Removal of cyanide by peroxydisulfate activated by copper ions inherently present in the electroplating wastewater

Cyanide is widely used in electroplating processes to ensure the quality of metal plating. Alkaline chlorination process is commonly employed to remove cyanide from electroplating wastewater. However, several drawbacks exist, including the formation of toxic intermediates, high consumption of chemical reagents, and low removal efficiency of metal-cyanide complexes. In this study, persulfate advanced oxidation process was explored for the removal of cyanide from electroplating wastewater. The metal ions typically present in the electroplating wastewater, including Cu2+, Zn2+, Fe2+, Ag+ and Ni2+, may activate peroxydisulfate (PDS) to oxidize cyanide. Among these metal ions, cyanide can be most effectively removed in the presence of Cu2+ and PDS, likely due to the activation of PDS by Cu2+ and Cu(II)-cyanide complex. The results of lab studies indicated that 99 % removal of cyanide (4 mM) could be achieved in the presence of 10 mM PDS and 1 mM Cu2+ within 20 min. The results of radical scavenging experiments, radical probe tests and electron paramagnetic resonance (EPR) studies showed that sulfate radical (SO4•−) and hydroxyl radical (OH•) were primarily responsible for cyanide removal and the complexation of cyanide and Cu2+ significantly enhanced the formation of SO4•−. Cyanate and nitrite were the main by-products detected. For real electroplating wastewater, the addition of 40 mM PDS could almost completely remove cyanide (19.5 mM) in 20 min.