Degradation mechanism and the toxicity assessment in TiO2 photocatalysis and photolysis of parathion
Introduction
Organophosphorus pesticides are widely used in agriculture and industry (Dzyadevych and Chovelon, 2002). Parathion (methyl parathion, CAS No. 56-38-2) is a non-systemic insecticide that is used in many countries worldwide. It is used as a fumigant and acaricide and as a pre-harvest soil and foliage treatment for a wide variety of crops, both outdoors and in greenhouses (WHO, 2004). It is acutely toxic to mammals, acting by inhibiting the enzyme acetylcholinesterase (AChE) in nerve tissue. Parathion is classified as an acutely toxic pesticide by the US EPA (2003). Fig. 1 shows the chemical structures and UV spectra of methyl parathion and its environmental degradation byproducts: methyl paraoxon and 4-nitrophenol.
For treating parathion residues in water, biological and physical processes such as wetland and thermal decomposition have limitations due to their land and energy constraints (Germain et al., 2000, Schulz et al., 2003). There is a pressing need for a readily available economical, on-site, low energy technology for this purpose. Advanced oxidation processes (AOPs) have been proposed as an alternative for the treatment of wastewater. Of particular interest is the use of TiO2 as a heterogeneous semiconductor catalyst, with solar light as the energy source (Malato et al., 2003). One of the potential advantages of this process is that photocatalysis can completely mineralize a variety of aliphatic and aromatic compounds under suitable conditions.
The degradation of parathion using TiO2 photocatalysis was recently investigated (Doong and Chang, 1998, Konstantinou et al., 2001, Konstantinou and Albanis, 2003). These studies focused on the kinetics and the identification of intermediates. However, the degradation mechanism and the toxicity of the treated water were not investigated.
Successful treatment of a pollutant by TiO2 photocatalysis must address the disappearance of the parent compound as well as identification of the intermediates and end-products to insure that harmful or toxic end-products are not produced. The toxicity of known end-products may be determined from literature surveys, but in the case of unknown end-products, a battery of different bioassays using organisms from different trophic levels can be used to insure that the end-products are not harmful.
This paper focuses on the kinetics, mechanisms and the end-product toxicity of parathion degradation in water using TiO2 and artificial UV light. Parathion degradation rates were compared for TiO2-alone, photolysis (UV-only) and photocatalysis (UV and TiO2). The mechanism of the photocatalytic degradation of parathion is discussed by referring to the analysis of intermediates, byproducts, and TOC. Also, two different bioassays were performed to evaluate the toxicity of the treated water after photocatalysis and photolysis, and these results were compared with measurements of the byproducts and TOC. Toxicity was measured at different stages of parathion degradation using a modified Microtox 81.9% test using Vibrio fischeri (V. fischeri) and a 48-h acute toxicity test using Daphnia magna (D. Magna).
Section snippets
Chemicals
Methyl parathion (99.4%), methyl paraoxon (98.3%), and 4-nitrophenol (99%) were purchased from Chem Service. TiO2 (Degussa P-25, with a BET surface of 50 ± 15 m2/g) was used with no pre-treatment. All other reagents were of analytical grade and were used without further treatment.
Photoreactor
The photocatalytic degradation experiments were performed in a circulating photoreactor system. The reactor system consisted of a temperature-controlled reservoir consisting of a stirred 2 l glass bottle, a metering pump
Comparison of parathion degradation between photolysis and photocatalysis
To confirm the role of TiO2 in the photocatalysis reaction, four sets of experiments were performed to compare parathion degradation rates with and without TiO2 catalysts. The first set was performed as a blank containing only parathion (10 mg/l). The second set was performed with parathion (10 mg/l) exposed to TiO2 (1 g/l), but no UV (TiO2-only condition). The third set was performed by exposing parathion (10 mg/l) to UV without TiO2 (photolysis condition). Then, the fourth set exposed parathion
Conclusions
This study examined the efficacy of the photocatalytic degradation of parathion by TiO2/UV photocatalysis. The conclusions were as follows:
- 1.
The rate of parathion degradation decreased with an increasing initial concentration of parathion, and the degradation kinetics followed pseudo-first-order kinetics based on the Langmuir–Hinshelwood kinetics.
- 2.
Photocatalysis was effective in degrading parathion and was much more effective than photolysis. TiO2 without UV light was ineffective and only 10% of
Acknowledgment
This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2003-003-D00217).
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