Dual adsorption–reduction mechanism of alginate-immobilized magnetic biochar beads for Cr(VI) removal

Hexavalent chromium [Cr(VI)] is a highly toxic and carcinogenic heavy metal commonly found in industrial wastewater. In this study, biochar, magnetic biochar, and alginate-immobilized biochar beads were prepared from pine and evaluated for Cr(VI) removal. The biochar produced at 600°C exhibited high aromaticity and microporosity, while Fe₃O₄ modification imparted magnetic properties and redox activity. Immobilization within alginate beads improved structural stability and reduced Fe leaching. The adsorption data fitted well to the Freundlich isotherm model (R² = 0.96–0.98), indicating heterogeneous multilayer adsorption, while kinetic analysis revealed that chemisorption dominated through electrostatic attraction and Fe²⁺/Fe³⁺-mediated reduction of Cr(VI) to Cr(III). Thermodynamic analysis confirmed that Cr(VI) adsorption onto the biochar (magnetic biochar) alginate beads was spontaneous and endothermic, accompanied by an increase in system randomness. Notably, the magnetic biochar alginate beads maintained over 91% removal efficiency even in the presence of competing anions (NO₃⁻, Cl⁻), exhibited minimal Fe leaching under acidic conditions, and retained over 70% removal rate after five regeneration cycles. These findings demonstrate that magnetic biochar alginate beads are stable and reusable adsorbents for Cr(VI) remediation in wastewater. • Alginate-immobilized biochar beads were successfully prepared from pine. • Fe₃O₄-loaded biochar gained magnetic properties and enabled Cr(VI) reduction via redox. • Alginate coating suppressed Fe leaching and improved the structural stability of the adsorbent. • Cr(VI) adsorption followed the Freundlich model through electrostatic attraction and redox. • Magnetic biochar/alginate beads resisted anions and retained >70% efficiency after reuse.