BiVO₄-assisted photocatalytic ozonation for efficient cyanide degradation in synthetic silver post-leaching effluents

Cyanide is extensively used in hydrometallurgical leaching due to its strong metal-chelating ability, yet its acute toxicity raises major environmental concerns. This study evaluates photoactivated BiVO₄ synthesized by the sol-gel method and combined with ozone for the degradation of metal-cyanide complexes in alkaline matrices (pH 10.5) containing silver, copper, iron, lead, and zinc, representative of effluents from silver leaching in the Mexican mining industry. Synthetic solutions were prepared and characterized to reproduce these conditions. Thermodynamic stability diagrams indicated that free cyanide and complexes such as Ag(CN)₂⁻, Cu(CN)₃²⁻, and Fe(CN)₆⁴⁻ persist at alkaline pH, while lead and zinc showed no tendency to form stable cyanide complexes. Oxidative treatments favored complex dissociation, with cyanate predicted as the main by-product; experimentally, ammonium was identified as the dominant degradation product. The BiVO₄/ozone system under irradiation demonstrated superior performance compared to individual processes, achieving faster cyanide abatement and improved efficiency. Kinetics followed a modified Langmuir-Hinshelwood model that accounted for both surface-mediated photocatalysis and bulk-phase ozone reactions. Complete cyanide removal was achieved in two consecutive cycles, though partial deactivation occurred in the third cycle due to BiVO₄ surface poisoning by adsorbed metals, confirmed by SEM/EDS and atomic absorption. Despite gradual loss of activity, the coupled process consistently removed both free and complexed cyanide while reducing ozone demand compared with conventional ozonation. These findings highlight photocatalytic ozonation with BiVO₄ as a promising and sustainable strategy for the treatment of metallurgical effluents, offering efficient pollutant degradation and lower oxidant consumption in comparison with conventional approaches.