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International Journal of Chemical Engineering
Volume 2012 (2012), Article ID 850468, 8 pages
http://dx.doi.org/10.1155/2012/850468
Research Article

Titanium Dioxide-Mediated Photcatalysed Degradation of Two Herbicide Derivatives Chloridazon and Metribuzin in Aqueous Suspensions

1Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India
2Centre of Advanced Materials and Nano-Engineering (CAMNE), Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia

Received 9 October 2011; Revised 20 February 2012; Accepted 7 March 2012

Academic Editor: Licínio M. Gando-Ferreira

Copyright © 2012 A. Khan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The aim of this paper is to find out the optimal degradation condition for two potential environmental pollutants, chloridazon and metribuzin (herbicide derivatives), employing advanced oxidation process using TiO2 photocatalyst in aqueous suspensions. The degradation/mineralization of the herbicide was monitored by measuring the change in pollutant concentration and depletion in TOC content as a function of time. A detailed degradation kinetics was studied under different conditions such as types of TiO2 (anatase/anatase-rutile mixture), catalyst concentration, herbicide concentration, initial reaction pH, and in the presence of electron acceptors (hydrogen peroxide, ammonium persulphate, potassium persulphate) in addition to atmospheric oxygen. The photocatalyst, Degussa P25, was found to be more efficient catalyst for the degradation of both herbicides as compared with two other commercially available TiO2 powders like Hombikat UV100 and PC500. Chloridazon (CHL) was found to degrade more efficiently under acidic condition, whereas metribuzin (MET) degraded faster under alkaline medium. All three electron acceptors tested in this study were found to enhance the degradation rate of both herbicides.

1. Introduction

Clean and safe drinking water is vital for human health, wildlife, and also for a stable environment. Yet, water is being polluted at alarming rates, with chemicals, nutrients, metals, pesticides, and other contaminants from industrial effluents, chemical spills, and agricultural runoffs [1, 2]. A wide variety of herbicides and other chemicals are applied to agricultural field and lawns mainly to control undesirable vegetation. A fraction of herbicides applied to these sites end up as runoff. This runoff goes into the streams, rivers, and lakes. Some of the herbicides also end up in groundwater systems by percolating down through the soil. As a result, herbicides are widely found in rivers, streams, lakes, and even in drinking water [3]. These chemicals due to their toxicity, stability to natural decomposition, and persistence in the environment have been the cause of much concern to the societies and regulatory authorities around the world [4]. The development of appropriate methods to treat these contaminated water is necessary before it is used for any useful purpose.

The photocatalysed degradation of various organic systems employing irradiated TiO2 is well documented in the literature [514]. Briefly, when a semiconductor such as TiO2 absorbs a photon of energy equal to or greater than its band gap energy, an electron may be promoted from the valence band to the conduction band () leaving behind an electron vacancy or “hole” in the valence band (), as shown in (1). If charge separation is maintained, and the reaction is carried out in water and oxygen, the electron and hole may migrate to the catalyst surface where they participate in redox reactions with sorbed species. Specially, may react with surface-bound H2O to produce the hydroxyl radical and is picked up by oxygen to generate superoxide radical anion (), as indicated in the following equations (1)–(3)

It has been suggested that the hydroxyl radicals and superoxide radical anions are the primary oxidizing species in the photocatalytic oxidation processes. These oxidative reactions would result in the degradation of the pollutants.

CHL is a herbicide currently used for selective control in beets. It belongs to the class of Hill reaction inhibitors, whereas MET is used primarily to discourage growth of broadleaf weeds and annual grasses among vegetable crops and turf grass [15, 16]. CHL and MET possess high mobility in soil and thus have great potential to leach and pollute surface and groundwater [17]. Few studies relating to the degradation of chloridazon and metribuzin in UV and sunlight have been reported recently in the literature [1820]. No major efforts have been made to study the detailed degradation kinetics of these two herbicide derivatives. Therefore, in this paper we present a detailed degradation of these compounds as shown in Scheme 1 in aqueous suspensions of TiO2 under a variety of conditions such as different types of TiO2, reaction pH, catalyst and substrate concentration, and also in the presence of electron acceptors like hydrogen peroxide, potassium persulphate, and ammonium persulfate besides atmospheric oxygen.

850468.sch.001
Scheme 1

2. Experimental

2.1. Reagents and Chemicals

Laboratory grade chloridazon was kindly supplied by Parijat Industries India Pvt. Ltd., Ambala whereas, the pesticide derivative metribuzin was obtained from Rallis India Ltd., and they were used as such for our studies without any further purification. All the solutions were made in double-distilled water for the irradiation experiments. Degussa P25 was used as catalyst in most of the experiments performed to carry out the photocatalytic degradation of these two model compounds. Other catalyst powders, namely, Hombikat UV100 (Sachtleben Chemie GmbH) and PC500 (Millennium Inorganic Chemicals), were used for comparative study. Degussa P25 contains 80% anatase and 20% rutile with a specific BET surface area of 50 m2g−1 and a primary particle size of 20 nm [21]. Hombikat UV100 consists of 100% pure anatase with a specific BET surface area of 250 m2g−1 and a primary particle size of 5 nm [22]. The photocatalyst PC500 has a BET-surface area of 287 m2g−1 with 100% anatase and primary particle size of 5–10 nm [23]. All other chemicals such as sodium hydroxide, nitric acid, hydrogen peroxide, potassium persulphate, and potassium bromate were obtained from Merck.

2.2. Procedure

An immersion well photochemical reactor consisting of inner and outer jacket made of Pyrex glass was used for the irradiation experiments. The detailed design of the photoreactor has been illustrated elsewhere [24]. Prior to illumination, stock solutions of CHL and MET were prepared in double-distilled water with desired concentration. The irradiation experiments were carried out in aqueous suspensions of TiO2 using a pyrex-filtered output of 125 W medium pressure mercury lamp (Philips). The light intensity was measured by UV light intensity detector (Lutron UV-340) and was found to be in the range of 4.86–4.88 mW/cm2. The experimental runs were carried out by using the following procedure for both the herbicide derivatives: firstly 125 mL solution of the herbicide derivative was taken into the reactor and required amount of photocatalyst was added. The suspension was magnetically stirred and purged with atmospheric oxygen in the dark for 10 min to attain adsorption-desorption equilibrium between herbicide derivatives and TiO2. The zero time reading was obtained from blank solution kept in the dark. Samples (5 mL) were taken at regular time intervals from the reactor and analysed after centrifugation. The pH of the reaction mixture was adjusted by adding dilute NaOH and HNO3.

2.3. Analysis

The analysis was carried out after removal of photocatalyst by centrifuging the samples at 4000 rpm for 1 h using Remi centrifuge (model R24). The degradation of CHL and MET was followed by measuring the change in absorption intensity at their respective   238 nm (CHL) and 295 nm (MET) using Shimadzu UV-Vis Spectrophotometer (Model 1601), and mineralization was monitored by measuring the depletion in Total Organic Carbon (TOC) content with a Shimadzu TOCVCSH Analyzer. The pH of the reaction mixture was measured using HI 2210 pH meter.

3. Results and Discussion

3.1. Photolysis of CHL and MET in Aqueous Suspensions of TiO2

Irradiation of an aqueous suspensions of CHL (0.18 mM, 125 mL, pH 6.2) and MET (0.30 mM, 125 mL, pH 6.5) in the presence of TiO2 (Degussa P25, 1 g L−1) using 125 W medium pressure mercury lamp in an immersion well photochemical reactor with constant stirring and bubbling of air led to a decrease in absorption intensity as a function of time as shown in the inset of Figures 1(a) and 1(b), respectively. The change in the concentration of the pollutant was calculated from standard calibration curve obtained from the absorption intensity of the herbicide derivatives at different concentration. The change in concentration in the presence and absence of photocatalyst as a function of time is shown in Figures 1(a) and 1(b), for herbicide derivatives, CHL and MET, respectively. The results demonstrate that 62% degradation of CHL and 55% degradation of MET could be achieved in 60 min and 28 min, respectively, whereas in the absence of photocatalyst no significant degradation was observed as shown in the Figure 1. Control experiments were carried out to show that there was no appreciable loss of the compound in unirradiated blank solutions and also due to adsorption on the surface of the photocatalyst. In order to see the effect of change in initial volume, experiments were carried out (data not shown), where samples were withdrawn initially at zero min and then directly after 60 min (CHL) and 28 min, (MET). There was no significant effect of volume alteration on the reaction rate.

fig1
Figure 1: (a) Change in concentration as a function of time on irradiation of an aqueous solution of CHL in the presence and absence of photocatalyst. Inset: Change in absorption intensity at 238 nm on irradiation of aqueous suspension of CHL containing TiO2. Experimental conditions: CHL (0.18 mM),  mL, 125 W medium pressure mercury lamp, light intensity: 4.88 mW/cm2, irradiation time: 60 min. (b) Change in concentration as a function of time on irradiation of an aqueous suspension of MET in the presence and absence of photocatalyst. Inset: Change in absorption intensity at 295 nm on irradiation of aqueous suspension of MET containing TiO2. Experimental conditions: MET (0.30 mM),  mL, 125 W medium pressure mercury lamp, light intensity: 4.88 mW/cm2, Irradiation time: 28 min.

For each experiment, the degradation rate constant of the compound was calculated from the linear regression of a plot of the natural logarithm of the compound concentration as a function of irradiation time. The degradation rate of the herbicide derivatives was calculated using the formula given below equals rate constant (min−1), equals concentration (mol L−1) of the pollutant, is order of reaction.

The degradation rate for both the herbicides was found to follow pseudo-first-order reaction kinetics, and the degradation rate was calculated in terms of mmol L−1 min−1.

3.2. Comparison of Photocatalytic Activity of Different TiO2 Powders

Titanium dioxide is the most widely used photocatalyst in heterogeneous photocatalysis due to its low cost, high photocatalytic activity, nontoxic nature, photostability, and chemical and biological inertness [14]. The photocatalytic activity of three commercially available TiO2 powders, namely, Degussa P25, Hombical UV100, and PC500, was tested on the degradation kinetics of herbicide derivatives CHL and MET. The rates obtained for the degradation of CHL and MET in the presence of different types of TiO2 powders by continuous purging of air are shown in Table 1. It has been observed that the degradation of CHL and MET proceeds much more rapidly in the presence of Degussa P25 as compared to other TiO2 powders. The photocatalysts UV100 and PC500 showed a comparable photocatalytic activity for the degradation of MET.

tab1
Table 1: (a) Comparison of degradation rate of CHL and MET in the presence of different types of TiO2 samples. Experimental conditions: CHL (0.18 mM), MET (0.30 mM),  mL, photocatalysts: Degussa P25 (1 gL−1), Sachtleben Hombikat UV100 (1 gL−1), PC500 (1 g−1), irradiation time: 60 min (CHL), 28 min (MET). (b) Comparison of mineralization rate of CHL and MET in the presence of TiO2 and TiO2/H2O2. Experimental conditions: CHL (0.55 mM), MET (0.11 mM),  mL, photocatalysts: Degussa P25 (1 gL−1), irradiation time: 60 min (CHL), 28 min (MET).

The photocatalyst Degussa P25 has been found to be a better photocatalyst for the degradation of a large number of compounds reported earlier [2527]. The reason for the better photocatalytic activity of Degussa P25 is attributed to its anatse/rutile mixture as reported earlier [28, 29]. In all following experiments, Degussa P25 was used since this material exhibited the highest photocatalytic activity as compared to other photocatalyst tested for the degradation of herbicides under investigation.

3.3. Effect of pH

The pH of the reaction mixture in surface reactions significantly influences the physicochemical properties of titanium dioxide, including the charge on its surface, the aggregation numbers of particles it forms, and the position of the conductance and valence bands [30]. The effect of pH on the degradation of CHL and MET employing Degussa P25 was studied in the pH range between 3 and 12. The rate obtained for the degradation of herbicide derivatives CHL and MET as a function of reaction pH is shown in Figure 2. It is interesting to note that in the case of compound CHL, the highest degradation rate was observed at pH 3.2, which slowly decreases with the increase in reaction pH, whereas in the case of MET, the degradation rate was found to increase with the increase in reaction pH.

850468.fig.002
Figure 2: Influence of pH on the degradation rate of CHL and MET. Experimental conditions: CHL (0.18 mM), MET (0.30 mM), initial reaction pH of CHL (3.2, 6.2, 9.2, and 12), initial reaction pH of MET (3.5, 6.5, 9.5, and 12),  mL, photocatalyst: TiO2 Degussa P25 (1 gL−1), irradiation time: 60 min (CHL), 28 min (MET).

The zero point of charge (pHzpc) of Degussa P25 has been reported as 6.25 [31]; hence, at more acidic pH values, the TiO2 particle surface is positively charged; while at pH values above pHzpc, it is negatively charged. The pKa for CHL and MET has been reported as 2.96 and 7.0, respectively [32, 33]. The higher degradation rate for CHL at lower pH values and MET at higher pH values could be explained on the basis of the fact that the structural orientation of the compound at these pH values are favoured for the reactive species.

3.4. Effect of TiO2 Concentration

The optimal amount of TiO2 has to be found out in order to avoid unnecessary excess catalyst and also to ensure total absorption of light for efficient photodegradation. In order to find out the optimal catalyst concentration, we have investigated the influence of different concentrations of Degussa P25 (0.5 to 4 g L−1) on the degradation kinetics of CHL and MET, and the results are shown in Figure 3. As expected, the rate of degradation for the model compound CHL was found to increase with the increase in catalyst concentration up to 2 gL−1 after which a further increase in catalyst concentration led to a decrease in degradation rate. The degradation rate for MET was found to increase linearly up to 3 gL−1 followed by a decrease in degradation rate on further increase in catalyst concentration.

850468.fig.003
Figure 3: Influence of Degussa P25 concentration on the degradation rate of CHL and MET. Experimental conditions: CHL (0.18 mM), MET (0.30 mM), TiO2 Degussa P25 (0.5, 1, 2, and 3 and 4 gL−1),  mL, irradiation time: 60 min (CHL), 28 min (MET).

The decrease in degradation rate at a higher catalyst concentration may be due to the fact that when the catalyst concentration is very high, after traversing a certain optical path, turbidity impedes further penetration of light in the reactor (incidence of the combined phenomena of particle masking and scattering), lowering the efficiency of the catalytic process. The results are in agreement with the studies reported earlier by our group [34].

3.5. Effect of the Initial Herbicide Concentrations

The study of the dependence of the photocatalytic degradation rate on the substrate concentration is very important for the application of the photocatalytic process to waste-water treatment. Hence the effect of initial herbicide concentration, varying from 0.14 to 0.20 for CHL and 0.24−0.33 for MET on the photocatalytic degradation was studied, and the results, are shown in Figure 4.

850468.fig.004
Figure 4: Effect of initial herbicide concentration on the degradation rate. Experimental conditions: Herbicide concentration: CHL (0.14, 0.16, 0.18, and 0.20 mM), MET (0.24, 0.27, 0.30, and 0.33 mM),  mL, photocatalyst: TiO2 Degussa P25 (1 gL−1), irradiation time: 60 min (CHL), 28 min (MET).

As expected, the rate of degradation of CHL was found to increase gradually with the increase in substrate concentration from 0.14 to 0.18 mM. Further increase in substrate concentration led to a slight decrease in the rate of the reaction. Similar results were obtained for the degradation of MET, that is, the rate was found to increase with the increase in substrate concentration from 0.24 to 0.30 mM, and a further increase in the concentration led to more or less same degradation rate within experimental error limits. The results are in agreement with earlier reported studies [35].

3.6. Effect of Electron Acceptors

Charge recombination of the photogenerated electron hole pairs is the major bottleneck in semiconductor photocatalysis. Due to the process of electron-hole recombination, both charge carriers annihilate each other. As a result, the rates of the photocatalytic transformations are limited by the rates of electron-hole recombination in the bulk of TiO2 or at the surface. In order to enhance the formation of hydroxyl radicals and also inhibit undesired electron/hole pair recombination, electron acceptors such as hydrogen peroxide (10 mM), ammonium per sulphate (3 mM), and potassium per sulphate (3 mM) were used in addition to Degussa P25 in order to see their effect on the degradation kinetics of the herbicide derivative CHL and MET. The rate obtained for the decomposition of herbicide derivatives in the presence and absence of additives is shown in Figure 5.

850468.fig.005
Figure 5: Influence of electron acceptors on the the degradation rate of CHL and MET. Experimental conditions: CHL (0.18 mM), MET (0.30 mM), electron acceptors, K2S8O8 (3 mM), (NH4)2S2O8 and H2O2 (10 mM),  mL, photocatalyst: TiO2 Degussa P25 (1 gL−1), irradiation time: 60 min (CHL), 28 min (MET).

All employed additives were found to enhance the degradation of both compounds, CHL and MET. The highest degradation rate for the decomposition of MET was observed in the presence of hydrogen peroxide as an electron acceptor in the presence of Degussa P25. In addition, the effect of different concentrations of hydrogen peroxide on the degradation kinetics of both herbicides have also been studied, and the rates are shown in the inset of Figure 5. In both compounds, the rates were found to increase as the H2O2 concentration increases from 5 to 15 mM (markedly in the case of MET). A further increase in the H2O2 concentration leads to a slight decrease in the degradation rate of MET and more or less same in the case of CHL.

The rate enhancement by H2O2 addition could be attributed to its better electron acceptance (5), thereby reducing electron hole recombination. Above the optimum value, the competitive reactions between hydroxyl radical and peroxide lead to the generation of less reactive hydroperoxide radicals, which does not contribute to the oxidative degradation of CHL  (6)

3.7. Photomineralization of CHL and MET in Aqueous Suspension of TiO2

The photocatalytic mineralization of CHL and MET was studied in aqueous suspensions of TiO2 in the presence and absence H2O2. Table 1 shows the mineralization rate of CHL and MET in aqueous suspension of Degussa P25 in the presence and absence of H2O2. Blanks experiments were carried out by irradiating the pesticide solutions in the absence of TiO2 containing H2O2 where no appreciable loss of the compounds was observed (data not shown). It is obvious from the table that the mineralization rate was enhanced in the presence of TiO2 containing hydrogen peroxide due to the efficient generation of hydroxyl radicals with the addition of H2O2.

4. Conclusion

TiO2 can efficiently catalyze the degradation and mineralization of herbicide derivatives chloridazon, and metribuzin in the presence of UV light. All parameters have been found to markedly influence the overall efficiency of degradation. Degussa P25 showed greater photocatalytic activity for the degradation of both herbicides, CHL and MET. In the case of chloridazon the highest efficiency was observed at pH 3.2, whereas metribuzin showed better degradation under alkaline condition. All the electron acceptors markedly enhanced the degradation of MET, and the highest rate was achieved with H2O2, whereas in the case CHL all acceptors enhanced the degradation at comparable rate. The observations of these investigations clearly demonstrate the importance of choosing the optimum degradation conditions to obtain high degradation and mineralisation, which is essential for any practical application of photocatalytic oxidation processes.

Acknowledgments

Financial support for the research projects from CSTUP, Lucknow, UGC, New Delhi, India, and DRS-1 (SAP) to the Department of Chemistry, AMU Aligarh, is gratefully acknowledged. Total Organic Carbon analyzer (TOC) used for the analysis of the samples was a gift instrument from the Alexander von Humboldt foundation, Bonn, Germany.

References

  1. G. D. Agrawal, “Diffuse agricultural water pollution in India,” Water Science and Technology, vol. 39, pp. 33–47, 1999.
  2. S. Mukherjee and P. Nelliyat, Groundwater Pollution and Emerging Environmental Challenges of Industrial Effluent Irrigation in Mettupalayam Taluk, Tamil Nadu International Water Management Institute, Colombo, Sri Lanka, 2007.
  3. D. Muszkat, M. Raucher, M. Mogaritz, and D. Ronen, “Groundwater contamination by organic pollutants,” in Groundwater Contamination and Control, M. Dekker and U. Zoller, Eds., pp. 257–272, Amer Society of Civil Engineer, New York, NY, USA, 1994.
  4. J. A. Graham, “Monitoring groundwater and well water for crop protection chemicals,” Analytical Chemistry, vol. 63, no. 11, pp. 631–622, 1991. View at Scopus
  5. C. Shifu and L. Yunzhang, “Study on the photocatalytic degradation of glyphosate by TiO2 photocatalyst,” Chemosphere, vol. 67, no. 5, pp. 1010–1017, 2007. View at Publisher · View at Google Scholar · View at Scopus
  6. S. Han, J. Li, H. Xi, D. Xu, Y. Zuo, and J. Zhang, “Photocatalytic decomposition of acephate in irradiated TiO2 suspensions,” Journal of Hazardous Materials, vol. 163, no. 2-3, pp. 1165–1172, 2009. View at Publisher · View at Google Scholar · View at Scopus
  7. M. M. Haque and M. Muneer, “Photodegradation of norfloxacin in aqueous suspensions of titanium dioxide,” Journal of Hazardous Materials, vol. 145, no. 1-2, pp. 51–57, 2007. View at Publisher · View at Google Scholar · View at Scopus
  8. M. N. Abellán, B. Bayarri, J. Giménez, and J. Costa, “Photocatalytic degradation of sulfamethoxazole in aqueous suspension of TiO2,” Applied Catalysis B, vol. 74, no. 3-4, pp. 233–241, 2007. View at Publisher · View at Google Scholar · View at Scopus
  9. H. K. Singh, M. Saquib, M. M. Haque, and M. Muneer, “Heterogeneous photocatalysed decolorization of two selected dye derivatives neutral red and toluidine blue in aqueous suspensions,” Chemical Engineering Journal, vol. 136, no. 2-3, pp. 77–81, 2008. View at Publisher · View at Google Scholar · View at Scopus
  10. J. C. Garcia, J. I. Simionato, A. E. C. da Silva, J. Nozaki, and N. E. D. Souza, “Solar photocatalytic degradation of real textile effluents by associated titanium dioxide and hydrogen peroxide,” Solar Energy, vol. 83, no. 3, pp. 316–322, 2009. View at Publisher · View at Google Scholar · View at Scopus
  11. W. Bahnemann, M. Muneer, and M. M. Haque, “Titanium dioxide-mediated photocatalysed degradation of few selected organic pollutants in aqueous suspensions,” Catalysis Today, vol. 124, no. 3-4, pp. 133–148, 2007. View at Publisher · View at Google Scholar · View at Scopus
  12. E. Evgenidou, E. Bizani, C. Christophoridis, and K. Fytianos, “Heterogeneous photocatalytic degradation of prometryn in aqueous solutions under UV-Vis irradiation,” Chemosphere, vol. 68, no. 10, pp. 1877–1882, 2007. View at Publisher · View at Google Scholar · View at Scopus
  13. E. Kusvuran, A. Samil, O. M. Atanur, and O. Erbatur, “Photocatalytic degradation kinetics of di- and tri-substituted phenolic compounds in aqueous solution by TiO2/UV,” Applied Catalysis B, vol. 58, no. 3-4, pp. 211–216, 2005. View at Publisher · View at Google Scholar · View at Scopus
  14. C. C. Chen, C. S. Lu, Y. C. Chung, and J. L. Jan, “UV light induced photodegradation of malachite green on TiO2 nanoparticles,” Journal of Hazardous Materials, vol. 141, no. 3, pp. 520–528, 2007. View at Publisher · View at Google Scholar · View at Scopus
  15. EPA, “Health effects support document for metribuzin U.S. Environmental Protection Agency Office of Water (4304T),” Tech. Rep. 822-R-03-004 , Health and Ecological Criteria Division, Washington, DC, USA, February 2003.
  16. R. Cremlyn, “Pesticides—preparation and mode of action,” in Herbicides, R. Cremlyn, Ed., pp. 140–172, John Wiley and Sons, Chichester, UK, 1978.
  17. M. F. Pérez, M. V. Sánchez, F. F. Céspedes, S. P. García, and I. D. Fernández, “Prevention of chloridazon and metribuzin pollution using lignin-based formulations,” Environmental Pollution, vol. 158, no. 5, pp. 1412–1419, 2010. View at Publisher · View at Google Scholar · View at Scopus
  18. D. M. Fouad and M. B. Mohamed, “Photodegradation of chloridazon using Coreshell Magnetic Nanocompsites,” Journal of Nanotechnology, vol. 2011, 7 pages, 2011.
  19. E. M. Scherer, Q. Q. Wang, A. G. Hay, and A. T. Lemley, “The binary treatment of aqueous metribuzin using anodic fenton treatment and biodegradation,” Archives of Environmental Contamination and Toxicology, vol. 47, pp. 154–161, 2004.
  20. L. Muszkat, L. Feigelson, L. Bir, and K. A. Muszkat, “Photocatalytic degradation of pesticides and bio-molecules in water,” Pest Management Science, vol. 58, no. 11, pp. 1143–1148, 2002. View at Publisher · View at Google Scholar · View at Scopus
  21. R. I. Bickley, T. G. Carreno, J. S. Lees, L. Palmisano, and R. J. D. Tilley, “A structural investigation of titanium dioxide photocatalysts,” Journal of Solid State Chemistry, vol. 92, no. 1, pp. 178–190, 1991. View at Scopus
  22. M. Lindner, D. W. Bahnemann, B. Hirthe, and W. D. Griebler, “Solar water detoxification: Novel TiO2 powders as highly active photocatalysts,” Journal of Solar Energy Engineering, Transactions of the ASME, vol. 119, no. 2, pp. 120–125, 1997. View at Scopus
  23. S. Rauer, Untersunchung von kommerziell erhaltlichen Titandioxiden hinsichtlich ihrer photokatalytischen Aktivtat, Diplomarbeit, Fachhochschule Hannover, Fachbereich Maschinenbau Vertiefung Umwelt-und Verfahrenstechnil, Ph.D. thesis, University of Hannover, Hannover, Germany, 1998.
  24. M. Qamar and M. Muneer, “Comparative photocatalytic study of two selected pesticide derivatives, indole-3-acetic acid and indole-3-butyric acid in aqueous suspensions of titanium dioxide,” Journal of Hazardous Materials, vol. 120, no. 1-3, pp. 219–227, 2005. View at Publisher · View at Google Scholar · View at Scopus
  25. H. K. Singh, M. Saquib, M. M. Haque, and M. Muneer, “Heterogeneous photocatalysed degradation of 4-chlorophenoxyacetic acid in aqueous suspensions,” Journal of Hazardous Materials, vol. 142, no. 1-2, pp. 374–380, 2007. View at Publisher · View at Google Scholar · View at Scopus
  26. M. Muneer, J. Theurich, and D. Bahnemann, “Titanium dioxide mediated photocatalytic degradation of 1,2-diethyl phthalate,” Journal of Photochemistry and Photobiology A, vol. 143, no. 2-3, pp. 213–219, 2001. View at Scopus
  27. M. M. Haque, M. Muneer, and D. W. Bahnemann, “Semiconductor mediated Photocatalysed degradation of a herbicide derivative chlorotoluron in aqueous suspensions,” Environmental Science & Technology, vol. 40, pp. 4765–4770, 2006.
  28. D. C. Hurum, A. G. Agrios, K. A. Gray, T. Rajh, and M. C. Thurnauer, “Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO2 using EPR,” Journal of Physical Chemistry B, vol. 107, no. 19, pp. 4545–4549, 2003. View at Scopus
  29. D. C. Hurum, K. A. Gray, T. Rajh, and M. C. Thurnauer, “Recombination pathways in the degussa P25 formulation of TiO2: Surface versus lattice mechanisms,” Journal of Physical Chemistry B, vol. 109, no. 2, pp. 977–980, 2005. View at Publisher · View at Google Scholar · View at Scopus
  30. C. Kormann, D. W. Bahnemann, and M. R. Hoffmann, “Photolysis of chloroform and other organic molecules in aqueous TiO2 suspensions,” Environmental Science and Technology, vol. 25, no. 3, pp. 494–500, 1991. View at Scopus
  31. J. Augustynski, Structural Bonding, chapter 1, Springer, Berlin, Germany, 1988.
  32. M. Zimpl, M. Kotouček, K. Lemr, J. Veselá, and J. Skopalová, “Electrochemical reduction of chloridazon at mercury electrodes, and its analytical application,” Analytical and Bioanalytical Chemistry, vol. 371, no. 7, pp. 975–982, 2001. View at Publisher · View at Google Scholar · View at Scopus
  33. D. C. Peek and A. P. Appleby, “Effect of pH on Phytotoxicity of Metribuzin and Ethyl-metribuzin,” Weed Technology, vol. 3, pp. 636–639, 1989.
  34. A. Khan, M. M. Haque, N. A. Mir, M. Muneer, and C. Boxall, “Heterogeneous photocatalysed degradation of an insecticide derivative acetamiprid in aqueous suspensions of semiconductor,” Desalination, vol. 261, no. 1-2, pp. 169–174, 2010. View at Publisher · View at Google Scholar · View at Scopus
  35. W. Bahnemann, M. Muneer, and M. M. Haque, “Titanium dioxide-mediated photocatalysed degradation of few selected organic pollutants in aqueous suspensions,” Catalysis Today, vol. 124, no. 3-4, pp. 133–148, 2007. View at Publisher · View at Google Scholar · View at Scopus