Bioavailability regulates seasonal dynamics and ecological risks of antibiotics-heavy metals mixtures in wastewater treatment.

Co-occurrence of antibiotics and heavy metals in domestic wastewater has caused seasonal instability in their removal in wastewater treatment plant (WWTP). Mechanisms for understanding such fluctuations remain uncovered due to the strong reliance on total concentrations rather than bioavailable fractions. Therefore, diffusive gradients in thin-films (DGT) technique devices were applied on the influent, effluent, and sludge of a domestic WWTP in Beijing from winter to spring (December 2024 to April 2025) for detecting 4 quinolones (QNs), 3 tetracyclines (TCs) and 3 heavy metals. DGT measurements demonstrated antibiotics dominated bioavailable fractions in winter, with QNs accounting for > 70% of total concentrations, while heavy metals (Cr, Cd and Cu) showed significantly increasing bioavailability in spring. QNs and TCs favoured coordination complexes with multivalent metal ions in sludge during winter, promoting immobilization of metals. With increasing temperature, destabilization of these complexes enhanced metal desorption into the aqueous phase and triggered remobilization of certain antibiotics into effluent. This temperature-dependent complexation-desorption behaviour revealed the seasonal dynamics of contaminant migration and uncertainties of WWTP removal performance. Ecological risk assessment based on DGT confirmed the shift in dominant risk drivers from antibiotics in winter to metals in spring. Wastewater-based epidemiology further confirmed that elevated winter antibiotic usage initiated these bioavailability-driven processes. Our study proved the bioavailability outperformed total concentration to characterize seasonal transport, removal efficiencies and environmental risk of antibiotic-metal mixtures in urban WWTPs, offering deep mechanistic insight for mixture-involved risk assessment and pollution control strategies.