This study evaluated the potential of montmorillonite as a reactive medium for Cr(VI) remediation through dithionite-mediated reduction of structural Fe(III) to Fe(II). Batch experiments revealed that dithionite treatment generated up to 19.1 mg/g Fe(II), representing 83% of total structural iron (23 mg/g). The reduction behavior was governed by redox potential equilibrium rather than reductant concentration. The generated Fe(II) effectively reduced Cr(VI) following a 3:1 stoichiometry, with all Fe(II) quantitatively consumed for Cr(VI) removal. Solid-phase analysis using SEM-EDS and XPS confirmed that reduced Cr(III) was immobilized as Cr(OH)₃ on clay surfaces, with no aqueous total chromium detected. In the column experiments, a 2 PV pulse injection of 50 mM dithionite lowered the ORP to -200 mV and generated structural Fe(II) within the medium. Following dithionite flushing, continuous injection of Cr(VI) solution exhibited delayed breakthrough up to 10 PV, indicating that the structural Fe(II) within the solid matrix served as an electron donor for Cr(VI) reduction. Sectional analysis and EPMA mapping confirmed immobilized Cr associated with montmorillonite particles. Dithionite reinjection successfully regenerated the reductive zone and restored the Cr(VI) removal capacity, thereby extending the operational lifetime of the reactive medium.

Keywords: Hexavalent chromium, Fe-bearing montmorillonite, Structural Fe(II), In situ reductive zone