Efficient and rapid removal of chromium(VI) from water via NH₂-MIL-101(Fe)/NiAl-LDH composite adsorbent

Due to the high toxicity and carcinogenicity of Cr(VI), wastewater should be treated to remove Cr(VI) before discharge. In this study, water purification was achieved through the adsorption of Cr(VI) using an Fe-MOF/NiAl-LDH composite (FML), where the Fe-MOF was grown in situ on the NiAl-LDH. This composite used AlOOH, a by-product of the aluminum-water hydrogen production system, as one of its precursors, thereby achieving resource reuse. Since the material has a higher specific surface area and more active adsorption sites than NiAl-LDH and Fe-MOF, with abundant anions and water molecules interspersed between the layers, the hexavalent chromium can be effectively removed by electrostatic adsorption on the surface of the material, interlayer anion exchange, and reduction reaction. The effects of composite material type, contact time, solution pH, adsorbent dosage, initial Cr(VI) concentration, competitive ions, and reuse on the adsorption efficiency were investigated in this study. The results indicated that when the initial concentration of Cr(VI) was 20 mg L^-1, the adsorbent dosage was 0.8 g L^-1, and pH was set at 6, 15-FML achieved the best removal efficiency for Cr(VI), with a removal rate of 91.08 % in 60 min. The adsorption isotherm of this material closely followed the Langmuir adsorption model, with a maximum adsorption capacity of 42.73 mg g^-1 for Cr(VI). The materials prepared in this study exhibited excellent regenerative capacity, with the Cr(VI)-loaded adsorbents effectively regenerated using a Na₂CO₃ solution. The regenerated materials could be reused multiple times without significant changes in adsorption capacity or structural integrity.