Abstract: Ethion, an insecticide, is widely used with fruit and vegetable crops. This research studied the reduction and oxidative degradation of standard ethion by TiO2 photocatalysis. A standard ethion solution (1 mg L-1) was treated with different concentrations of TiO2 powder (5, 10, 20, 40 and 60 mg mL-1) for 0, 15, 30, 45 and 60 min. The amount of ethion residue was detected by gas chromatography with flame photometric detection (GC-FPD) and the concentration of anions produced as major degradation products was determined by Ion Chromatography (IC). The TiO2 photocatalysis efficiently reduced ethion concentrations, with the highest degradation rate occurring within the first 15 min of reaction. The reaction produced sulphate and phosphate anions. The TiO2 photocatalysis reduced 1 mg L-1 ethion to 0.18 mg L-1 when treated with 60 mg mL-1 TiO2 powder for 60 min. The lethal concentration (LC50) of standard ethion was also estimated and compared to the treated ethion. All treatments, especially 60 mg mL-1 TiO2 powder, markedly detoxified ethion, as tested with brine shrimp (Artemia salina L.), with an LC50 value of 765.8 mg mL-1, compared to the control of 1.01 mg mL-1.
INTRODUCTION
Ethion (O,O,O',O'-tetraethyl S,S'-methylene bisphosphorodithioate) is an organophosphate pesticide widely used as an insecticide and acaricide to control aphids, spiders, mites and insects in fruit and vegetable production (Desouky et al., 2013). Organophosphorus insecticides (OPIs), including ethion, contaminate many components in environment. It accumulates in biological systems and its presence in agricultural products and aquatic environments is potentially harmful to humans and other organisms. Ethion is highly toxic to freshwater and marine fish. The half-life of ethion in soil is range from 1.3-49 weeks depending on soil conditions and repeated usage of ethion (Extonet, 1996).
Titanium dioxide (TiO2) belongs to the family of transition metal oxides (Jian, 1997; Hashimoto et al., 2005). Four commonly known polymorphs of TiO2 are found in nature: Anatase (tetragonal), brookite (orthorhombic), rutile (tetragonal) and TiO2 (B) (monoclinic) (Gupta and Tripathi, 2011; Florencio et al., 2004). This study reports on TiO2 powder only. The TiO2 is relatively economical, photostable in solution, highly stable chemically, nontoxic, redox selective and strongly oxidizing (Fujishima et al., 2000; Gupta and Tripathi, 2011). TiO2 powder is highly photocatalytic due to its high specific surface area. When TiO2 absorbs Ultra Violet (UV) radiation from sunlight or an illuminated light source (fluorescent lamps), pairs of electrons and holes are produced (Philippopoulos and Nikolai, 2010). The positive-hole of TiO2 breaks apart the water molecule to form hydrogen gas and hydroxyl radical. The negative-electron reacts with the oxygen molecule to form a super oxide anion. This cycle continues when light exists. Hydroxyl radicals have been reported as extremely powerful oxidizing agents due to their oxidizing strength (Stasinakis, 2008). Environmentally, this mechanism has been utilized to oxidize hazardous organic pollutants into nontoxic materials (Ravelli et al., 2010). Many studies have reported TiO2’s ability to remove toxic substances (Konstantinou and Albanis, 2004), such as malathion, methamidophos, chlorfenapyr, phoxim, dichlofenghion, bromophos ethyl, bromophos methyl, atrazine, cyanazine, irgarol, prometryne, propazine, chlorotoluron, metobromuron, isoproturoncinosulfuron, triasulfuron (Gupta and Tripathi, 2011), fenamiphos (Gupta and Tripathi, 2011), pirimiphos-methyl (Herrmann and Guillard, 2000), diquat, paraquat (Tariq et al., 2007), triclopyr, daminozid (Lomora et al., 2011), parathion (Carp et al., 2004), 4-bromoaniline, 3-nitroaniline, pentachlorophenol, 1,2,3-trichlorobenzene and diphenylamine (Rahman and Muneer, 2005). However, the potential and appropriate applications of TiO2 need further study, including oxidative degradation. Currently, the use of TiO2 photocatalysis as an oxidation process for water treatment is increasing. The advantage of photocatalysis is that the photogeneration of ∙OH radicals are not harmful to the environment. This study was conducted to determine the oxidative degradation of ethion by TiO2 photocatalysis.
MATERIALS AND METHODS
Chemical standard and preparation
Preparation of ethion standard: Ethion (Fig. 1) of analytical standard and 98.7% purity, was purchased from Sigma-Aldrich Pte Ltd. (Singapore, Fluka®). Ethion stock solution (1,000 mg L‾1) was prepared by dissolving standard ethion in 99.8% acetone. The solution was diluted with distilled water to the appropriate concentrations.
Photocatalyst: Commercial titanium dioxide (TiO2) powder was purchased from Ajax Finecham®.
Photoreactor and light source: Photocatalytic experiments (Fig. 2) were all carried out in a dark acrylic box (30×30×60 cm) with the upper cover at room temperature. Two UV lamps at 10 W each supplied the UV radiation.
Reduction of standard ethion solution by TiO2 photocatalysis: A standard ethion solution (25 mL, 1 mg L‾1) was subjected to TiO2 photocatalys is using different concentrations of TiO2 powder (5, 10, 20, 40 and 60 mg mL‾1). The titanium dioxide particles and ethion solution were adequately mixed using an air pump throughout the experiment. Samples were taken at 15 min intervals for 1 h. Three replicates of ethion samples for each treatment were conducted. Ethion concentrations were determined by gas chromatography. The analysis was performed in an Agilent Technologies (Wilmington, DE) model 6890 gas chromatograph equipped with a flame photometric detector (GC-FPD). The GC column was a fused silica capillary column HP-5, 5% phenylmethylsiloxane, with the dimensions of 30 m×0.32 mm i.d. and a 0.25 μm film thickness(Agilent Technologies).
| Fig. 1: | Chemical structure of ethion |
| Fig. 2: | Schematic diagram of the TiO2 photocatalysis for ethion reduction in vitro |
The temperature was programmed to increase at 10°C min‾1 from an initial 100-200°C and then at 4°C min‾1 until the final temperature of 220°C was reached. A purified helium carrier gas was used at a flow rate of 3.6 mL min‾1. The detector temperature was set at 250°C. Sample solution (1.0 μL) was injected in splitless mode. Quantification of ethion was performed using an ethion standard as reference.
Oxidative degradation of treated ethion by TiO2 photocatalysis: Concentrations of sulphate and phosphate anions of treated ethion solutions by TiO2 photocatalysis were determined. Anions, released from the decomposition of ethion, were analyzed at 60 min by Ion Chromatography (IC). The amount of these ions corresponded to the degradation of ethion in this study. The analysis was performed using an ion chromatograph equipped with a Metrosep A Supp 5 250/4.0. Eluent composition was A Supp 5 eluent 3.2 mM Na2CO3 and 1.0 mM NaHCO3. Samples were filtered by syringe filters of size 13 mm, 0.45 μm (Vertical®). A 10.0 μL sample was automatically injected in conductivity mode and ethion was quantified using calibration curves with external standards at a flow rate of 0.70 mL min‾1, pressure of 12.71 MPa and recording time of 28 min.
Toxicity test by bioassay method: Toxicity was estimated using brine shrimp (Artemia salina L.). Brine shrimp eggs were hatched in artificial sea water (3% marine salt in water) oxygenated by air pump and sown for three weeks to reach adult stage. Ten adult brine shrimps were put into a vial containing 5 mL of TiO2 of 60 mg mL‾1 with ethion concentration at 1.0 mg L‾1 and then subjected to photocatalysis as described in experiment 1. The experiments were conducted in five replications. Mortality of adult brine shrimp was checked every 6 h. The LC50 of the ethion solution was calculated.
Statistical analysis: All experiments were evaluated with a regression procedure, using SPSS version 17 while the differences among various treatments were calculated using one-way analysis of variance followed by Duncan’s New Multiple Range test (p<0.05).
RESULTS
Reduction of standard ethion solution by TiO2 photocatalysis: Reduction of standard ethion by TiO2 photocatalysis is shown in Fig. 3. The cncentration of ethionsolution decreased as the amount of TiO2 powder and time of exposure increased. Within the first 15 min, a rapid reduction in ethion concentration was observed for all treatments. However, the most effective method of reducing the amount of ethion was observed using 60 mg mL‾1 TiO2 powder for 60 min; it was able to reduce 1-0.28 mg L‾1.
Ethion treated with TiO2 photocatalysis resulted in increased anion concentrations. As shown in Fig. 4a the amount of phosphate ions increased abruptly for the first 15 min but slowed down from 30-60 min. During TiO2 photocatalysis, the phosphate concentration increased from 5.73-7.01 ppm.
| Table 1: | Bioassay toxicity test with brine shrimp (Artemia salina L.) of ethion solution after TiO2 photocatalysis treatment for 60 min |
| Fig. 3: | Concentration of ethion after treatment with TiO2 photocatalysis |
TiO2 photocatalysis also induced the release of sulphate anion from ethion. The sulphate anion increased during the first 15 min of reaction time, then stabilized from 30 min until 60 min of reaction (Fig. 4b).
Treatment reduced toxicity as evidenced by the increasing LC50 value with increasing time (Table 1). The highest value was 765.8 mg mL‾1 after treatment with TiO2 for 60 min, compared with the control (1.01 mg mL‾1). Toxicity evaluation indicated the toxicity of ethion decreased after treatments. Ethion concentration may be degraded by TiO2 photocatalysis process.
DISCUSSION
Results obtained from this study conformed to the one where an organophosphate pesticide decomposes better the longer it is exposed to UV light (Mountacer et al., 2013).
| Fig. 4(a-b): | (a) Phosphate and (b) Sulphate anion released from ethion solution during TiO2 photocatalysis using 60 mg mL‾1 TiO2 powder |
This is because as irradiation time of the UV lamp increases lots of free radicals had formed in the liquid, causing much decomposition on the pesticides (Konstantinou and Albanis, 2002; Shayeghi et al., 2012).
During the reaction while the formation of sulphate from the stoichiometric concentration is achieved, the remaining amount is still linked to the parent molecule (Calza et al., 2004; Rajeswari and Kanmani, 2009).
Many previous studies have shown several intermediates of photocatalytic degradation that have been identified corresponding to the reaction that taking place to break apart the ethion molecules (Herrmann et al., 1999). Oxidant attack of OH on the P = S bond occurred first, resulting in the formation of oxon derivatives. The continuous attack of OH, followed by the rupture of the P-O bond, resulted in the formation of corresponding phenols and different alkyls or phosphate esters (Mountacer et al., 2013; Konstantinou and Albanis, 2004).
The result of toxicity tested against brine shrimp (Artemia salina L.) in this research was also confirmed by Tsuda et al. (1997) that the contamination of fish and other aquatic organisms by the oxidation products in the environment is very low.
CONCLUSION
TiO2 photocatalysis applied to ethion solution reduced ethion concentration. Most of the decomposition appeared within the first 15 min. The best condition for reducing ethion was to use 60 mg mL‾1 TiO2 powder exposed for 60 min. During degradation, sulphate and phosphate anions were released from the ethion structure, due to the TiO2 photocatalysis that generated •OH. The treatment of TiO2 photocatalysis significantly lowered the toxicity of ethion. The results proved that TiO2 photocatalysis offers potential to chemically reduce residues and by-products also have low toxicity tested while tested against Artemia salina L. However, continuous efforts to improve the photocatalytic properties should be considered.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the Agricultural Research Development Agency (Public Organization, ARDA) and the Postharvest Technology Innovation Center, Commission on Higher Education (Bangkok) for their financial support (Scholarships for the Ph.D. program). This research was conducted at the Postharvest Technology Research Institute, Graduate School, Chiang Mai University and the Office of Agricultural Research and Development, Region 1 (OARD1) which provided the laboratory facilities.