Three-in-One: Coupling Chemical Reduction, Photoreduction, and Ion-Exchange Mechanisms in Greigite/Red Soil-Based Analcime Zeolite Composites for Cr(VI) Remediation in Groundwater

Simultaneously reducing and immobilizing Cr(VI) anions in groundwater are crucial to minimize their hazardous effects. Herein, benefiting from the feasible chemical-/photo-reduction capability of greigite, together with the ion-exchange functionality of zeolites, the integrated three-in-one function in greigite/Re-ANA composites (GRAs) material has been employed for removing and immobilizing Cr(VI) in groundwater remediation. GRA-3 achieved a 99.4% Cr(VI) removal efficiency in a Cr(VI) solution with 1 mg/L. Cr(VI) residue concentration is 0.006 mg/L, complying with the control guideline in drinking water according to WHO regulations. A comprehensive mechanism study reveals that the chemical reduction and photoreduction processes are involved within greigite, which drives the process of reducing Cr(VI) to Cr(III). Subsequently, the as-reduced Cr(III) is either adsorbed on a greigite surface or ion-exchanged with zeolite microporous channels, thus getting immobilized. Furthermore, environmentally relevant Cr(VI) contaminated groundwater sampling from the chromate plant has been employed for the fixed-bed column study, which achieved the discharge standard for practical Cr(VI) polluted groundwater. This work reported a composite material matrix with great potential and significance for Cr(VI) remediation, which could be employed in permeable reactive barriers for Cr(VI)-polluted groundwater remediation in the future.