Roles of soluble minerals in Cd sorption onto rice straw biochar

Transforming to biochar provides an environmentally friendly approach for crop residue reutilization, which are usually applied as sorbent for heavy metal removal. As typical silicon-rich material, the specific sorptive mechanisms of rice straw derived biochar (RSBC) are concerned, especially at the low concentration range which is more environmentally relevant. In the present study, Cd sorption onto RSBCs at the concentration of ≤ 5 mg/L was investigated. The sorptive capacity was positively correlated with the pyrolytic temperature of the biochar and the environmental pH value. Water soluble minerals of the RSBCs played the dominant roles in Cd sorption, contributing 29.2%, 62.5% and 82.9% of the total sorption for RSBCs derived under 300°C, 500°C and 700°C, respectively. Increased number of cations, dominantly K⁺, were exchanged during the sorption. Coprecipitation with cations and carbonates may also be contributive to the sorption. The dissolution of silicon-containing minerals was found to be declined during sorption, suggesting its involvement in the sorption process, possibly through precipitation. Whilst, the sparingly soluble silicate crystals may impose ignorable role in the sorption. Complexation with organic groups is only a minor mechanism in Cd sorption, compared to the much more dominant roles of the inorganic ashes.