Enhanced irreversible stabilisation of hexavalent chromium in field-scale industrial waste disposal

Numerous industrial processes generate hazardous by-products, including persistent pollutants like chromium which pose a threat. Safe and cost-effective management of chromium is a major challenge in developing countries. This study investigates the selection and efficacy of chemical stabilizing agents for reducing hexavalent chromium (Cr(vi)) to trivalent chromium (Cr(iii)) in contaminated soil from a mining region in India. Various combinations of stabilizing reagents-including ferrous sulphate (FeSO₄), sodium sulphide (Na₂S), sodium sulphite (Na₂SO₃), sodium metabisulphite (Na₂S₂O₃), and sodium thiosulphate (Na₂S₂O₃)-in conjunction with binding agents (fly ash and lime) were evaluated using response surface methodology. The optimized reagent combinations resulting from the response outcome were applied to the contaminated soil samples to assess for irreversibility, leachability, and longevity, ensuring adherence to landfill disposal standards. The study established sodium thiosulphate as the most potent stabilizing reagent, requiring a mere 3.00% (by weight) dosage for treating Cr(vi) contaminated soil by 99.56% while maintaining irreversibility. Analytical determinations using X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), and Fourier transform infrared spectroscopy were performed to determine the crystallinity, surface morphology, and functional groups present in the stabilized material synthesized with Na₂S₂O₃ to devise a stabilization reaction mechanism of Cr(vi) to Cr(iii) transformation. Ultimately, an economic analysis comparison supported the establishment of a combination of Na₂S₂O₃, lime, and fly ash in the ratio of 3.00% : 35.00% : 29.00% as the most cost-effective solution, surpassing conventional reagents' expenses by up to 356.00%.