Photodegradation of Chlorophenoxyacetic Acids by ZnO / γ-Fe 2 O 3 Nanocatalysts : A Comparative Study

ZnO/γ-Fe2O3 nanocomposite was synthesized via simple precipitation. The synthesized nanocatalysts underwent heat treatment at 450 C for an hour. The characteristics of the nanocomposite were investigated by XRD, TEM, and BET surface area measurement. Zeta potential analysis was used to examine the surface charge properties of the nanocatalysts. The synthesized nanocomposite has an average particle size of 11 nm and a surface area of 20 m g. The potential of ZnO/γ-Fe2O3 as a photocatalyst was evaluated by photodegrading chlorophenoxyacetic acids (PAA, 2,4-D, 2,4,5-T and 4CA). The decomposition of chlorophenoxyacetic acids by ZnO/γ-Fe2O3 followed 4CA > 2,4,5-T ≈ 2,4-D > PAA. The result indicates the applicability of ZnO/γ-Fe2O3 nanocomposite as a photocatalyst in removing organic pollutants in wastewater.


Introduction
Rapid industrialisation has resulted in, among other things, problems with water pollution.The discharge of wastewater and chemical spillage into water channels has exacerbated the water quality crisis (Richardson, 2008;Wintgens, Salehi, Hochstrat, & Melin, 2008).Although many conventional methods have been applied in removing organic contaminants, those methods do not lead to total mineralisation of the pollutants.Moreover, further treatment is needed since the methods employed are unable to convert the pollutants into harmless products such as CO 2 and H 2 O, which indirectly increases the operational cost (Allegre, Maisseu, Charbit, & Moulin, 2004;Gaya & Abdullah, 2008;Lu, Zhang, Ma, & Chen, 2009;Padmanabhan et al., 2006;Yang & Cheng, 2007).Therefore, the development of an efficient wastewater treatment solution is imperative.
One of the major drawbacks in semiconductor photocatalysis is the recombination of photogenerated holes (h + ) and electrons (e -).This recombination step lowers the quantum yield and wastes energy.Therefore, the e --h + recombination process should be inhibited to ensure efficient photocatalysis.Coupled semiconductors have been proven to enhance the charge separation of electron-hole pairs, which increases the lifetime of the charge carriers and consequently reduces the recombination of electron-hole pairs (Serpone, Maruthamuthu, Pichat, Pelizzetti, & Hidaka, 1995).This is based on the fact that the photogenerated electrons can flow from one semiconductor with a higher to a lower conduction band minimum.Under such conditions, the separation of electron-hole pairs was promoted while keeping reduction and oxidation reactions at two different sites (Peng, Xie, Lu, Fan, & Wang, 2010;Shang et al., 2009;Wang et al., 2009).As a result, the efficiency of semiconductors improved (Jing, Xu, Sun, Jing, & Cai, 2001;Lin, Wu, Zhang, & Pan, 2009;Liu, Ye, Xiong, & Liu, 2010;Vaezi, 2008;Zheng et al., 2009).
The widespread use of chlorophenoxyacetic acids as herbicides in agriculture has raised public concern due to the accumulation of disposed residues in natural waters (Bucheli, Grüebler, Müller, & Schwarzenbach, 1997).
There have been many studies on the removal of chlorophenoxy herbicides using titanium dioxide (Bahnemann, Muneer, & Haque, 2007;Giri, Ozaki, Ishida, Takanami, & Taniguchi, 2007;Kamble, Sawant, & Pangarkar, 2006;Singh, Saquib, Haque, & Muneer, 2007;Tanaka & Reddy, 2002) Very little work involved ZnO (Djebbar & Sehili, 1998).Since ZnO has almost the same band gap energy (3.2 eV) as TiO 2 , its photocatalytic capability is anticipated to be similar to that of TiO 2 .Moreover, ZnO is relatively cheaper compared to TiO 2 whereby the usage of TiO 2 and platinum catalyst are uneconomic for large scale water treatment operations (Daneshvar, Salari, & Khataee, 2004).The greatest advantage of ZnO over TiO 2 is the ability to absorb a wide range of solar spectrum (Behnajady, Modirshahla, & Hamzavi, 2006).Based on these factors, we synthesized ZnO/γ-Fe 2 O 3 and evaluated its potential as photocatalyst.To our best knowledge, no works have been carried out by ZnO/γ-Fe 2 O 3 on the removal of the selected chlorophenoxyacetic acids.In this paper, ZnO/γ-Fe 2 O 3 nanocomposites were obtained via precipitation.The characteristics of the resulting catalysts and their efficiency in degrading chlorophenoxyacetic acids were studied.The ease of degradation of chlorophenoxy herbicides by ZnO/γ-Fe 2 O 3 was also studied.

Reagents and Apparatus
Zinc acetate dihydrate (Zn(CH 3 COO) 2 •2H 2 O), ammonia solution (25% v/v, NH 3 •H 2 O) and 4-chlorophenoxyacetic acid (C 8 H 7 ClO 3 ) were obtained from Merck.Iron acetate (Fe(CH 3 COO) 2 ) and phenoxyacetic acid (C 8 H 8 O 3 ) were purchased from Aldrich.2,4-dichlorophenoxyacetic acid (C 8 H 6 Cl 2 O 3 ) and 2,4,5-trichlorophenoxyacetic acid (C 8 H 5 Cl 3 O 3 ) were obtained from Fluka.All the chemicals were used without any purification.Deionized water was used throughout the study.The obtained products were characterized by XRD (Shimazdu XRD-6000 Diffractometer), TEM (Hitachi 7100) and BET surface area (Autosorb AS-1, Quantachrome).ImageJ, a free software package was used to measure the particle size of the catalyst based on TEM micrographs while SPSS software (IBM) was applied in constructing the particle size distribution histogram.The band gap of the synthesized catalysts was determined by UV-Vis diffuse reflectance spectrometry.The surface charge of ZnO/γ-Fe 2 O 3 was measured by a zeta potential analyzer (Zetasizer Nano series, Malvern Instruments).

Photodegradation Experiments
The photodegradation efficiency of the synthesized ZnO/γ-Fe 2 O 3 nanocomposite was evaluated by photodegrading chlorophenoxyacetic acids in a photoreactor under 6 W UV-A irradiation (365 nm, Hitachi).In a typical experiment, appropriate amounts of photocatalyst were suspended into 1 L solution of phenoxyacetic acid in a photoreactor.Where required, the pH of the solution was adjusted by the addition of HNO 3 or NaOH.The solution was stirred for 15 min at 280 rpm to attain adsorption equilibrium prior to irradiation.During irradiation, agitation was maintained by a magnetic stirrer and air was bubbled into the reaction medium to ensure a constant supply of oxygen (2 L min -1 ).Other factors such as temperature and light intensity were held constant.5 mL samples were withdrawn at regular time intervals for 4 hours and were immediately filtered by 0.45 μm cellulose nitrate filters to remove catalyst particles.The concentration of chlorophenoxyacetic acids in test samples was determined by a Perkin Elmer Lambda 35 UV-Vis spectrophotometer.This agrees with TEM analysis, where small particle sizes result in a large surface area.The synthesized ZnO/γ-Fe 2 O 3 catalyst has band gap energy of 3.11 eV (Figure 3).In order to study the surface properties of the ZnO/γ-Fe 2 O 3 nanocomposite, the electrokinetics of the composite were measured as zeta potentials as a function of pH.The pH of point of zero charge (pH zpc ) of the nanocatalyst was 7.3, indicating that the catalyst surface is positively charged below pH 7.3 and is negatively charged when the pH value is greater than 7.3 (Figure 4).

Preliminary Experiments
In order to validate the significant role of photocatalysis (UV/ZnO/γ-Fe 2 O 3 / chlorophenoxyacetic acids) in the removal process, photolysis (UV/chlorophenoxyacetic acids) and adsorption (ZnO/γ-Fe 2 O 3 / chlorophenoxyacetic acids) was performed as background experiments.The removal percentage of chlorophenoxyacetic acids was summarized in Table 1.It can be seen that there is no noticeable change in the concentration of chlorophenoxyacetic acids either in photolysis or adsorption test except for photocatalysis.Thus, there is a synergistic effect between the ZnO/γ-Fe 2 O 3 catalyst and UV irradiation for the photodegradation process to work effectively.Experimental conditions: Photocatalyst loading = 0.20 g L -1 , pollutant concentration = 20 mg L -1 .

Effect of Photocatalyst Loading
The effect of ZnO/γ-Fe 2 O 3 dosage on the removal of chlorophenoxyacetic acids was examined at an initial concentration of 20 mg L -1 .A series of experiments were performed while varying only the amount of photocatalyst from 0.20 to 0.60 g (Figure 5).The percentage of photodegradation initially increased with the amount of catalyst loaded and then decreased beyond optimum mass.This phenomenon is based on the fact that the increase in catalyst dosage also increases the total active surface area and the number of reaction sites (L.Wei, Shifu, Z. Wei, & Sujuan, 2009).As a result, the number of hydroxyl and superoxide radicals increases as well, which facilitates the degradation of the pollutant.Thus, the degradation percentage is enhanced.However, a reduction in photodegradation was observed when the photocatalyst dosage went beyond the optimum concentration (0.4 g L -1 for 2,4-D and 2,4,5-T and 0.5 g L -1 for PAA and 4CA).This may be due to the opacity of the solution, reducing UV light penetration, which consequently reduces degradation of the pollutant (Pardeshi & Patil, 2008).Furthermore, the low percentage of degradation may due to the agglomeration of ZnO/γ-Fe 2 O 3 which causes a reduction in catalyst surface area available for light and pollutant adsorption and suppresses the generation of hydroxyl radicals (Daneshvar et al., 2004).

Effect of Substrate Concentration
The influence of pollutant concentration on the degradation rate of the chlorophenoxyacetic acids was studied in the range of 10 to 50 mg L -1 at their respective optimum catalyst dosages.The rate of reaction is given by: where r 1 , C and k 1 are the rate of reaction, the concentration of chlorophenoxyacetic acids and the first-order rate constant, respectively.Integration of Equation 1gives: where C o is the initial concentration of the chlorophenoxyacetic acids and t is irradiation time.
The first-order rate constant (k 1 ) is determined from the slope of the linear plots of ln (C 0 /C) versus irradiation time, t.The first-order rate constant, k 1 and correlation factor, R 2 values are shown in Table 2.The high value of R 2 indicates the degradation of chlorophenoxyacetic acids follow first-order kinetics.The rate of reaction increased with increasing chlorophenoxyacetic acid concentration up to the optimum concentration and then decreased at higher concentrations (Figure 6).This may be due to the screening effect of the pollutant molecules, which reduced light penetration of the solution.Moreover, the generated intermediates may also compete with the pollutant molecules for the hydroxyl radicals.Thus, the photodegradation efficiency reduced as the substrate concentration increased (Khataee & Zarei, 2011).Additionally, the active sites are also covered by adsorbed pollutant molecules and intermediates that can reduce hydroxyl radical generation (Konstantinou & Albanis, 2004).Therefore, the concentration of hydroxyl radicals is insufficient to photodegrade the pollutants at higher concentrations.Another possibility is that the catalyst dosage, light intensity and irradiation period are constant as the initial concentrations of pollutants increases.Thus, the number of hydroxyl radicals formed on the catalyst surface is also constant.Hence, the hydroxyl radicals available to degrade substrate molecules would decrease at higher pollutant concentrations leading to a decrease in degradation.Another possible factor that could lead to the above phenomena is the competition between adsorbed pollutant molecules and water molecules for the photogenerated holes (Gaya & Abdullah, 2008).
Table 2. First-order rate constant, k 1 and R 2 values of various initial chlorophenoxyacetic acids concentrations The influence of initial pH levels on the degradation performance of chlorophenoxyacetic acids was investigated at pH 5 to 10 (Figure 7).The results indicated that the photodecomposition rates of the chlorophenoxyacetic acids were low at pH 5 due to a substantial loss of ZnO.A maximum degradation rate was observed at pH 7 and further increases in pH resulted in a lower removal efficiency.The pK a of 2,4-D, 2,4,5-T, PAA and 4CA is 2.64, 2.88, 3.12 and 3.56, respectively.Therefore, the chlorophenoxyacetic acids are negatively charged above their respective pK a values.As expected, the pH that shows optimal degradation must fall in between pK a < pH < pH zpc , due to the enhanced electrostatic interaction among the anions of the chlorophenoxyacetic acids and the positively charged catalyst surface (Daneshvar, Aber, Seyed-Dorraji, & Khataee, 2007).Above pH 7, the repulsive force between the negatively charged catalyst surface and the hydroxyl ions reduces the formation of hydroxyl radicals and consequently decreases the degradation rate (Qamar, Muneer, & Bahnemann, 2006).Is it notable that the hydroxyl radicals might act as a scavenging agent at high pH levels, which inhibits its reaction with the pollutant substrate (Davis & Huang, 1989).Hence, low removal percentage was observed at pH 10. Figure 8 shows the chemical structures of PAA, 2,4-D, 2,4,5-T and 4CA.Among the parameters studied, the percentage of photodegradation is in the following order: 4CA > 2,4,5-T ≈ 2,4-D > PAA.All chlorinated phenoxyacetic acids were degraded more than PAA.The corresponding byproducts of the reaction detected by UPLC were 4-chlorophenol, 2,4,5-trichlorophenol, 2,4-dichlorophenol and phenol, respectively.This indicates that the degradation process is driven by hydroxyl radicals.The attack of •OH on the CO position at the aromatic ring of PAA is not favorable, due to the stability of the benzene ring hence resulting in a low percentage of degradation.The present of chlorine, an ortho-and para-directing deactivators, will cause the •OH to attack the CO position of benzene ring, which is at the para position of the molecule in the case of 4CA.Conversely, 4CA was found to be the most degraded molecule among the studied chlorinated phenoxyacetic acids.Therefore the hydroxylation is the first elementary step which precedes the dissociation of chlorine atoms which is in agreement with Tang and Huang (1996).The increase in the number of chlorine atoms in the aromatic ring resulted in decreased degradation due to the steric effect.Additionally, the symmetrical and non existence of steric effect of 4CA may also contribute to the higher percentage of degradation (Kluson et al., 2008).

Conclusion
The highly photoactive ZnO/γ-Fe 2 O 3 catalyst was successfully synthesized via precipitation followed by calcination at 450 °C for an hour.The photocatalyst produced was nearly spherical, has a surface area of 20 m 2 g -1 , an average particle size of 11 nm and a point of zero charge at pH 7.3.The efficiency of the synthesized ZnO/γ-Fe 2 O 3 as a photocatalyst in photodegrading chlorophenoxy herbicides from aqueous solutions under UV irradiation increased with increasing mass of ZnO/γ-Fe 2 O 3 up to an optimum loading but decreased at higher initial concentrations of the herbicides.The highest degradation percentage of chlorophenoxyacetic acids was achieved at pH 7. The ease of degradation of chlorophenoxyacetic acids was in the following order: 4CA > 2,4,5-T ≈ 2,4-D > PAA.Therefore, ZnO/γ-Fe 2 O 3 nanocomposites have potential use in removing organic-polluted wastewater.
/γ-Fe 2 O 3 Nanocomposite 0.01 mole of Zn(CH 3 COO) 2 •2H 2 O and 0.001 mole of Fe(CH 3 COO) 2 were dissolved in 100 mL deionized water at room temperature.Ammonia solution was added drop-wise into the zinc-iron solution with vigorous stirring to ensure homogenous formation of precipitate.The precipitate was centrifuged, washed with deionized water and oven-dried overnight at 110 o C. The resulting powder was then calcined at 450 o C for an hour in a box furnace (Carbolite, CWF 1200).

Figure 5 .
Figure 5.Effect of ZnO/γ-Fe 2 O 3 dosage on the degradation percentage of chlorophenoxyacetic acids

Figure 6 .
Figure 6.First-order rate of chlorophenoxyacetic acids at different concentrations

Figure 7 .
Figure 7. Effect of initial solution pH on the degradation rate of chlorophenoxyacetic acids

Table 1 .
Removal of chlorophenoxyacetic acids under various experimental conditions