Purpose <p>Cadmium (Cd) contamination in reservoir sediments poses significant ecological risks due to its high toxicity and mobility. This study aimed to investigate the adsorption-desorption behavior and speciation transformation of Cd at the sediment-water interface in Luhun Reservoir, China, and to elucidate the driving mechanisms under key environmental variables (pH, ionic strength, temperature).</p> Materials and methods <p>Surface sediments with low, medium, and high background Cd levels were collected. Batch equilibrium experiments were conducted under varying pH (5.0–9.0), ionic strength (0.01–0.10&#xa0;mol L^− 1 NaCl), and temperature (15–35℃). Cd adsorption-desorption kinetics and isotherms were analyzed, and chemical speciation was determined using the BCR sequential extraction procedure. Kinetic and isotherm models were applied to elucidate the mechanisms.</p> Results and discussion <p>The average Cd concentration (2.01&#xa0;mg kg^− 1) exceeded background values. Although the residual fraction (F4) dominated (68.3%), labile fractions (F1 + F2) comprised ~ 30%, indicating potential ecological risk. Cd adsorption followed pseudo-second-order kinetics (R²=0.99) and Freundlich isotherms, suggesting multilayer adsorption. Higher pH enhanced Cd retention by shifting speciation from F1 to F2, while increased ionic strength competitively inhibited adsorption and promoted release. Temperature minimally affected adsorption but significantly increased desorption ratios (e.g., from 27.6% at 15℃ to 65.2% at 35℃ in low-Cd sediment).</p> Conclusions <p>Cadmium in Luhun Reservoir sediments presents a notable remobilization risk, particularly under acidification and salinity stress. Environmental factors, especially pH, critically regulate Cd mobility and speciation. The findings provide mechanistic insights and a scientific basis for assessing and managing Cd pollution risks in reservoir environments.</p> Graphical Abstract <p></p>