Fate of pesticides in wastewater treatment plant (WWTP) and mixture toxicity of pesticides and their transformation products (TPs)

Abstract

Pesticides play a crucial role in agriculture by controlling pests and enhancing crop production. However, due to their toxicity and persistence, these chemicals impact non-target organisms and are considered as one of the concerns to be potential threats to aquatic ecosystems. Pesticides that enter WWTPs are often not completely removed and are discharged into aquatic environments through effluents. Consequently, WWTP can serve as a significant source of pesticides in aquatic environments. The pesticides in environments undergo various degradation and metabolic processes, leading to the formation of transformation products (TPs). Particularly, pesticides discharged into aquatic environments after treatment in WWTPs may undergo microbial transformations. Often, these TPs are found to be more abundant and exhibit greater mobility, persistency, and toxicity compared to their parent compounds, potentially posing similar or greater risks to the aquatic environments. Despite their importance, studies on the environmental impacts of pesticides and their TPs, as well as their toxicity, are limited. The main objective of this dissertation is to identify the TPs of pesticides in WWTPs and to assess the mixture toxicity of these pesticides and their TPs. This research consisted of four main areas: ⅰ) monitoring and conducting environmental risk assessment of pesticides in WWTP effluents, ⅱ) prioritizing pesticides in WWTP effluents based on the source of pesticides in surface water and their potential risks to aquatic environments, ⅲ) identifying TPs of priority pesticides that are formed by activated sludge from WWTP, ⅳ) assessing the single and mixture toxicity of pesticides and their TPs and conducting a mixture risk assessment of pesticides and their TPs in WWTP effluent. In the first part, a year-long monitoring of 87 pesticides in the effluents of three WWTPs located in the Yeongsan River basin. This monitoring detected 59 pesticides, with four identified as priority substances under Directive 2013/39/EC. Risk quotients (RQs) calculated based on measured environmental concentrations (MEC) and predicted no-effect concentrations (PNEC) varied across monitoring periods and locations, with most substances showing negligible to low risks. However, some compounds exceeded the RQ value of 1, indicating moderate to very high risks. An optimized RQ (RQf)-based prioritization, which considering the frequency exceeding the PNEC, suggested that 13 pesticides potentially pose a risk to the aquatic environments. Notably, the herbicide metribuzin was found to contribute approximately 40% to the overall RQf for the mixture (RQf, mix). In the second part, a year-long monitoring campaign was conducted at six sites within the Yeongsan River basin to evaluate the contribution of WWTP effluents as potential sources of pesticide pollution in the aquatic environment. The hierarchical cluster analysis based on the normalized concentrations of pesticides in surface water and WWTP effluents identified pesticides as likely originating from WWTP effluents. By integrating potential sources of pesticides in the aquatic environment with the result of RQf-based ERA from the first part of the research, this study prioritized the pesticides in the WWTP effluents. This study concluded that pesticides exhibiting higher risks and likely derived from WWTP effluents, such as metribuzin, 3-phenoxybenzoic acid, atrazine, and atrazine-2-hydroxy, should be regulated with priority in WWTP effluents. In the third part, the TPs of the herbicide metribuzin, which exhibited the highest RQf among the priority pesticides, were identified using LC-HRMS-based suspect and non-target screening analysis. This study identified ten TPs formed through various reactions such as desulfuration, deamination, methylation, and oxidation. The major TPs identified were metribuzin-diketo (MET-DK) and metribuzin-desamino (MET-DA). Notably, it was observed that the toxicophore of metribuzin was not degraded during these transformations, suggesting that the TPs might exhibit a mode of action (MoA) and toxicity similar to that of the parent compounds. In the last part, the individual and mixture toxicity of metribuzin and its TPs — MET-DA, MET-DK, and MET-N-methyl — to freshwater algae were evaluated. Metribuzin exhibited the highest toxicity, followed by MET-N-methyl and MET-DA, while MET-DK showed no toxicity to algae. The mixture toxicity assessment confirmed that the concentration addition (CA) model appropriately predicts the mixture toxicity of metribuzin and its TPs, with a model deviation ratio (MDR) of 0.793. The mixture risk assessment based on the toxic unit to algae (TUalgae) in the WWTP effluent indicated that while metribuzin potentially poses a risk to aquatic environments, MET-DA and MET-N-methyl were found to be non-hazardous. However, due to the shared toxicophore between metribuzin and its TPs, which indicates a similar MoA, and the presence of other similarly acting pesticides in the effluent, careful management of metribuzin and its TPs is necessary to protect aquatic environments. In this study, priority pesticides in WWTP were identified through a risk assessment of pesticides in WWTP effluents and the selection of major pollution sources in the aquatic environments. The information on the transformation and toxicity of these priority pesticides, which occur during the wastewater treatment process, was also established. The finding indicated that various pesticides in WWTP effluents, as well as their TPs, potentially pose a risk to the aquatic environments. Moreover, these substances, when present in a mixture, can produce an additive effect, highlighting their significance as environmental pollutants. In conclusion, this study suggested that for effective management of pesticides and their TPs in WWTPs, it is necessary to address not only the removal of individual substances but also the detoxification of the TPs and their mixtures.