Dual-oxidant chlorination (NaClO/ClO₂) for enhanced degradation of metal–cyanide complexes

To mitigate the risk of toxic cyanide emissions from metal-plating wastewater, the development of treatment strategies capable of addressing recalcitrant metal-cyanide complexes is essential. In this study, a dual-oxidant system combining chlorine dioxide (ClO 2 ) with the conventional sodium hypochlorite (NaClO)-based alkaline chlorination process was investigated to enhance the degradation of metal-cyanide complexes, including tetracyanonickelate (Ni-CN), zinc cyanide (Zn-CN), and ferricyanide (Fe-CN). Under practical operating conditions (initial cyanide concentration of 50 mg/L, total chlorine of 50 mM, pH 10), the combined NaClO/ClO 2 system achieved significantly higher degradation efficiencies for all target pollutants (Ni-CN = 93.6 ± 1.3%, Zn-CN = 96.8 ± 0.0%, Fe-CN = 44.6 ± 1.1%) compared with the single NaClO system (Ni-CN = 27.8 ± 1.1%, Zn-CN = 87.1 ± 3.2%, Fe-CN = 13.9 ± 1.1) after 60 min of reaction. Electrochemical analysis showed that ClO 2 exhibited a higher redox potential, whereas NaClO primarily existed as the less oxidative hypochlorite ion at pH 10. The system demonstrated strong resistance to interference from inorganic anions and metal ions commonly found in plating wastewater. Furthermore, its application to real plating wastewater validated the practical feasibility of the system. These findings offer insight into a practical, adaptable, and easily integrable strategy for treating cyanide-bearing effluents using existing treatment facilities.