Background Cadmium (Cd) pollution in agricultural soils has become a priority environmental concern globally. A reasonable application of passivators is critical to address the problem. In this study, we examined the remediation effects of rice husk biochar (rBC) and sepiolite (Sep) as single and combined (rBC + Sep) treatments on Cd pollution in a weakly alkaline soil using three maize cultivars (Liyu 16, Zhengdan 958, and Sanbei 218) as test crops. We also explained the mechanisms involved in the remediation effects. Results The pseudo-second-order kinetic equation and Langmuir model could well describe the adsorption process of rBC + Sep for Cd2+. Compared with the control treatment (CK), soil available Cd concentration decreased by 29.51-36.34% under rBC + Sep treatment (p< 0.05) and the Cd concentrations in maize grains of Liyu 16, Zhengdan 958, and Sanbei 218 decreased by 38.08-47.85%, 37.25-45.61%, and 33.96-46.15%, respectively (p< 0.05). Following passivation treatment, soil available Cd concentration decreased and gradually changed from the exchangeable and carbonate binding forms to the Fe/Mn oxide and residual forms. The bioconcentration factors of Liyu 16 (0.05-0.09) and Sanbei 218 (0.05-0.09) were lower than those of Zhengdan 958 (0.07-0.13). In addition, rBC +Sep treatment increased soil pH and soil electrical conductivity, but the differences were not significant (p> 0.05). Conclusions The application of 0.2% rBC + 0.5% Sep composite passivation material to weakly alkaline Cd-contaminated soil can effectively reduce the Cd concentration of soil and maize.
