Availability and desorption kinetics of correlative cadmium and manganese in soils from karst areas

Cadmium (Cd) contamination in soils poses substantial environmental and health risks globally, with manganese (Mn) playing a crucial role in regulating Cd mobility through soil adsorption processes and shared crop uptake pathways. While the importance of understanding Cd-Mn dynamics in soils is widely recognized, quantitative assessments of their correlated desorption processes remain limited. This study employed diffusive gradients in thin-films (DGT) technique combined with DGT-induced fluxes in soils (DIFS) modeling to investigate Cd and Mn availability and desorption dynamics in karst soils from Guangxi, southwestern China. The soil solution concentrations ranged from 0.23–1.82 µg/L for Cd and 1.29–8.41 mg/L for Mn. DGT measurements demonstrated nonlinear accumulation patterns for both metals over 48 h duration. DIFS modeling yielded distribution coefficients (K_dl) ranging from 2.50 to 807 mL/g and response time (T_c) between 1.27 and 425 s for both metals. Solid phase resupply was limited by desorption rates of 5.38–229 × 10⁻⁵/s, providing unprecedented insight into the kinetics of metal release in these soils. Analysis of metal desorption rate ratios (k₋₁-Mn/k₋₁-Cd) indicated that soil organic matter content, clay content, pH, and metal contents collectively control Cd and Mn desorption kinetics, leading to distinct desorption patterns across soils with varying physicochemical properties. These findings demonstrate rapid equilibrium reestablishment and desorption-limited resupply characteristics of Cd and Mn in karst soils, advancing understanding of correlative metal behaviors in these unique geological settings.