Synergistic adsorption and reduction for efficient Cr (VI) removal by sulfur-functionalized magnetic porous biochar

Cycad leaves were used as biomass feedstock to prepare a series of sulfur-functionalized magnetic biochar ( SMBC ) composites via KOH activation combined with impregnation-pyrolysis, which were applied for efficient hexavalent chromium Cr VI removal from aqueous solutions. The regulatory mechanism of sulfur doping on the microstructure, surface physicochemical properties, and Cr VI adsorption performance of magnetic biochar was systematically explored, focusing on clarifying the synergistic effect of coexisting Fe 3 O 4 and FeS phases in the composites. Results showed that sulfur doping significantly increased the material’s specific surface area, refined its pore structure, and constructed a Fe 3 O 4 − FeS synergistic system. SMBC exhibited excellent Cr VI adsorption under acidic conditions, with the synergistic mechanism providing key support for improved adsorption capacity and accelerated reaction kinetics. The adsorption process fitted well with the pseudo-second-order kinetic model and Elovich model ( R 2 > 0.97 ), confirming dominant chemisorption on heterogeneous surfaces. The Langmuir model best described the adsorption isotherm ( R 2 > 0.97 ), indicating a typical monolayer adsorption mechanism. Thermodynamic analysis revealed the adsorption was a spontaneous and endothermic process. Cr VI removal was attributed to the synergy of physical adsorption (e.g. pore filling, electrostatic attraction) and redox reaction. This study provides theoretical support for developing efficient, recyclable Fe 3 O 4 − FeS synergistic magnetic biochar adsorbents for Cr VI -contaminated water remediation.