Influence of Coexisting Copper and Zinc on the Adsorption and Migration of Sulfadiazine in Soda Saline–Alkali Wetland Soils: A Simulation Approach

This study investigates the adsorption and migration of sulfadiazine (SDZ) in soda saline–alkali soils under Cu/Zn co-pollution using equilibrium adsorption and soil column experiments. Freundlich and Langmuir isothermal models, combined with Hydrus-1D two-site modeling, revealed concentration-dependent interactions. Low Cu (10–100 mg kg−1) and Zn (10–100 mg kg−1) enhanced SDZ adsorption via charge regulation and complexation, while high concentrations (300 mg kg−1) suppressed adsorption through competitive adsorption and hydroxide precipitation. Synergistic Cu-Zn coexistence further reduced adsorption to 3.035 mg kg−1. Freundlich modeling (R2 = 0.922–0.995) outperformed Langmuir, confirming adsorption site heterogeneity. Column experiments showed Cu (300 mg kg−1) and Zn (300 mg kg−1) accelerated SDZ migration (peaks 0.93–0.94), delaying breakthrough versus Br−. Hydrus-1D simulations (R2 ≥ 0.915, RMSE < 0.1) effectively quantified nonlinear dynamics between instantaneous adsorption sites (f = 0.101–0.554) and metal concentrations. Results demonstrate heavy metals critically regulate antibiotic fate via concentration-dependent mechanisms in saline–alkali ecosystems.