Biodegradation-driven reconfiguration of sediment dissolved organic matter controls pollutant-specific binding pathways of lead and tetracycline at the sediment-water interface.

Sediment dissolved organic matter (SDOM) is a dynamic mediator of contaminant fate in lake sediments, yet how its biodegradation-driven reorganization alters pollutant binding under regional heterogeneity remains unresolved. Here, we examined lead (Pb) and tetracycline (TC) binding to SDOM from 18 sites across three environmentally distinct zones of Lake Nansi, a hydro-regulated shallow lake, before and after a 28-day biodegradation experiment. Before biodegradation, the upper lake area contained more protein-enriched SDOM, with the relative contribution of the protein-like component C3 averaging 52.0%, compared with 38.2% and 41.8% in the near-dam and lower lake areas, respectively. Biodegradation reduced C3 by 47.2%, increased the humification index by 17.2%, and decreased total fluorescence intensity by 30.8%, indicating substantial restructuring of SDOM composition and microbial assemblages. This restructuring produced pollutant-specific responses. Pb showed stronger regional heterogeneity than TC in both binding sequence and apparent binding response, and its dominant binding contribution shifted away from the protein-like fraction after biodegradation. TC displayed a more conserved binding sequence but became increasingly associated with transformed fulvic-like fractions after biodegradation. Piecewise structural equation modeling further revealed distinct association pathways: Pb apparent binding response was mainly associated with the regional environmental gradient (β = 0.364, p = 0.034), whereas TC apparent binding response was more closely linked to C3 (β = 0.702, p < 0.001) and biodegradation (β = 0.498, p = 0.012). These findings demonstrate that biodegradation-driven SDOM reorganization creates distinct binding pathways for metals and antibiotics, and highlight the importance of incorporating SDOM quality, microbial reorganization, and spatial heterogeneity into process-based assessment of contaminant fate in regulated lake systems.