Efficient and selective removal of chromium (VI) by sulfide assembled hydrotalcite compounds through concurrent reduction and adsorption processes

Abstract

It is usually difficult to design selective, stable and efficient adsorbent for the reduction and adsorption of Cr(VI) oxoanions from wastewater. Herein, we report an effective, selective and easily recyclable adsorbent for the elimination of Cr(VI) oxoanions using sulfide assembled hydrotalcite of Fe/Mg/Al hydroxides. Various characterization tools such as XRD, FTIR, SEM, XPS, CHN and ICP were applied to confirm the layered structure and chemical compositions. The prepared adsorbent displayed fast kinetics, high adsorption capacity and outstanding selectivity to Cr(VI) removal. The factors affecting Cr(VI) removal including pH, adsorbent dosage, contact time and initial Cr(VI) concentration was studied. Under the optimum conditions, >99.99% Cr(VI) was removed in 180 min with remarkable distribution coefficients of (Kd > 107). The maximum removal capacity (135.59 mg/g) placed the prepared adsorbent on the top of materials list known for Cr(VI) removal. Pseudo-second-order kinetic model expresses well the adsorption behavior towards Cr(VI), indicating a chemisorption process, while the sorption isotherm agreed with the Langmuir model, suggesting monolayer adsorption. The (S)2− in MoS42− was involved in reducing Cr(VI) to Cr(III) while itself simultaneously being oxidized to SO42− and the redox reaction was evidenced by XPS analysis. Moreover, due to the unique memory effect of layered double hydroxides, significant stability and excellent reusability were also achieved. Based on the results obtained, hybrid adsorbent by embedding sulfide into hydrotalcite of Fe/Mg/Al hydroxides can be considered as one of a compelling candidate for the remediation of hexavalent chromium.