Enhanced remediation of cadmium-polluted soil and water using facilely prepared MnO2-coated rice husk biomass

Metal oxide-modified biochar can effectively remediate or adsorb heavy metals in polluted soil and wastewater. However, the interaction between cellulose and hemicellulose on the surface of biochar/biomass and metal oxides is unclear, and the loading stability, adsorption capacity and mechanism of metal oxides remain to be explored. In this work, weakly crystalline birnessite (δ-MnO2) rice husk biomass composites (MBC) were prepared via hydrothermal impregnation and then used to immobilize heavy metals in cadmium-polluted soil and wastewater. The surface of MBC was covered by sheet-like manganese oxides with a regular layered stacking structure. The adsorption of Cd(II) by MBC could be mainly attributed to monolayer chemisorption, which could achieve a maximum adsorption capacity of 115.04 mg g−1. The addition of 1.0 % MBC reduced the effective Cd concentration in CaCl2 extractant in the soil from 0.24 mg kg−1 to 0.09 mg kg−1, and the highest decreasing rate reached 62.5 %. Additionally, MBC could also reduce the concentration of H2O-leachable Cd from 22.44 μg kg−1 to 9.50 μg kg−1. MBC facilitated the transformation of exchangeable Cd (EX-Cd) to iron-manganese bound Cd (OX-Cd) mainly comprising crystalline Mn oxide and Fe oxide. In addition, the mechanisms for MBC immobilization of Cd in polluted-soil and aqueous systems may include complexation-dominated and ion exchange-assisted processes, which contribute to a better understanding of the chemical structures and characteristics of metal oxides used to directly modify biomass. Overall, the findings indicate the possibility of applying MBC for soil and water remediation.