Underestimated risks of conjugated estrogens in wastewater: New insights into transformation products and mass balance

Neglected conjugated estrogens (CEs) may lead to significantly increased ecological risks following transformation. Wastewater treatment plants (WWTPs) act as critical transformation hubs for CEs. However, the transformation behavior and ecological risks of CEs and their transformation products (TPs) remain poorly understood. In this study, the degradation and transformation profiles of CEs by activated sludge were investigated under laboratory conditions, and the distribution, transformation behaviors, and potential risks of CEs and their TPs in WWTPs were systematically examined via field investigations. The results showed that the degradation of CEs followed a biphasic exponential decay pattern, with G-type conjugates degrading significantly faster than the corresponding S-type counterparts. A total of 85 TPs were tentatively identified, 33 of which were unique and primarily formed through oxidation, deconjugation, sulfation, and hydroxylation. The accumulation of TPs exhibited a distinct time-dependent pattern, and moderately extending the incubation time effectively reduced the abundance of most TPs. Transformation networks revealed extensive interconnections among natural CEs, with free estrogens (FEs) and hydroxylated TPs serving as central nodes linking different pathways. Field investigations across 12 WWTPs detected over 80 % of CEs and quantified TPs in influent, along with 20 additional TPs identified through suspect screening. Mass fluxes of CEs and quantified TPs exhibited distinct spatial patterns along the treatment process. CEs decreased in the anaerobic and aerobic tanks, whereas TPs were mainly removed in the aerobic and advanced treatment stages. >70 % of the TPs detected in the effluent retained estrogenic activity, resulting in a substantial increase in the overall estrogenic risk when both CEs and their TPs were considered. These findings underscore the importance of considering CEs and their TPs in risk assessments to avoid underestimating estrogen-associated risks.