Photocatalytic degradation of malathion in aqueous solution using an Au–Pd–TiO2 nanotube film

Journal of Hazardous Materials 184 (2010) 753–758

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Photocatalytic degradation of malathion in aqueous solution using an Au–Pd–TiO2 nanotube film Hongbin Yu, Xinhong Wang, Hongwei Sun, Mingxin Huo ∗ School of Urban and Environmental Sciences, Northeast Normal University, Changchun 130024, China

a r t i c l e

i n f o

Article history: Received 13 May 2010 Received in revised form 21 August 2010 Accepted 25 August 2010 Available online 19 September 2010 Keywords: Photocatalytic degradation Organophosphorus pesticide Malathion Au–Pd–TiO2

a b s t r a c t The extensive use of pesticides has promoted the agricultural production, but a series of subsequent environmental issues have drawn the concern of governments and people worldwide, such as groundwater and surface water pollutions. In order to remove these pollutants, photocatalysis has emerged as a powerful method. In this paper, the photocatalytic degradation of an organophosphorus pesticide malathion was investigated using an Au–Pd co-modified TiO2 nanotube film (Au–Pd–TiO2 ). This film was fabricated by simultaneously photo-depositing Au and Pd precursors on a self-organized TiO2 nanotube film. Its morphology and structures were well characterized by a scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The photocatalytic experiments revealed that the malathion elimination rate increased by 172% when the photocatalyst of the naked TiO2 nanotube film was replaced by Au–Pd–TiO2 . Additionally, the amount of H2 O2 yielded on the Au–Pd–TiO2 film in 60 min was 2.89 times that on the naked TiO2 . The enhanced photocatalytic performance could be attributed to both the effective separation of photo-generated charge carriers and the higher synthesis rate of H2 O2 . The possible photocatalytic mechanism was discussed. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Malathion, an organophosphorus pesticide with a broad range of target pests, has been widely used in agriculture. It was highly toxic toward aquatic organism [1], amphibian [2], vertebrate, and even human beings [3]. As far as fish tissues were concerned, a concentration-dependent increase in DNA damage with the duration of exposure demonstrated malathion was genotoxic [4]. Additionally, the experiments of embryo development and in vitro fertilization [5] indicated that malathion was an endocrine disruptor and could interfere with reproductive functions. However, due to its chemical stability and high toxicity, malathion resisted to biodegrade [6]. Therefore, it was important to explore a pragmatic approach to eliminate these pesticide pollutants. Heterogeneous photocatalysis was experimentally proved to be a promising method to degrade a large variety of pesticides in water [7,8]. In the field of photocatalysis, TiO2 has caught tremendous attention in both industrial and environmental applications because of its easy availability, nontoxicity, inexpensiveness and chemical stability. However, the poor quantum yield caused by the rapid recombination of photo-generated electrons and holes has become the bottleneck of its widespread use in practical applications.

∗ Corresponding author. Tel.: +86 431 85099550. E-mail address: [email protected] (M. Huo). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.08.103

On the surface of photocatalysts, to modify with noble metals, such as Au [9,10], Pd [11,12], Ag [13,14], and Pt [15], was a feasible approach to enhance their photocatalytic efficiency. These small metal particles could accelerate the photoelectrons to be scavenged to generate superoxide radical anion O2 −• [16], and thus the separation of photo-generated charge carriers was improved. Metallic gold, an excellent electronic conductor, could facilitate the rapid transfer of photoelectrons on TiO2 . Consequently, the recombination between photoelectrons and holes was decreased and a high quantum yield could be obtained [9,10,17]. In addition, the platinum group metal Pd has attracted great attention [11,12,18,19]. Pd deposited on TiO2 could not only enhance the photocatalytic performance and promote O2 to scavenge photogenerated electrons, but also significantly inhibit photocatalyst deactivation [18]. Pd-modified TiO2 has been successfully used in gas-phase photocatalysis [18,19] and wastewater treatment [11]. It was reported that, compared with the corresponding gold or palladium monometallic catalysts [20,21], the combination of them exhibited an obvious synergistic effect and greatly enhanced their catalytic activity and selectivity [22–24]. The catalytic behaviors of Au–Pd bimetal catalysts in the absence of light illumination have already been extensively studied [25–29]. However, the use of Au–Pd for improving photocatalysis in wastewater treatment has been researched little. To some extent, a better photocatalytic activity might be expected if Au and Pd were simultaneously used to modify TiO2 , especially the TiO2 nanotube film. On one hand, the nanotube film could be repeatedly used requiring no complicated

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recycling process which was necessary for the conventional powder photocatalysts; on the other hand, the nanotube layers could provide a large specific surface area and exhibit greater photocatalytic ability. In this work, we designed to fabricate an Au–Pd co-modified TiO2 nanotube film photocatalyst (Au–Pd–TiO2 ) by simultaneously photo-depositing Au and Pd precursor ions on a TiO2 nanotube film. The microstructure and characteristics of the proposed photocatalyst were investigated by a FESEM, XRD, XPS, and UV–Vis absorption spectra. In addition, malathion, an organophosphorus pesticide widely existing in environment and still being used in agriculture, was chosen as a model pollutant to examine the photocatalytic performance of Au–Pd–TiO2 . The yield of H2 O2 was also monitored. Besides, the possible photocatalytic mechanism was discussed. 2. Experimental 2.1. Materials A titanium foil (0.5 mm thickness, purity > 99.5%) was purchased from Baoji Baoye Titanium–Nickel Industry Co. Ltd. Auric chloride acid (HAuCl4 ·4H2 O) and palladium chloride (PdCl2 ) from Sinopharm Chemical Reagent Co. Ltd., China were selected as Au and Pd precursors, respectively. Malathion (95%) was purchased from Huludao Linyun Group of Pesticides Chemical Co. Ltd. The standard substance of malathion was from Institute of Environmental Protection and Monitoring, the Ministry of Agriculture. All other reagents from Beijing Chemical Works, such as glycerol and NH4 F, were of analytical reagent grade and used without further purification. Twice-distilled water was used throughout the experiments. 2.2. Preparation of catalysts The preparation of titania nanotube was similar to the method reported in the literature with a little modification [30]. A titanium foil was polished with abrasive papers and ultrasonically degreased in acetone and distilled water for 10 min in turn, and then dried in nitrogen flow. This pretreated titanium foil was anodized for 2 h at 30 V in a two-electrode system equipped with a platinum cathode (at room temperature). The electrolyte was a mixture of glycerol and 0.4 M ammonium fluoride solution (60:40 vol%). The obtained titania nanotube film was annealed to crystallize TiO2 in air at 500 ◦ C for 4 h with heating and cooling rates of 2 ◦ C min−1 . The annealed film was immersed into an acid solution (pH 3, containing 1 mM HAuCl4 and 1 mM PdCl2 ) and irradiated for 1 h by a 450 W high-pressure Hg lamp under magnetic stirring. Ethanol was selected as the hole scavenger. The desired product (Au–Pd–TiO2 ) was washed repeatedly with distilled water before photocatalytic experiments. For the purpose of comparison, the naked titania nanotube film was also fabricated according to the same processes mentioned above, but Au and Pd were not deposited. 2.3. Characterization The surface morphologies of the prepared photocatalysts were characterized by a Field Emission Scanning Electron Microscope (FESEM, JSM 7600-F, JEOL, Japan). The solid phases were identified by powder X-ray diffractometry (XRD) with Cu K␣ radiation (D/maxZ200PC, Rigaku, Japan) and X-ray photoelectron spectroscopy (XPS) with Al K␣ radiation (ESCALAB MK II, VG Scientific, England). The absorbance spectra were recorded on a UV–Vis diffuse reflectance spectrophotometer (DRS, Cary 50, Varian, America).

Fig. 1. Schematic diagram of the reactor. (a) Bracket for fixing photocatalyst, (b) quartz reactor, (c) photocatalysts, (d) magnetic bar, and (e) UV light source.

2.4. Photocatalytic experiments The photocatalytic degradation of malathion on Au–Pd–TiO2 was carried out in a rectangular quartz reactor (40 mm × 50 mm × 60 mm) under the UV light irradiation. The schematic diagram of the reactor was shown in Fig. 1. A 450 W high-pressure mercury lamp (principal wavelength 365 nm) served as the UV light source and the luminous intensity was 0.4 mW cm−2 . The active area of the photocatalyst was 10 cm2 . At a given time interval, 1.0 mL aliquots were sampled to examine the residual malathion by a Shimadzu LC-20A high performance liquid chromatography (HPLC) equipped with a C18 column (Shim-pack VP-ODS, 4.6 mm × 250 mm). Methanol and water (v:v = 70:30) at a flow rate of 1.0 mL min−1 served as the mobile phase. The detective wavelength was 230 nm. A total organic carbon (TOC) analyzer (TOC-VCPN, Shimadzu, Japan) was employed for mineralization degree analysis. Additionally, the formation of H2 O2 was investigated in a diluted sulfuric acid solution (pH 2.0). The experimental conditions were the same to what mentioned above. Hydrogen peroxide yield was titrated by acidified Ce(SO4 )2 (1 × 10−3 mol L−1 ) using ferroin as the indicator. 3. Results and discussion 3.1. Characterization of photocatalysts 3.1.1. SEM Fig. 2(a) and (b) showed the morphologies of the prepared nanotube films before and after the Au–Pd alloy was deposited. From the SEM images, it could be seen that the morphologies of these two films seemed no significant difference except that there were many bright rings in Fig. 2(b). We presumed that the bright ring was the Au–Pd bimetal deposited on the top of some nanotubes. The EDS result (Fig. 2(b) insert) confirmed the existence of Au and Pd, and revealed that Au was rich in this Au–Pd alloy. The reason for this phenomenon will be investigated in detail later. The deposited Au–Pd alloy could be further analyzed by the following sections (XRD patterns and XPS analysis). 3.1.2. XRD patterns Fig. 3 displayed the XRD patterns of the naked TiO2 and Au–Pd–TiO2 . As could be seen, the diffraction peaks of anatase and rutile were observed in both samples, but anatase was predominant. Based on the Scherrer’s equation and the full width half-maximum peak of anatase (1 0 1) crystal plane, the average

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Fig. 4. XPS spectrum of Au–Pd–TiO2 . The insert (a) and (b) were Au 4f and Pd 3d spectra, respectively.

Fig. 2. SEM images of (a) the naked TiO2 nanotube film and (b) Au–Pd–TiO2 . The insert was the EDS of Au–Pd–TiO2 .

crystallite size was around 32 nm. Besides the diffraction peaks of TiO2 and the Ti substrate, two additional peaks (2 = 38.9, 45.2◦ ) appeared in the Au–Pd deposited TiO2 film, which might be the diffractograms of the Au–Pd bimetal. This was because the broad peak (2 = 38.9◦ ) locating between metallic gold (1 1 1) (JCPDS No. 89-3697) and metallic palladium (1 1 1) (JCPDS No. 88-2335) was indicative of the formation of alloy [31,32]. For the same reason, the diffraction peak at 2 = 45.2◦ existed between metallic gold (2 0 0) and metallic palladium (2 0 0).

Fig. 3. XRD patterns of (a) the naked TiO2 nanotube film and (b) Au–Pd–TiO2 .

3.1.3. XPS analysis XPS analysis was performed to identify the existence of metal elements and elucidate the chemical state of Au and Pd deposited on the TiO2 nanotube films. As shown in Fig. 4, besides the evident peaks of Ti and O, the small peaks of Au and Pd were also detected. The signal strength of Pd was weaker than Au. This consisted with the EDS results and indicated that Au was rich in this Au–Pd alloy. The C 1s peak was used to calibrate the binding energy scale for XPS measurements. It was worth while to note from Fig. 4 insert (a) that the peaks at 83.4 eV and 87.1 eV were lower than the binding energies of Au 4f7/2 (83.8 eV) and Au 4f5/2 (87.5 eV), respectively. The variation of the binding energy might be attributed to the charge transfer from Pd to Au [23], which further confirmed the formation of the Au–Pd alloy and bolstered the XRD results. 3.1.4. Photoabsorbance In order to determine the photoabsorbance properties, the naked TiO2 and Au–Pd–TiO2 were analyzed by DRS in the wavelength range of 200–800 nm. The absorption spectra were shown in Fig. 5. The band gap (Eg ) of an indirect gap semiconductor (TiO2 for instance) could be determined by the equation: (˛h)1/2 = A(h − Eg ), where h was the photon energy, ˛ was the absorption coefficient, A was a constant relative to the material [33]. The corresponding (˛h)1/2 ∼ h curves were illustrated in Fig. 5 (insert). The absorption spectra illuminated that there was no sig-

Fig. 5. UV–Vis diffuse reflectance spectra of the naked TiO2 nanotube film and Au–Pd–TiO2 . The insert was the corresponding variation of (˛h)1/2 with h.

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Fig. 7. The photocatalytic degradation of malathion in five reaction cycles using Au–Pd–TiO2 as a photocatalyst.

Fig. 6. The variation of (a) malathion and (b) ln(C0 /Ct ) vs. reaction time with or without photocatalysts.

nificant difference between the naked TiO2 and Au–Pd–TiO2 in the UV region, and the band gaps were almost the same (around 3.2 eV). In addition, the spectra also demonstrated a high absorbance for Au–Pd–TiO2 in the visible region, which might result from the light absorption of the deposited Au and Pd. However, this improvement of visible light absorption would contribute little to the photocatalytic activity in the UV region. 3.2. Photocatalytic degradation of malathion The photocatalytic activities of the naked TiO2 and Au–Pd–TiO2 were evaluated by degrading malathion. As seen in Fig. 6(a), malathion was decomposed slowly by direct photolysis and the elimination rate reached around 43.1% in 4 h. As compared with direct photolysis, 73.8% of malathion could be removed in the presence of the naked TiO2 . However, a higher degradation rate (98.2%) was obtained when the naked TiO2 was replaced by Au–Pd–TiO2 . This result indicated that the deposited Au–Pd alloy enhanced the photocatalytic degradation of malathion. Its elimination followed pseudo first-order kinetics. Namely, ln(C0 /Ct ) was linear with reaction time. It could be seen in Fig. 6(b) that the kinetic constant for Au–Pd–TiO2 was around 2.72 and 6.58 times the naked TiO2 and direct photolysis, respectively. TOC is a measure of organically bound carbon and usually selected as the criterion for evaluating environmental quality. The photocatalytic ability of catalysts can also be assessed by TOC removal. In this work, 50.7% of TOC was mineralized after 4 h irradiation in the presence of Au–Pd–TiO2 , whereas only 30.3% of TOC was transformed into CO2 in the case of the naked TiO2 . The results evidently demonstrated that higher mineralization was achieved using Au–Pd–TiO2 as the photocatalyst but some carbonaceous substances still existed in the solution. The major reason for the improvement of photocatalytic activities

was that small metal clusters on TiO2 promoted the separation of photo-generated charges, and thus more holes could involve in the malathion oxidation reactions [9,10,17]. Furthermore, the Au–Pd alloy facilitated the formation of superoxide radical anion O2 −• [16] and H2 O2 (the detailed information was discussed in Section 3.3). In addition, the dark control was carried out and the results indicated that the adsorbed malathion could be negligible. In an electrically driven oxidation process, the energy consumption necessary to degrade contaminants was an interesting factor. A parameter usually used to determine the removal efficiency of contaminants was the energy per order (EEO kWh m−3 order−1 ) [34,35]. The EEO could be calculated according to: EEO = P × t/(V × log(C0 /Ct )), where P was the rated power (kW), t was the time (h), V was the volume (m3 ) treated, C0 and Ct were the initial and final concentrations of the contaminant, respectively. In the case of malathion, the value of EEO in the presence of Au–Pd–TiO2 (0.12 kWh m−3 order−1 ) was considerably lower than the naked TiO2 (0.33 kWh m−3 order−1 ). This indicated that the deposition of the Au–Pd alloy on TiO2 gave rise to a substantial improvement in the malathion degradation. Moreover, these EEO values obtained here were similar to some other pesticides in the literatures, such as atrazine and carbaryl [34,35]. In order to examine the stability of Au–Pd–TiO2 , five successive experiments were conducted under the same experimental conditions. As can be seen in Fig. 7, malathion was almost completely degraded in each reaction cycle. The removal rates were all higher than 96.0%. Therefore, it might be safely affirmed that the prepared Au–Pd–TiO2 was rather stable and its photocatalytic activity changed little after five reaction cycles. 3.3. Synthesis of H2 O2 As discussed above, the improved photocatalytic activity of Au–Pd–TiO2 should be attributed to the deposition of the Au–Pd alloy. This modification could not only promote the rapid transfer of photoelectrons on TiO2 , but also accelerate the generation of hydrogen peroxide. The combination of H2 O2 with TiO2 under the UV illumination could greatly enhance the degradation of phenol compounds [36]. The addition of H2 O2 could also benefit the photocatalytic oxidation of organophosphorus pesticides [8]. However, the extra addition of H2 O2 undoubtedly increased the cost of wastewater treatment. Therefore, a large amount of H2 O2 produced in situ would facilitate the malathion degradation and simultaneously decrease the cost. In this study, the formation of H2 O2 was investigated using both the naked TiO2 and Au–Pd–TiO2 as photocatalysts. As shown in Fig. 8, the amount of H2 O2 yielded on the Au–Pd–TiO2 film (44.0 ␮M) in 60 min was 2.89 times that on the naked TiO2 (15.2 ␮M). This confirmed that a higher yield of hydrogen peroxide could be produced on the Au–Pd–TiO2 film, and thus a higher photocatalytic activity was obtained.

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ity of gold and palladium was concerned, the electron density of gold in the Au–Pd alloy increased, while palladium possessed relatively positive charge. This nanostructure resulted in a superior catalytic activity for hydrogenating 4-pentenoic acid [41]. Anyhow, the deposition of Au–Pd bimetal could effectively delay the recombination of photo-generated charges and promote the synthesis of H2 O2 as discussed in Section 3.3. All these excellent characteristics greatly enhanced the photocatalytic activities of TiO2 and leaded to the rapid degradation of malathion. 4. Conclusions

Fig. 8. The synthesis of H2 O2 vs. reaction time using the naked TiO2 nanotube film and Au–Pd–TiO2 as photocatalysts.

3.4. Short discussion on photocatalytic mechanism The conventional photocatalytic mechanism was that electrons in the valence band of TiO2 could be excited to the conduction band under the UV light illumination. The photo-generated holes could oxidize the species absorbed on the surface of TiO2 to form hydroxyl or organic radicals. The photoelectrons could reduce molecular oxygen to form superoxide radical (O2 −• ) which could participate in the synthesis of hydrogen peroxide (O2 −• + 2H+ → H2 O2 ) [8,18]. Unfortunately, the poor quantum yield caused by the rapid recombination of photo-generated charges was a bottleneck that had to be settled. Metal deposition was a promising approach to improve the photocatalyst activity. For example, it was experimentally proved that Au could induce the localization of photo-generated electrons (electron localization effect) and prevent the fast electron–hole recombination in photocatalysis [37]. It was also reported that the deposited metal could facilitate the uptake of photoelectrons by molecular oxygen to form O2 −• [38]. The capture of photoelectrons was the limiting step in photocatalytic processes [39]. Moreover, compared with the TiO2 immobilized with a mixture of Au and Pd monometallic nanoparticles, an obvious synergistic effect was observed in the photocatalytic hydrogen evolution when TiO2 was immobilized with Au–Pd bimetallic nanoparticles [40]. As shown in Fig. 9, the excited electrons could effectively migrate to gold and palladium because the work functions of both noble metals were lager than that of TiO2 [18]. This would liberate more positive holes to oxidize organic pollutants. As far as the electronegativ-

Fig. 9. Schematic diagram representing the charge-carrier transfer on Au–Pd–TiO2 and its interaction with the adsorbed O2 .

An Au–Pd co-modified TiO2 film (Au–Pd–TiO2 ) was fabricated by photo-depositing Au and Pd precursors on a self-organized TiO2 nanotube film. The photocatalytic experiments revealed that, compared with a naked TiO2 film, malathion could be more efficiently degraded by using Au–Pd–TiO2 as a photocatalyst. The enhanced photocatalytic activity might be attributed to both the effective separation of photo-generated charge carriers and the higher synthesis rate of H2 O2 . Additionally, it had to be emphasized that many unknown details still remained to be solved, such as the determination of “x” and “y” in the Aux Pdy alloy and its nanostructures (core–shell and cluster-in-cluster). All of these were important factors in photocatalysis [40,41]. Besides, it had been reported in the literature that the intermediates containing organophosphorus functions could be formed during the photocatalytic degradation of malathion [42]. They were still toxic or even more toxic. For example, the toxicity of malaoxon was approximately 100-times higher than the parent pure malathion [42]. Therefore, from the degradation mechanism and toxicological point of view, to identify the possible intermediates is a meaningful task. The corresponding investigations are in progress. Acknowledgements This work was supported by the Training Fund of NENU’S Scientific Innovation Project (NENU-STC08004), the Postdoctoral Foundation of Northeast Normal University, Analysis and Testing Foundation of Northeast Normal University, and the National Natural Science Foundation of China (No. 50908038). References [1] S.A. Budischak, L.K. Belden, W.A. Hopkins, Relative toxicity of malathion to trematode-infected and noninfected Rana palustris tadpoles, Arch. Environ. Contam. Toxicol. 56 (2009) 123–128. [2] H.P. Gurushankara, S.V. Krishnamurthy, V. Vasudev, Effect of malathion on survival, growth, and food consumption of Indian cricket frog (Limnonectus limnocharis) tadpoles, Arch. Environ. Contam. Toxicol. 52 (2007) 251–256. [3] M. Ahmed, J.B.T. Rocha, C.M. Mazzanti, A.L.B. Morsch, D. Cargnelutti, M. Corrêa, V. Loro, V.M. Morsch, M.R.C. Schetinger, Malathion, carbofuran and paraquat inhibit Bungarus sindanus (krait) venom acetylcholinesterase and human serum butyrylcholinesterase in vitro, Ecotoxicology 16 (2007) 363–369. [4] R. Kumar, N.S. Nagpure, B. Kushwaha, S.K. Srivastava, W.S. Lakra, Investigation of the genotoxicity of malathion to freshwater teleost fish Channa punctatus (bloch) using the micronucleus test and comet assay, Arch. Environ. Contam. Toxicol. 58 (2010) 123–130. [5] Y. Ducolomb, E. Casas, A. Valdez, G. González, M. Altamirano-Lozano, M. Betancourt, In vitro effect of malathion and diazinon on oocytes fertilization and embryo development in porcine, Cell Biol. Toxicol. 25 (2009) 623–633. [6] B. Kumari, A. Gha, M.G. Pathak, T.C. Bora, M.K. Roy, Experimental biofilm and its application in malathion degradation, Folia Microbiol. 43 (1998) 27–30. [7] S. Devipriya, S. Yesodharan, Photocatalytic degradation of pesticide contaminants in water, Sol. Energy Mater. Sol. Cells 86 (2005) 309–348. [8] R. Doong, W. Chang, Photoassisted titanium dioxide mediated degradation of organophosphorus pesticides by hydrogen peroxide, J. Photochem. Photobiol. A 107 (1997) 239–244. [9] C. Yogi, K. Kojima, T. Takai, N. Wada, Photocatalytic degradation of methylene blue by Au-deposited TiO2 film under UV irradiation, J. Mater. Sci. 44 (2009) 821–827. [10] Y. Wu, H. Liu, J. Zhang, F. Chen, Enhanced photocatalytic activity of nitrogendoped titania by deposited with gold, J. Phys. Chem. C 113 (2009) 14689–14695.

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