Adsorption behaviour and mechanisms of cadmium and nickel on rice straw biochars in single- and binary-metal systems

Adsorption mechanisms and competition between Cd²⁺ and Ni²⁺ for adsorption by rice straw biochars prepared at 400 °C (RB400) and 700 °C (RB700) were investigated in this study. Based on the Langmuir model, the maximum adsorption capacities (mg g^-1) of Cd²⁺ and Ni²⁺ on RB400 and RB700 were in the order of Cd²⁺ (37.24 and 65.40) > Ni²⁺ (27.31 and 54.60) in the single-metal adsorption isotherms and Ni²⁺ (25.20 and 32.28) > Cd²⁺ (24.22 and 26.78) in the binary-metal adsorption isotherms. Cd²⁺ competed with Ni²⁺ for binding sites at initial metal concentrations >10 mg L^-1 for RB400 and > 20 mg L^-1 for RB700. The adsorption sites for Cd²⁺ and Ni²⁺ on the biochars largely overlapped, and the binding of Cd²⁺ and Ni²⁺ to these sites was affected by the occupation sequence of these metals. For Cd²⁺ and Ni²⁺ adsorption in the binary system, cation exchange and precipitation were the dominant adsorption mechanisms on RB400 and RB700, respectively, accounting for approximately 36% and 60% of the adsorption capacity. Competition decreased the contribution of cation exchange but increased that of precipitation and other potential mechanisms. Results from this study suggest that types and concentrations of metal ions should be taken into account when removing metal contaminants from water or soil using biochars.