Mechanistic insights into the simultaneous removal of As(V) and Cr(VI) oxyanions by a novel hierarchical corolla-like MnO₂-decorated porous magnetic biochar composite: A combined experimental and density functional theory study

A novel hierarchical corolla-like MnO₂-decorated porous magnetic biochar composite (c-PMB/MnO₂) was synthesized and used for the removal of As(V) and Cr(VI) from aqueous solutions. The experimental results indicated that the adsorption affinity order of c-PMB/MnO₂ in the single-component system was As(V) (0.414 mmol/g) < Cr(VI) (0.421 mmol/g), whereas it was reversed in the binary-component system as As(V) (0.446 mmol/g) ≫ Cr(VI) (0.185 mmol/g), which were more pronounced in sequential adsorption systems. XPS results revealed that all components of c-PMB/MnO₂ (i.e., Fe₃O₄, MnO₂, and biochar) contributed to As(V) and Cr(VI) adsorption, while the selective reduction of adsorbed Cr(VI) to Cr(III) occurred via the redox reaction between Fe₃O₄ and Cr(VI). Density functional theory calculations further indicated that As(V) and Cr(VI) compete for the available binding sites in binary-component system, although the presence of reduced Cr(III) as a majority species serves as a strong binding site for As(V) via the formation of covalent bonding between Cr(III) and the O atom in As(V) with binding energies of − 123.1 and − 125.6 kcal/mol, thereby enhancing competitive As(V) adsorption in binary-component and sequential adsorption systems. These results may provide important information to better understand the competitive adsorption mechanisms for the simultaneous removal of As(V) and Cr(VI) in water.