Fate of emerging contaminants in drinking water treatment plants along the Yangtze and Han rivers: From occurrence to advanced removal by UV and NaClO governed by molecular descriptors

Emerging contaminants (ECs) in drinking water sources pose significant risks to both human health and ecological security. However, their occurrence patterns and removal efficiencies in conventional drinking water treatment processes remain insufficiently studied, limiting the development of effective mitigation strategies. This study conducted a systematic investigation of major ECs in the Yangtze River and its largest tributary, the Han River. Among the analyzed ECs, pesticides and pharmaceuticals were identified as the predominant contaminants, with the Han River exhibiting particularly elevated concentrations. Monitoring across the treatment trains of two drinking water treatment plants (DWTPs) revealed that most ECs were removed with efficiencies below 50%, demonstrating the superior performance of chemical oxidation over physical methods. Lab-scale experiments further verified that UV irradiation and NaClO oxidation were the most effective treatment techniques, though both exhibited notable compound-specific selectivity. Structure-activity relationship analysis of 18 ECs indicated that direct photolysis by UV and direct oxidation by NaClO served as the primary removal mechanisms, with UV proving significantly more efficient. Removal selectivity was closely associated with molecular structural features: UV photolysis efficiency correlated with triplet excitation energy and the energy gap between the lowest unoccupied and highest occupied molecular orbitals (LUMO-HOMO), whereas NaClO oxidation efficiency was linked to excited-state energy and HOMO energy levels. These findings offer valuable insights for assessing source water risks and optimizing treatment strategies to enhance drinking water safety.