Cyanide speciation as the determinant of treatment performance: A critical review of removal, destruction and recovery routes for cyanide-bearing wastewater

Cyanide-containing wastewater is a multicomponent system: cyanide occurs as free cyanide, within weak acid dissociable (WAD) cyanide and stable metal-cyanide complexes, and alongside thiocyanate, metals, and other impurities.The chemical form of cyanide and the composition of the aqueous matrix govern treatment performance, which makes direct comparison between technologies difficult.This review covers physical and physicochemical, chemical, electrochemical, photochemical, and biological methods; for each group, it examines the process mechanism, the target cyanide species, the treatment conditions, the secondary streams generated, the scope for recovery, and the level of technological maturity.Removal, destruction, and recovery of cyanide are treated as distinct outcomes because an identical removal efficiency may conceal a fundamentally different final fate of the pollutant.No universal treatment technology exists, and data obtained on model solutions cannot always be transferred to real multicomponent effluents.For concentrated process streams, the feasibility of cyanide extraction and reuse should be assessed first; where recovery is not viable, destructive processes followed by deep polishing are applied.Further work requires testing on real wastewater, lower energy and reagent consumption, smaller volumes of secondary waste, scale-up of promising processes, and integrated schemes in which recovery, destruction, and polishing are matched to the characteristics of the specific stream.