Immobilization mechanisms of Cr(VI)/Cr(III) in red mud-based binder: Insights from Cr-S-Fe interactions and hydration...

Chromium (Cr)-contaminated soils from electroplating and metallurgical industries have emerged as a critical environmental challenge. In this study, a novel RGCD (RM-GGBFS-CS-DG) binder composed of red mud, ground granulated blast furnace slag, carbide slag, and desulfurization gypsum was developed for the stabilization/solidification of Cr. The results revealed that at 0.25 wt% Cr(VI) and Cr(III) dosages, the leached concentrations of total Cr decreased to 2.25 mg/L and 10.2 μg/L after 28 d of curing, respectively, with the immobilization efficiencies reaching 99.1 % and 99.9 %. Even at elevated 1.00 % dosage, the immobilization efficiencies remained above 98 %. The pseudo-second-order kinetic model accurately described the immobilization processes for both Cr(VI) and Cr(III). The low-valence sulfur and reactive Fe^2 + in the binder exhibited strong reductive capacity, effectively reducing highly toxic Cr(VI) into less toxic Cr(III). In the system, Cr(VI) primarily formed CaCrO₄ precipitates or was incorporated into ettringite via anion exchange (CrO₄^2-/SO₄^2-). Meanwhile, Cr(III) predominantly formed Fe-Cr hydroxide coprecipitates and may partially substitute for Al³⁺ in C-(A)-S-H gels through ion exchange (Cr³⁺/Al³⁺). ²⁹Si MAS NMR analysis revealed that highly polymerized, long-chain C-(A)-S-H gels exhibit superior Cr immobilization capacity, which may be attributed to their enhanced structural stability. However, higher Cr(VI) and Cr(III) dosages inhibited hydration gel formation and disrupted the development of layered gel networks.