Sequential chelator-assisted extraction and FeCl₃/CaO-induced immobilization of hexavalent chromium from contaminated...

Hexavalent chromium (Cr(VI)) poses persistent environmental risks due to its high solubility and toxicity in alkaline solid wastes. This study introduces a sequential chelator-assisted extraction and dual-step immobilization strategy integrating a biodegradable chelator (HIDS), ferric chloride (FeCl₃), and calcium oxide (CaO) for efficient Cr(VI) remediation in solid wastes, including contaminated soil and alkaline biomass fly ash. During the chelator-assisted washing step, optimal Cr(VI) extraction from soil was achieved using 10 mmol L^-1 HIDS at pH 3, with a liquid-to-solid ratio of 10:1 and a contact time of 1 h. However, the HIDS-washed soil residues exhibited an elevated total Cr leachability due to surface-bound residual chelator species acting as secondary extractants. Subsequent FeCl₃ treatment (20 mmol L^-1) partially reduced total Cr leaching, whereas sequential CaO addition (25 mmol L^-1) further suppressed the water-leachable total Cr concentration to below the regulatory threshold for Cr(VI) (0.05 mg L^-1). Application of the optimized protocol to two biomass-derived fly ashes confirmed its robustness, yielding water-leachable total Cr concentrations of 0.021 and 0.037 mg L^-1, both well below the regulatory standard. Sequential extraction and spectroscopic analyses (FE-SEM/EDX, XRD, FT-IR, XPS) revealed that Cr(VI) stabilization primarily governed by Fe(III) oxyhydroxide formation and co-precipitation within Ca-bearing mineral matrices. Overall, the environmentally benign HIDS-FeCl₃/CaO system provides a sustainable remediation route that couples efficient extraction with durable immobilization, offering a practical framework for mitigating Cr(VI) risks in soils and alkaline wastes.