The synergistic immobilization mechanism of Cr(VI) and Ni(II) in red mud-based ceramsite fabricated by multi-waste co-sintering

Confronting the dual crisis of hazardous waste accumulation and heavy metal pollution, this study employs a sustainable co-sintering technique at 1150°C for 15 min to transform red mud (RM), excavated soil (ES), and steel slag (SS) into high performance ceramsite. The occurrence states and migration transformation behavior of Cr and Ni in the ceramsite matrix were systematically investigated using multiple analytical techniques, including ICP, XRD, and XPS. The synthesized material demonstrates robust mechanical integrity with a 16.8 MPa compressive strength and minimal 0.81% water absorption. Under aggressive pH 1 leaching conditions, chromium and nickel release remains exceptionally low at 0.96 mg L⁻¹ and 0.81 mg L⁻¹ , achieving immobilization efficiencies of 99.3% and 96.6% respectively. Analytical characterization reveals that hexavalent chromium undergoes reduction to its trivalent form, subsequently stabilized within FeCr₂O₄ spinel structures, while divalent nickel integrates into CaNiSi₂O₆ and NiFe₂O₄ crystalline networks. This integrated approach converts problematic industrial residues into viable construction aggregates, simultaneously mitigating landfill pressures and establishing a sustainable model for resource recovery and environmental protection through atomic-scale contaminant immobilization.