Photocatalytic degradation of a fungicide difenoconazole and residual toxicity of its degradation products

Abstract

Worldwide consumption of pesticide active ingredients reaches 2.7 million tons per year [1]. After being released into the environment significant portion of applied pesticides ends up in surface water bodies affecting nontarget organisms. Due to high environmental pollution, improvement in technological practices for water treatment has been a demand, and advanced oxidation processes stand out as a prominent method. In this study, photocatalytic degradation of a fungicide difenoconazole was investigated and residual toxicity was assessed using zebrafish embryos (Danio rerio). Difenoconazole has been subjected to photocatalytic degradation in presence of the TiO2 Degussa P-25 catalyst and artificial illumination (Osram Ultra Vitalux 300 W). Influence of different anions (carbonate, sulphate and nitrate), concentration of pesticide and mass of catalyst, on degradation process has been assessed. Decrease in absorbance (Shimadzu 1800 UV spectrophotometer) served as an indicator of the pesticide concentration in the reaction system. The environmental acceptability of the specified degradation process was assessed for optimal circumstances of total photodegradation. Residual toxicity of degradation product was assessed on the basis of toxicity comparison of initial, partly and completely degraded samples, according to OECD 210 [2]. Embryotoxicity assays were performed in triplicate and were terminated 120 hours post fertilization (hpf). According to the results of assays engaged in assessment of the influence of the ions on photocatalytic processes, it was found that all ions have comparable catalytic effects on the kinetics of the difenoconazole degradation process. The presence of sulphates and carbonates had the greatest catalytic effect, while the nitrate ions showed the lowest catalytic capacity (Fig. 1). The most suitable conditions for photodegradation were obtained by using 0.2 g/l of TiO2 and 5 mg/l of difenoconazole solution at atmospheric conditions. The embryotoxicity assays followed the analytical examination. Comparison of results obtained in embryotoxicity assays of the initial solution, partly and completely degraded samples confirmed suitability of applied degradation method. Significant decrease in toxicity, compared to the initial solution, was registered in both degraded samples. Based on cumulative mortality (120 hpf), medium lethal concentration (LC50) of difenoconazole is 1.4 mg/l, while the same parameter could not be determined in partly and completely degraded samples, due to the lack of mortality (Fig. 2). According to the obtained results it can be concluded that engaged photocatalytic degradation process is suitable for pesticide contamination water management.