Removal of hexavalent chromium in water by chitosan-modified Enteromorpha prolifera biochar loaded with iron-manganese oxides: Application performances and reaction mechanisms

This study successfully synthesized magnetic iron-manganese biochar (F1M3BC) and iron-manganese chitosan biochar (Chi-F1M3BC) using Enteromorpha prolifera (EP) as the raw material, aiming to efficiently remove hexavalent chromium (Cr(VI)) from water. Compared to F1M3BC, Chi-F1M3BC, after chitosan cross-linking encapsulation, exhibited significant advantages in terms of specific surface area, pH adaptability, and adsorption performance. The specific surface area of Chi-F1M3BC reached 101.47 m³/g, approximately double that of F1M3BC (52.89 m³/g), making it rich in reactive groups for Cr(VI) removal. The addition of chitosan elevated the pHpzc value of Chi-F1M3BC from 5.5 to 9, and the abundant active groups enabled it to efficiently remove Cr(VI) over a wider pH range. Under 30 °C conditions, 0.5 g/L of Chi-F1M3BC could completely remove 30 mg/L of Cr(VI) solution within 1 h. Adsorption isotherm and kinetic studies indicated that Chi-F1M3BC complied with the Langmuir model and pseudo-second-order kinetics. At 30 °C, the adsorption capacity of Chi-F1M3BC reached 104.5 mg/g, approximately twice that of F1M3BC (58.93 mg/g). Moreover, after five consecutive usage cycles, the removal efficiency of Cr(VI) by Chi-F1M3BC only slightly decreased by 3%, demonstrating its superior stable removal performance. Subsequent characterization and experimental investigations elucidated the elimination of Cr(VI) by Chi-F1M3BC, achieved through the synergistic interplay of ion exchange, electrostatic attraction involving surface hydroxyl and amino functional groups, chelation, and the reduction of iron and manganese oxides. In conclusion, the synthesized Chi-F1M3BC has demonstrated its efficiency and stable adaptability in removing Cr(VI), coupled with the additional benefit of utilizing waste Enteromorpha prolifera.