Enhanced Cr(VI) removal from acidic solutions using biochar modified by Fe₃O₄@SiO₂-NH₂ particles

Fe₃O₄@SiO₂-NH₂ magnetic particles with core-shell structure were attached on carboxylated biochar derived from phoenix tree leaves to synthesize a novel magnetic biochar for removing Cr(VI) ions from acidic solutions. FSEM, FTEM, XRD characterizations of the synthesized magnetic biochar revealed that the Fe₃O₄@SiO₂-NH₂ magnetic particles distributed uniformly on the surface or macrospores of carboxylated biochar by strong chemical bonding. The Cr(VI) ions adsorption capacity of magnetic biochar was 27.2mg·g^-1, surpassing original carboxylated biochar (18.2mg·g^-1). VSM and XPS characterizations demonstrated that the attached Fe₃O₄@SiO₂-NH₂ magnetic particles not only endowed biochar with perfect magnetic property (23emu·g^-1) but also provided complexing sites for binding Cr(III) cations reduced from Cr(VI) anions. The Cr(VI) ions removal by magnetic biochar contained three steps: (1) adsorption of Cr(VI) anions by protonated functional groups; (2) reduction of Cr(VI) anions to Cr(III) cations by electron-donor groups; and (3) chelation of Cr(III) cations by amine groups. The adsorption recycling test showed that magnetic biochar kept 85% of its initial Cr(VI) adsorption capacity at the sixth cycle, and the Fe leakage under pH1.0 was smaller than 0.25mg·L^-1. The results indicated that this novel magnetic biochar was applicable for the practical treatment of Cr(VI)-containing wastewater.