Redox-mediated Cr(VI) removal in anaerobic passive columns using waste Fe-sludge containing indigenous Fe-reducing bacteria

This study evaluates the long-term performance of a passive column system for Cr(VI) remediation using waste Fe-sludge composed mainly of poorly crystalline FeOOH and harboring indigenous Fe(III)-reducing bacteria. Batch tests confirmed higher Cr(VI) adsorption than Cr(III), but Cr(VI)-loaded sludge exceeded toxicity characteristic leaching procedure (TCLP) limits (7.7 mg/L), underscoring the need for reductive immobilization. Two continuous-flow columns were operated for 202 days: an Fe/Compost-column, in which compost and later sake lees were added as slow-release electron donors, and an Fe-only control. The Fe/Compost-column consistently maintained effluent Cr(VI) < 0.2 mg/L and total Cr < 2.0 mg/L at hydraulic retention times (HRTs) ≥ 6 h, whereas the Fe-only column lost capacity within ∼46 days but recovered transiently with yeast extract, indicating carbon-limited microbial activity. Depth-resolved solid analyses revealed a depth-dependent adsorption-reduction mechanism, with strong inverse correlations between total Cr and Fe (R² > 0.99) and progressive Cr(VI) reduction with depth; Cr K-edge X-ray absorption near-edge structure (XANES) showed up to 93 % Cr(III) in the lower Fe/Compost section compared to 56 % in the control. Fe K-edge XANES and X-ray diffraction (XRD) confirmed sustained Fe(III)/Fe(II) cycling without major mineralogical changes. TCLP confirmed that Cr immobilized in middle and lower sections met the U.S. disposal limit (≤ 5 mg/L). These results demonstrate that waste Fe-sludge, coupled with waste-derived organics, enables redox-mediated Cr(VI) detoxification and long-term immobilization, offering a practical strategy for the passive treatment of acidic Cr-contaminated waters.