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

Abstract

A Ni2+-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 Fe3+ wastewater was added instead of Ni2+. XRD and FTIR characterizations indicated that in the first step Ni2+ converted Zn–CN complex and free CN− into Ni(CN)42−. In the second step, ZnAl-layered double hydroxide (LDH) precipitated in 10min after the adding of Al. These results demonstrated that Ni(CN)42− formation improved CN removal as the adsorption of Ni(CN)42− 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)42−-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.