Sustainable leaching of nickel and cobalt from asbestos waste using deep eutectic solvents: Kinetic modeling and recovery performance

• Asbestos waste valorized via ligand-functionalized deep eutectic solvent. • Selective Ni/Co recovery (>72%) with <8% Fe/Mg/Si co-dissolution achieved. • Leaching mechanism scrutinized by FTIR, 1 H NMR, and kinetic modeling. • Autocatalytic Prout–Tompkins kinetics confirmed by R 2 > 0.99. • Diffusion-limited process evidenced by low E a (13.7–15.7 kJ/mol). A sustainable hydrometallurgical route was developed for the selective recovery of nickel and cobalt from asbestos production waste using a novel deep eutectic solvent (DES) composed of ethylene glycol and hydroxylamine hydrochloride. Magnetic separation yielded a Ni–Co-enriched fraction with a 10–12-fold increase in target metal content relative to the raw waste, as confirmed by WD-XRF, SEM/EDS, and chemical analysis. Under optimized conditions (80 °C, 4 h, L/S = 4:1), leaching efficiencies of 72.4% for Ni and 79.6% for Co were achieved, while co-dissolution of Fe, Mg, and Si remained below 8%, demonstrating exceptional selectivity. Kinetic analysis revealed a dual-control mechanism: the excellent fit to the Prout–Tompkins model (R 2 > 0.99) reflects autocatalytic, ligand-assisted surface reactions driven by Ni 2+/ Co 2+ coordination with DES components, whereas the low activation energies (15.73 kJ/mol for Ni, 13.68 kJ/mol for Co) and strong agreement with the Jander diffusion model indicate that mass transfer through the evolving porous residue governs the overall rate. Critically, 1 H NMR spectroscopy of the spent DES shows pronounced signal broadening and attenuation due to paramagnetic Ni 2+/ Co 2+ ions, providing direct, solution-phase evidence of metal - ligand complexation - a key advancement over indirect FTIR data alone. Complementary SEM and XRD analyses confirm selective phase dissolution without bulk matrix degradation. These results establish the DES as an environmentally benign, ligand-functionalized leaching medium that enables the valorization of hazardous asbestos waste into a secondary source of critical metals, aligning with green and circular hydrometallurgical principles.