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

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.