Hierarchically porous magnetic biochar as an efficient amendment for cadmium in water and soil: Performance and mechanism

Three types of hierarchically porous magnetic biochars (HMBs) were prepared by pyrolyzing low-cost wheat straw and potassium ferrate (K₂FeO₄) under a nitrogen atmosphere at 600, 700 and 800 °C, respectively, which could be used as amendments for cadmium (Cd) in water and soil. HMB fabricated at 700 °C (HMB700) had the best remediation performance for Cd in water and soil, which was mainly due to its largest specific surface area and micropore volume. Batch sorption experiments showed that Cd(II) sorption onto HMBs were well-described by a pseudo-second-order model and Sips (Freundlich-Langmuir) model, indicating that HMBs removed Cd(II) mainly through chemical adsorption. MINTEQ modeling evidenced that HMBs adsorbed Cd(II) mainly through precipitation rather than surface complexation. The adsorption behavior of HMB700 to Cd(II) could be explained by surface complexation (-OCd, -COOCd), precipitation (Cd(OH)₂ and CdCO₃), physical adsorption (rich pore structure) and ion exchange (K⁺, Ca²⁺, Mg²⁺). Furthermore, adding HMBs (1 wt%) (incubation 60 days) could also significantly increase soil pH and electrical conductivity (EC), and significantly reduce the available Cd content in soil (47.97%-61.38%). Adding HMBs could promote the conversion of bioavailable to less bioavailable Cd forms. These results provided a new idea for fabricating agricultural waste-based HMBs to remediate Cd in water and soil.