Transformation of sulfonylurea herbicides in simulated drinking water treatment processes

National trends in pesticides in drinking water and water sources in Japan

Although agricultural activities-especially paddy rice cultivation-are prominent in watersheds in Asian countries, few comprehensive studies have examined pesticide concentrations in water in these areas. Here, we report the concentrations of 162 pesticides in treated drinking water and source water (14,076 samples) in Japan, where rice cultivation is common, along with trends in sales of herbicides, fungicides, and insecticides from 1963 to 2016. Herbicides and fungicides-especially those used in rice farming were frequently detected in drinking water sources. The herbicide bromobutide, which is not listed in drinking water quality standards or guidelines except in Japan, as well as the widely used-and-detected bentazone, were frequently detected in source water (bromobutide and bentazone were detected at concentrations >0.1 μg/L in 31.1% and 33.8% of samples, respectively). Dymron and tefuryltrione were also detected in over 10% of samples at concentrations >0.1 μg/L. The highest observed concentration of bromobutide was 10 μg/L, and 7.5% of samples had concentrations >1 μg/L. High concentrations were also observed for halosulfuron methyl (7.9 μg/L), pyroquilon (7.0), molinate (6.8), and metominostrobin (4.6). Some of the pesticides frequently detected in source water were not detected at all in drinking water, but the main cause of the non-detection appeared to be degradation by chlorine. From the 1970s onward, sales of herbicides and fungicides with higher acceptable daily intakes (ADIs; i.e., with lower toxicity) have increased. However, the percentage of herbicides with very low ADIs (<10^-2.5 mg kg^-1 d^-1) being shipped has also increased. Tefuryltrione, which was detected at normalized concentrations >0.1 in 8% of samples, is an example of this type of herbicide. The average log-Kow of herbicides has decreased from the 1970s to the present, due to the strong trend towards the application of hydrophilic herbicides, such glyphosate. The need for increased monitoring of pesticides used in rice paddy farming is highlighted.

Study on Electrochemical Degradation of Nicosulfuron by IrO₂-Based DSA Electrodes: Performance, Kinetics, and Degradation Mechanism

The widely used sulfonylurea herbicides have caused negative effects on the environment and human beings. Electrochemical degradation has attracted much attention in the treatment of refractory organic compounds due to its advantage of producing no secondary pollution. Three kinds of IrO₂-based dimensionally stable anodes (DSAs) were used to degrade nicosulfuron by a batch electrochemical process. The results showed that a well-distributed crack network was formed on the Ti/Ta₂O₅-IrO₂ electrode and Ti/Ta₂O₅-SnO₂-IrO₂ electrode due to the different coefficients of thermal expansion between the Ti substrate and oxide coatings. The oxygen evolution potential (OEP) increased according to the order of Ti/RuO₂-IrO₂ < Ti/Ta₂O₅-SnO₂-IrO₂ < Ti/Ta₂O₅-IrO₂. Among the three electrodes, the Ti/Ta₂O₅-IrO₂ electrode showed the highest efficiency and was chosen as the experimental electrode. Single factor experiments were carried out to obtain the optimum electrolysis condition, shown as follows: currency intensity 0.8 A; electrode spacing 3 cm, electrolyte pH 3. Under the optimum conditions, the degradation of nicosulfuron followed first-order kinetics and was mainly due to indirect electrochemical oxidation. It was a typical diffusion-controlled electrochemical process. On the basis of the intermediate identified by high performance liquid chromatograph-mass spectrometry (HPLC-MS), two possible degradation routes were proposed.