Simultaneous adsorption and immobilization of As and Cd by birnessite-loaded biochar in water and soil

A novel biochar was prepared by loading birnessite to improve its capability to simultaneously adsorb As(III), As(V), and Cd(II) in water and soil. Layer sheet-structured birnessite was successfully loaded onto the biochar surface with increased functional groups. SEM, XRD, and FTIR combining with XPS analysis were utilized to characterize birnessite-loaded biochar and its adsorption mechanisms for As and Cd(II). The saturated adsorption capabilities of the birnessite-loaded biochar (BRB) for As(III), As(V), and Cd(II) were as large as 3543, 2412, and 9068 mg/kg (calculated by Langmuir isotherm model), much higher than for the corresponding non-loaded biochar (no adsorption of As, 4335 mg/kg for Cd). Adsorption of Cd and As onto BRB was controlled by multi mechanisms; Cd(II) appeared to coordinate to vacant sites of birnessite, while As formed surface complex with functional groups. Furthermore, BRB showed higher abilities for co-adsorption of As(III) and Cd or As(V) and Cd, which may be due to the formation of Cd₃(AsO₄)₂ surface precipitate as well as synergistic reaction between anions and cations. After conditioning to soil, BRB showed potential for Cd and As remediation under both flooded and unflooded conditions. These results suggested that BRB can be used as an effective sorbent for simultaneous immobilization of heavy metals, especially As and Cd, in environmental and agricultural systems.