Synchronous cyanide purification with metals removal in the co-treatment of Zn-CN and Ni electroplating wastewaters via the Ni2+-assisted precipitation of LDH

A Ni-assisted method has been developed to remove cyanide and heavy metals synchronously from electroplating wastewater (Zn-EW). This process involved in the introduction of Ni wastewater in Zn-EW, followed by the addition of Al at pH = 12.0. After treatment, over 90% of cyanide was removed, much higher than 65% in our previous work where Fe wastewater was added instead of Ni. XRD and FTIR characterizations indicated that in the first step Ni converted Zn-CN complex and free CN into Ni(CN). In the second step, ZnAl-layered double hydroxide (LDH) precipitated in 10 min after the adding of Al. These results demonstrated that Ni(CN) formation improved CN removal as the adsorption of Ni(CN) on LDH was more effective than that of CN. Simultaneously, Ni, Zn and Al were removed and their residual concentrations were lower than regulated limitations. Furthermore, the trace contaminates in treated water was purified by the calcined Ni(CN)-LDH, after which the amounts of residual cyanide and metals met the discharging limitation. Therefore, the current work has proposed a self-purification process of cyanide and heavy metals in wastewaters, suggesting a promising strategy for wastewaters co-treatment