Synergistic Roles for Highly Efficient Removal of Cr(VI) from Aqueous Solution by Attapulgite-Loaded Sulfidized Nanoscale Zero-Valent Iron

Abstract

Recognizing the unique roles of attapulgite (ATP)-loaded and FeS_x in enhancing the dispersion and osxidation resistance of nanoscale zero-valent iron (nZVI) as well as facilitating a high electron transfer rate for removal of target pollutants is important but challenging, especially in hexavalent chromium (Cr(VI))-containing wastewater systems. Herein, S-nZVI@ATP, a composite material consisting of sulfidized nZVI loaded onto ATP, was utilized to remove Cr(VI), and the corresponding reaction mechanisms was explored. The findings revealed that the removal efficiency of Cr(VI) (R_Cr) for S-nZVI@ATP was 97.93% at S/Fe molar ratio (S/Fe_MRR) of 0.12, S-nZVI/ATP mass ratio (S-nZVI/ATP_MSR) of 4:1, pH of 3, and an initial Cr(VI) concentration of 20 mg/L. In the pH range of 3 to 7, S-nZVI@ATP exhibited excellent removal performance for Cr (VI), with the highest R_Cr 99.71% at pH 3. Coexisting ions such as SO₄²⁻, CO₃²⁻, PO₄³⁻, and HCO₃⁻ showed varying degrees of inhibition on the removal of Cr(VI). HCO₃⁻ displayed positive effects at concentrations of 10 and 15 mmol/L (R_Cr = 99.99%). The removal process followed the Pseudo-second-order kinetic model and Freundlich adsorption isothermal model, with an adsorption amount reaching 19.25 mg/g at equilibrium. Thermodynamic calculations revealed that the material adsorbed Cr(VI) onto the S-nZVI@ATP by spontaneous heat absorption. By studying the kinetics, thermodynamics, and adsorption isothermal model, analyzing the morphology of Fe and Cr, and characterizing the materials before and after the reaction, the removal mechanism of Cr(VI) was determined as adsorption-redox-co-precipitation.