Developing water quality criteria and assessing ecological risks for 6PPD and 6PPD-Q in freshwater ecosystems

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its derivative N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), originating from tire wear particles (TWPs), are emerging contaminants (ECs) of growing concern. This study focused on the development of water quality criteria (WQC) for 6PPD and 6PPD-Q in freshwater ecosystems, alongside an ecological risk assessment (ERA). We first derived the hazardous concentration for 5 % of species (HC₅) for both compounds by constructing species sensitivity distribution (SSD) models based on experimental toxicity data, interspecies correlation estimation (ICE) models, and quantitative structure-activity relationships (QSAR) models. The short-term and long-term WQC for 6PPD were 26.02 μg/L and 4.30 μg/L, respectively, while those for 6PPD-Q were 0.20 μg/L and 0.15 μg/L, respectively. Subsequently, we used reported surface water concentration data of 6PPD and 6PPD-Q to assess their risk to freshwater ecosystems by calculating risk quotients (RQs) and determining risk probabilities via Monte Carlo simulation and joint probability curves. In contrast to 6PPD, which posed a negligible aquatic ecological risk, 6PPD-Q presented ecological risks in some urban-influenced surface water bodies, particularly after rainfall events. This study provided a theoretical foundation for establishing water quality standards and managing the risks of 6PPD and 6PPD-Q in aquatic ecosystems, while also serving as a valuable methodological reference for the future development of WQC and ERA of other ECs.