Solidification and stabilization of Cr( VI ) contaminated soil by alkaline industrial waste

Basic oxygen furnace slag (BOFS), a calcium‐rich alkaline industrial waste, can effectively immobilize Cr(III) but shows limited remediation efficacy for Cr(VI)‐contaminated soil. In this study, a synergistic remediation system (VC–BCF) combining a BOFS‐based composite (BCF, produced from BOFS, carbide slag (CCR), and flue‐gas desulfurization (FGD) gypsum) with ascorbic acid (VC) was developed. CCR and FGD synergistically activated BOFS, enhancing hydration and promoting the formation of ettringite (AFt), C‐S‐H, and C‐A‐S‐H, while VC efficiently reduced Cr(VI) to Cr(III). The remediation performance was evaluated by unconfined compressive strength (UCS) and leaching tests. Under the optimum parameters (60% liquid‐to‐solid ratio, 0.8% VC, 15% BCF), the 28‐day leaching concentrations reached minima of 0.108 mg/L (total Cr) and 0.037 mg/L (Cr(VI)), and the UCS reached 1.47 MPa. A quantitative synergy analysis showed that the VC–BCF combination immobilized Cr far more effectively than the additive prediction of the individual reagents (synergy factor ≈2.3), confirming a significant synergistic interaction between chemical reduction and matrix encapsulation. The Risk Assessment Code (RAC) of the treated soil decreased from 49.34% (high risk) in the untreated soil to 13.84%–14.65% (medium risk), indicating substantially reduced environmental mobility. Mechanistic analyses suggest that Cr immobilization proceeds through Cr(OH) 3 precipitation following VC reduction, encapsulation/adsorption by C‐S‐H/C‐A‐S‐H/AFt, pore filling, and ion exchange; the assignment of isomorphous substitution of Cr(III) for Al(III) in AFt is inferred from indirect evidence and requires direct spectroscopic confirmation (e.g., XPS, XANES/EXAFS, HRTEM‐SAED) in future work.