5 shows the change in absorption intensity and depletion in TOC as a function of time on irradiation of an aqueous solution of isoproturon (0.5 mM, 250 ml) in the presence and absence of photocatalyst (Degussa P25, 1 gL -1 ) by the "Pyrex" filtered output of a 125 W medium Pressure mercury lamp (radiant flux 4.860 mW/cm 2 ). Experimental conditions: 0.5 mM isoproturon, V=250 mL, photocatalyst: TiO 2 (Degussa P25, 1 gL -1 ), immersion well photoreactor, 125 W medium pressure Hg lamp, absorbance was followed at 238 nm after 75% dilution, cont. O 2 purging and stirring, irradiation time = 80 min.
It could be seen that 94.8% degradation and 78.45 % mineralization of the isoproturon takes place after 80 min of illumination. On the other hand no observable loss of the compound was found when the irradiations were carried out in the absence of the photocatalysts (Haque & Muneer, 2003). Both the mineralization (depletion of TOC Vs irradiation time) and decomposition (decrease in absorption intensity/concentration Vs irradiation time) curves can be fitted reasonably well by an exponential decay curve suggesting the first order kinetics. Figure 6 shows the linear regression curve fit for the natural logarithm of the degradation of isoproturon Vs irradiation time for first order reaction. For each experiment, the degradation rate constant was calculated from the plot of the natural logarithm of the TOC depletion or absorbance / concentration of the pesticide as a function of irradiation time (the concentration of the pollutant was calculated by taking the absorbance of the pollutant at its λ max of standard concentration and then by plotting the graph of absorbance Vs standard concentration of the pollutant). The degradation rate for the mineralization and for the decomposition of the pollutants for first order reaction was calculated using formula given below; www.intechopen.com TOC = Total Organic Carbon, k = rate constant, c = concentration of the pollutant, n = order of reaction.

Comparison of different photocatalysts
Titanium dioxide is known to be the semiconductor with the highest photocatalytic activity and stable in aqueous solution. Several reviews have been written, regarding the mechanistic and kinetic details as well as the influence of experimental parameters. It has been demonstrated that degradation by photocatalysis can be more efficient than by other wet-oxidation technique (Weichrebe and Vogelpohl, 1995). We have tested the photocatalytic activity of four different commercially available TiO 2 powders (namely Degussa P25, Sachtleben Hombikat UV100, Milenium Inorganic PC500 and Travancore Titanium Product, India) on the degradation kinetics of the pollutants. In most of the cases it has been observed that the degradation of pollutants under investigation proceed much more rapidly in the presence of Degussa P25 as compared with other TiO 2 samples (Muneer and Bahnemann, 2001;Haque & Muneer, 2003;Bahnemann et al., 2007) as shown in Figure 7 (isoproturon). The better photocatalytic activity of Degussa P25 has also been reported by Pizzaro et al., in 2005. While in some cases Hombikat UV 100 is found to be better than Degussa P25 for the degradation of benzidine, eosine yellowish, and Remazol brilliant blue R Muneer, 2002, 2003) as shown in Figure 8 (eosine yellowish). In an earlier study Lindner et al. 1995, showed that Hombikat UV100 was almost four times more effective than P25 when dichloroacetic acid was used as the model pollutant.
www.intechopen.com (1 gL -1 ), Sachtleben Hombikat UV100 (1 gL -1 ) and PC500 (1 gL -1 ). www.intechopen.com The differences in the photocatalytic activity of TiO 2 are likely to be due to differences in the BET-surface, impurities, lattice mismatches or density of hydroxyl groups present on the catalyst's surface. Since they will affect the adsorption behaviour of a pollutant or intermediate molecule and the lifetime and recombination rate of electron-hole pairs. The reason for the better photocatalytic activity of Degussa P25, could be attributed to the fact that P25 being composed of small nano-crystallites of rutile being dispersed within an anatase matrix. The smaller band gap of rutile "catches" the photons, generating electronhole pairs. The electron transfer, from the rutile conduction band to electron traps in anatase phase takes place. Recombination is thus inhibited allowing the hole to move to the surface of the particle and react (Hurum et al., 2003). The better efficiency of photocatalyst Degussa P25 may also due to 'quantum size effect' (Nozik et al., 1993;weller, 1993). When the particles become too small, there is a 'blue shift' with an increase of the band gap energy, detrimental to the near UV-photon absorption, and an increase of the electron-hole recombination.

Influence of pH on the degradation kinetics
An important parameter in the photocatalytic reactions taking place on the particulate surfaces is the pH of the solution, since it dictates the surface charge properties of the photocatalyst and size of aggregates it forms. Employing Degussa P25 as photocatalyst the decomposition and mineralization of pollutant in aqueous suspensions of TiO 2 was studied as a function of pH. The degradation rate for the decomposition and mineralization of acid red 29 was found to increase with the increase in pH from 3 to 11 as shown in Figure 9   while the degradation rate for the decomposition and mineralization of the pesticide derivative propachlor was found to decrease with the increase in pH from 3 to 11 as shown in Figure  10  . Similar results on pH effect have also been reported earlier by Vaz et al., 1998 (for the degradation of uracil and 5-halogenouracil) and Lu et al., 1995 (for the degradation of dichlorvos, propoxur and 2,4-D).
The interpretation of pH effects on the photocatalytic process is very difficult task because of its multiple roles such as electrostatic interactions between the semiconductor surface, solvent molecules, substrate and charged radicals formed during the reaction process. The ionization state of the surface of the photocatalyst can be protonated and deprotonated under acidic and alkaline conditions, respectively, as shown in following equations: The point of zero charge (pzc) of the TiO 2 (Degussa P25) is widely reported at pH ~ 6.25 (Augustynski, 1988). Thus, the TiO 2 surface will remain positively charged in acidic medium (pH < 6.25) and negatively charged in alkaline medium (pH > 6.25). The functional group present on the pollutants can be protonated and deprotonated depending on the pH of the reaction mixture. The better degradation rate in acidic or basic pH may also be attributed on the basis of the fact that the structural orientation of the molecule is favoured for the attack of the reactive species under that condition.

Effect of substrate concentration
It is important both from mechanistic and from application point of view to study the dependence of substrate concentration in the photocatalytic reaction rate. Effect of substrate concentration on the degradation of the pollutants was studied at different concentrations. As a representative example Figure 11 shows the degradation rate for the TOC depletion and for the decomposition of pollutant as a function of substrate concentration employing Degussa P25 as photocatalyst . It was found that the degradation rate for the decomposition and for the mineralization increases gradually with the increase in substrate concentrations. Similar trend was found in most of the colourless organic pollutants as reported earlier (Sabin et al., 1992;Krosley et al., 1993;O'Shea et al., 1997). In coloured compound it has been found that the degradation rate increase upto a certain limit and after that a further increase in substrate concentration lead to decrease in the degradation rate. This may be due to the fact that as the initial concentrations of the pollutant increases, the irradiating mixture becomes more and more intense which prevents the penetration of light to the surface of the catalyst. Hence, the generation of relative amount of OH and O 2 on the surface of the catalyst do not increase as the intensity of light and irradiation times are constant. Conversely, their concentrations will decrease with increase in concentration of the pollutant as the light photons are largely absorbed and prevented from reaching the catalyst surface by the substrate molecules. As oxidation proceeds, less and less of the surface of the TiO 2 particle is covered as the pollutant is decomposed. Evidently, at total decomposition, the rate of degradation is zero and a decreased photocatalytic rate is to be expected with increasing irradiation time. It has been agreed, with minor variation that the expression for the rate of photodegradation of www.intechopen.com organic substrates with irradiated TiO 2 follows the Langmuir Hinshelwood (L-H) law for the four possible situations, i.e., (1) the reaction takes place between two adsorbed substances, (2) the reaction occurs between a radical in solution and an adsorbed substrate molecule, (3) the reaction takes place between a radical linked to the surface and a substrate molecule in solution, and (4) the reaction occurs with both the species being in solution. In all cases, the expression for the rate equation is similar to that derived from the L-H model, which has been useful in modelling the process, although it is not possible to find out whether the process takes place on the surface, in the solution or at the interface. Our results, on the effect of the initial concentration on the degradation rate are in agreement with the assumption of the Langmuir Hinshelwood model.

Effect of catalyst concentration
The effect of photocatalyst concentration on the degradation kinetics of pollutant under investigation was studied employing different concentrations of Degussa P25 varying from 0.5 to 7.5 gL -1 . As a representative example the degradation rate for the TOC depletion and for the decomposition of the tebuthiuron as function of catalyst loading is shown in Figure  12  . It was observed that the degradation rate was found to increase with the increase in catalyst concentration upto 5 gL -1 and on subsequent addition of catalyst lead to the levelling off the degradation rate. Whether in static, slurry, or dynamic flow reactors, the initial reaction rates were found to be directly proportional to catalyst concentration, indicating a heterogeneous regime. However, it was observed that above a certain concentration, the reaction rate decreases and becomes independent of the catalyst concentration. This limit depends on the geometry and working conditions of the photoreactor and for a definite amount of TiO 2 in which all the www.intechopen.com particles, i.e., surface exposed, are totally illuminated. When the catalyst concentration is very high, after travelling a certain distance on an optical path, turbidity impedes further penetration of light in the reactor. In any given application, this optimum catalyst concentration [(TiO 2 ) OPT ] has to be found, in order to avoid excess catalyst and insure total absorption of efficient photons. Our results on the effect of catalyst concentration on the degradation rate for the TOC depletion and decomposition of pollutants under investigation are in agreement with numerous studies reported in the literature (Shifu andYunzhang 2007, Daneshvar et al., 2004). It is believed that both the number of photons absorbed as well as the solute molecules adsorbed increases with increase in number of TiO 2 particles upto the optimum value. Any further increase in TiO 2 concentration beyond optimum value may cause scattering and screening effects which reduces the specific activity of the catalyst (Evgenidou et al., 2007). The highly turbid suspension may prevent the catalyst farthest from being illuminated (Rahman and Muneer, 2005). Higher amount of catalyst may lead to aggregation of TiO 2 particles which may decrease the catalytic activity (Garcia and Takashima, 2003). The optimum value of catalyst has been found to vary with different initial solute concentrations (Sakthivel et al., 2003).

Effect of different electron acceptors
One practical problem in using TiO 2 as a photocatalyst is the undesired electron / hole recombination, which, in the absence of proper electron acceptor or donor, is extremely efficient and represent the major energy -wasting step thus limiting the achievable quantum yield. One strategy to inhibit electron -hole pair recombination is to add other (irreversible) electron acceptors to the reaction. They could have several different effects such as, i.e., (1) to increase the number of trapped electrons and, consequently, avoid recombination, (2) to generate more radicals and other oxidizing species, (3) to increase the oxidation rate of intermediate compounds and (4) to avoid problems caused by low oxygen concentration. In highly toxic wastewater where the degradation of organic pollutants is the major concern, the addition of electron acceptors to enhance the degradation rate may often be justified. With this view, the electron acceptor such as potassium persulphate, potassium bromate and hydrogen peroxide were added in the solution. As a representative example Figure 13 shows the degradation rate for the mineralization and decomposition of the pollutants in the presence of different electron acceptors.
The electron acceptors such as hydrogen peroxide, bromate and persulphate ions are known to generate hydroxyl radicals by the mechanisms shown in equations 10-14;  (Wardman, 1989). From the thermodynamic point of view all employed additives should therefore be more efficient electron acceptors than molecular oxygen.
(10) (11) (12) (13) (14) www.intechopen.com The effective electron acceptor ability of KBrO 3 has been observed in number of studies before (Nevim et al., 2001;Muneer and Bahnemann, 2001). The reason can be attributed to the number of electrons it reacts as shown in eq. 12. Another possible explanation might be a change in the reaction mechanism of the photocatalytic degradation. Since the reduction of bromate ions by electrons does not lead directly to the formation of hydroxyl radicals, but rather to the formation of other reactive radicals or oxidizing agents eg. BrO 2 and HOBr. Furthermore, bromate ions by themselves can act as oxidizing agents. Linder has proposed a mechanism for the photocatalytic degradation of 4-chlorophenol in the presence of bromate ions considering direct oxidation of the substrate by bromate ions (Linder, 1997). The enhanced effect of persulphate ion on the degradation of pollutants may be accounted on the basis that persulphate is a beneficial oxidizing agent in photocatalytic detoxification because SO 4 i s f o r m e d f r o m t h e o x i d a n t b y r e a c tion with the electron generated at conduction band (ecb ) of the semiconductor as shown in eq. 13. This sulphate radical anion (SO 4 ) is a strong oxidant (E 0 = 2.6 eV) and it can react with the organic pollutants in three possible modes (1) by abstracting a hydrogen atom from saturated carbon, (2) by adding to unsaturated or aromatic carbon and (3) by removing one electron from the carboxylate anions and from certain neutral molecules. In spite of this sulphate radical anion can trap the photogenerated electrons and/ or generated hydroxyl radical as shown in eq. 14. The formation of sulphate radical anion and hydroxyl radical are powerful oxidant, which can degrade the organic pollutants at a faster rate. The effect of H 2 O 2 has been investigated in numerous studies and it was observed that it increases the photodegradation rates of organic pollutants (Hallamn, 1992). The enhancement of the degradation rate on addition of H 2 O 2 can be rationalized in terms of several reason. Firstly, it increase the rate by removing the surface-trapped electrons, www.intechopen.com thereby lowering the electron-hole recombination rate and increasing the efficiency of hole utilization for reactions such as ( OH + h + OH). Secondly, H 2 O 2 may split photolytically to produce hydroxyl radicals ( OH) directly, as cited in studies of homogeneous photooxidation using UV / (H 2 O 2 + O 2 ) (Peyton and Glaze 1988). Thirdly, the solution phase may at times be oxygen starved, because of either oxygen consumption or slow oxygen mass transfer, and peroxide addition thereby increases the reaction rate. During the photocatalytic degradation the free radical formed serve a dual function. They are not only the strong oxidants but also at the same time their formation and subsequent rapid oxidation reactions inhibit the electron hole pair recombination.

Comparison of degradation rate under UV and sunlight
For practical applications of wastewater treatment based on these processes, the utilization of sunlight is preferred. Hence the aqueous suspension of TiO 2 containing organic pollutants was exposed to solar radiation. As a representative example Figure 14 shows the change in concentration as a function of irradiation time on illumination of an aqueous suspension of acetamiprid (0.1 mM) in the presence of TiO 2 (Degussa P25, 1gL -1 ) under sunlight and UV light source (Khan et al., 2010). It was found that the degradation of the model compound proceeds reasonably fast under sunlight as well.
Fig. 14. Change in concentration on irradiation of an aqueous suspension in the presence and absence of TiO 2 (Degussa P25, 1 gL -1 ) containing acetamiprid (0.1 mM), Light source: Pyrex filtered output of a medium pressure mercury lamp and sunlight.

Characterization of intermediate products
The identification and characterization of by-products formed during the photodegradation of organic pollutants has been of great interest among the people working in this area around the world. A brief summary showing the starting material and by-products under photolytic condition is shown below in Chart 2.

S. No.
Compound By-products Reference 1.

www.intechopen.com
We have also made an attempt to identify the intermediate products formed in the photocatalytic degradation of variety of pesticide derivatives in aqueous suspensions of titanium dioxide through GC-MS analysis technique. Results on the photocatalytic degradation of few selected pesticide derivatives for product analysis are shown below;

Photocatalysis of propham (1)
Irradiation of an aqueous solution of propham (1) in the presence of Degussa P25 TiO 2 and analysis of the irradiated mixture at different time intervals through GC-MS analysis showed the formation of several intermediate products out of which, two products appearing at retention times (t R ) 12.95 and 14.81 min., respectively were characterized as products 3 and 8, on the basis of molecular ion and fragmentation pattern . A probable mechanism for the formation of these products from propham (1) involving electron transfer reactions and reactions with hydroxyl radical and superoxide radical anions formed in photocatalytic system, is shown in Scheme 1. The model compound 1, upon the transfer of an electron can form the radical anion 2, which may undergo addition of a hydroxyl radical either at ortho or para position followed by loss of a proton to give the observed product 3. This compound on further transfer of an electron followed by addition of a hydroxyl group can undergo cleavage reaction to give 4. This intermediate on subsequent transfer of an electron can undergo addition of a hydroxyl group to give the observed product 8.

Photocatalysis of propachlor (9)
The GC-MS analysis of an irradiated mixture of propachlor (9) in the presence of TiO 2 showed the formation of several by-products. A plausible mechanism for the formation of the different products such as N-Isopropyl-N-phenyl-acetamide (11), 2-Hydroxy-Nisopropyl-N-phenyl acetamide (13), hydroxyl inserted product (15) and N-phenyl-acetamide (16) from propachlor (9) involving similar reactive species, is proposed in Scheme 2 . The model compound, 9 on transfer of an electron can form the radical species 10 on removal of chloride ion. This radical species can lead to the observed products 11 and 13, on abstraction of a hydrogen atom or hydroxyl radical. The product 11, on further transfer of an electron can form the radical species 14, which may lose isopropyl to give the observed product 16. The formation of 15 could be understood in terms of pathways shown in Scheme 2 through electron transfer to give radical anion species 12 followed by addition of hydroxyl radical.

Photocatalysis of chlorotoluron (17)
The steady state photolysis of an aqueous suspension of chlorotoluron (17) in the presence of TiO 2 under constant bubbling of atmospheric oxygen for 2 h showed the formation of several products, of which, three products such as 3-(3-Hydroxy-4-methylphenyl)-1,1dimethylurea (23) (3-chloro-4-methylphenyl) urea (24) and 3-chloro-4-methylphenylamine (25) appearing at retention times (t R ) 10.310 min, 10.255 min and 5.520 min., respectively were identified on the basis of molecular ion and fragmentation pattern with those reported in the GC/MS NIST library, along with some unchanged starting material (17) appearing at (t R ) 5.097 min. (Haque et al., 2006). A possible mechanism for the formation of products 23, 24 and 25 from chlorotoluron (17) involving electron transfer reaction and reaction with hydroxyl radicals could be understood in terms of pathways as shown in Scheme 3. The model compound 17, upon the transfer of an electron followed by the loss of methyl radical and abstraction of a proton to give species 21. This species on subsequent similar reaction can give the observed product 24. The observed product 25 from 17 may be arising through the transfer of an electron to give radical anionic species 19, which may undergo addition of a hydroxyl radical to give 22. This species may lose N(CH 3 ) 2 COOH followed by abstraction of a proton to give the observed product 25 as shown in the Scheme 3. The product 23 could be formed upon the transfer of an electron followed by the loss of chloride ion with subsequent addition of hydroxyl radical as shown in scheme below. Scheme 3.

Photocatalysis of phenoxyacetic acid (26)
The desired concentration of an aqueous solution of phenoxyacetic acid (26) (Singh et al., 2007a). These products were identified by comparing its molecular ion and mass fragmentation peaks with those reported in the NIST library. A plausible mechanism for the formation of these products involving reactions with hydroxyl radical and superoxide radical anion formed in the photocatalytic process is shown in Scheme 4. The model compound undergoes addition of a hydroxyl radical leading to the formation of a radical species 27, which may undergo loss of ( • OCH 2 COOH) to give the observed product phenol (28). Alternatively, the model compound 26 on addition of hydroxyl radical followed by loss of CO 2 may lead to the formation of the radical species 29. This species can either lose formaldehyde molecule followed by addition of hydroxyl radical led to the formation of phenol or can combine with another radical species leading to the formation of the 1,2-diphenoxyethane (30).

Photocatalysis of 2,4,5-trichlorophenoxy acetic acid (31)
Analysis of the irradiated aqueous mixture of 2,4,5-trichlorophenoxy acetic acid (31) in the presence of photocatalyst showed the formation of three intermediate products namely, 2,4,5-trichlorophenol (33), 2,4-dichlorophenol (37) and 1,2,4-trichloro-5-methoxy benzene (35) appearing at retention times 4.11, 6.3 and 12.86 min., respectively and were identified by comparing their molecular ion and mass fragmentation peaks with those reported in the NIST library (Singh et al., 2007a). The model compound 31 on addition of a hydroxyl radical can lead to the formation of a radical species 32, which may undergo loss of ( • OCH 2 COOH) group to give the observed product 2,4,5-trichlorophenol (33). Alternatively, the model compound 31 upon the transfer of an electron followed by loss of CO 2 may lead to the formation of the radical species 34. This species abstracts a hydrogen atom to form the observed product 1,2,4-trichloro-5-methoxy benzene (35). The product, 33 on further transfer of an electron may undergo loss of chlorine atom followed hydrogen atom abstraction giving 2,4-dichlorophenol (37) as shown in Scheme 5.

Photocatalysis of 4-chlorophenoxy acetic acid (38)
Irradiation of 4-chlorophenoxyacetic acid (4-CPA, 38) in the presence of titanium dioxide showed the formation of 4-chlorophenol (40), which was identified by comparing its molecular ion and fragment ion peak with those reported in the NIST library (Singh et al., 2007b). A plausible mechanism for the formation of product, 40 involving reactions with hydroxyl radicals formed in the photocatalytic process is shown in Scheme 6. The model compound, 4-CPA undergoes addition of a hydroxyl radical formed in the photocatalytic process leading to the formation of a radical species 39 as an intermediate, which may undergo loss of ( • OCH 2 COOH) group to give the observed product 4-chlorophenol (40). Alternatively, the model compound 4-CPA (38) on addition of hydroxyl radical followed by loss of CO 2 may lead to the formation of the intermediate radical species 41, which upon loss of formaldehyde molecule followed by addition of hydroxyl radical led to the formation of the observed product 4-chlorophenol (40).

Conclusions
The results of these studies clearly indicate that TiO 2 can efficiently catalyse the photodegradation and photomineralization of the pollutants in the presence of light and oxygen. In most of the cases the photocatalyst Degussa P25 was found to be more efficient as compare to other photocatalyst TiO 2 powders. But in few cases Hombikat UV100 has also found to be more efficient photocatalyst. The addition of electron acceptor enhanced the degradation rate of the pollutants. The results also indicate that degradation rates could be influenced not only by the different parameters such as type of photocatalyst, catalyst concentration, substrate concentration, pH and additives and their concentration but also by the model pollutants. The observations of these investigations clearly demonstrate the importance of choosing the optimum degradation parameters to obtain high degradation rate, which is essential for any practical application of photocatalytic oxidation processes. The intermediate products formed during the process are also a useful source of information for the degradation pathways. Ten years after coming into force of the Stockholm Convention on Persistent Organic Pollutants (POPs), a wide range of organic chemicals (industrial formulations, plant protection products, pharmaceuticals and personal care products, etc.) still poses the highest priority environmental hazard. The broadening of knowledge of organic pollutants (OPs) environmental fate and effects, as well as the decontamination techniques, is accompanied by an increase in significance of certain pollution sources (e.g. sewage sludge and dredged sediments application, textile industry), associated with a potential generation of new dangers for humans and natural ecosystems. The present book addresses these aspects, especially in the light of Organic Pollutants risk assessment as well as the practical application of novel analytical methods and techniques for removing OPs from the environment. Providing analytical and environmental update, this contribution can be particularly valuable for engineers and environmental scientists.