Removal of arsenic from aqueous solution using a surface-functionalized magnetic biochar derived from Chlorella

Magnetic biochar microspheres were designed as a promising material for arsenic remediation, specifically for the challenging removal of inorganic As(III). In this study, Fe₃O₄/BC-α-MnOOH@A microspheres, which combine redox and adsorption on a single regenerable stage. Experimentally, Fe₃O₄ nanoparticles were produced by solvothermal decomposition, using the Algal biomass of Chlorella as BC source, while As(III) served as a representative heavy metal contaminant. A key innovation involved functionalizing the thermal decomposition particles with α-MnOOH and sodium alginate (SA), thereby altering their surface characteristics to strengthen interactions with As(III). Characterization by SEM, XRD, FTIR, Raman, and XPS confirmed the successful integration of α-MnOOH nanoneedles, revealed the formation of inner-sphere complexes, and verified the oxidative conversion of As(III) to As(V) upon adsorption. Fe₃O₄/BC-α-MnOOH@A exhibited rapid adsorption kinetics, reaching equilibrium within 2 h, and achieved 99.6% As(III) removal at pH 7 using a sorbent dosage of 2 g L^-1 (C₀ = 6 mg L^-1), significantly outperforming Fe₃O₄/BC-α-MnOOH@A, Fe₃O₄/BC, and pristine BC. Performance was robust from pH 3-10, with modest declines at high pH due to OH^- competition. Co-existing ions Cl^-, SO₄^2-, Na⁺, Ca²⁺, and Mg²⁺ had negligible effects, while PO₄^3- reduced uptake, but the Fe₃O₄BC-α-MnOOH@A remained superior to controls. The actual drinking, tap water, and seawater trials demonstrated an As(III) uptake of 2.94, 2.92, and 2.91 mg g^-1, while four adsorption-desorption cycles maintained good performance with minimum leaching of Fe/Mn, confirming the stability of the operation. Overall, these results established a multifunctional and scalable As(III) remediation approach from an aqueous system.