Purifying cyanide-bearing wastewaters by electrochemical precipitate process using sacrificial Zn anode

With the rapid development of society, wastewaters, such as cyanide-containing wastewaters (CBWs) have caused environmental problems. In the present work, an electrochemical approach using sacrificial Zn anode was investigated for the removal of cyanides from CBWs. The effects of operational parameters, such as current density, pH, initial cyanide concentration, and ionic strength, on the cyanides removal from synthetic solution were discussed in turn. Under the optimal conditions obtained, the treatment of industrial CBWs was considered. Subsequently, more attentions were paid to elucidate the removal mechanisms of cyanide ions and the corresponding metal-cyanide (copper and iron) complexes by a combination of cyclic voltammetry (CV), pHPZC, X-Ray diffractometer (XRD), scanning electron microscope with energy disperse spectroscopy (SEM/EDS) and X-ray photoelectron spectrometer (XPS) characterizations. Experimental results demonstrated that the removal efficiency of total cyanide (CNT), Cu, and Fe from industrial CBWs are 98%, 91%, and 96%, respectively, with an anode consumption of 1.78 kg/m3 and energy consumption of 2.50 kW·h/m3, of which 72% of Cu was collected on the cathode and almost all of Fe was in the precipitate. Removal mechanisms suggested that free cyanide (CN–) mainly presented as Zn(CN)2 into the electrolytic precipitate. The replacement of Zn2+ and electroreduction promoted 72% of Cu(I) to be reduced, while 57% of the remaining portion of Cu(I) was oxidized to Cu(II) and 43% of Cu(I) could form CuSCN into the precipitate by XPS analysis. With respect to Fe, it was mainly ascribed to the formation of Zn2Fe(CN)6 precipitate to be removed. From above, this work provided a method for enriching cyanides from wastewaters into the precipitate, while valuable metal Cu was deposited on the cathode, facilitating the separation and recovery of valuable resources.