precipitation chelation of cyanide complexes in electroplating industry wastewater

electroplating industry wastewater (eiw) characterized by high chemical oxygen demand (cod) is a big source of water and air contamination with heavy metals. the formation of cyanide complex with heavy metals is responsible for its elevated cod. the concentration of heavy metals in eiw can be removed by the use of different precipitating agents (sulfide and hydroxide). but the major bottleneck in the removal of these metals is the presence of cyanide in eiw resulting in chelation with all the metals that are soluble in water. the present work focused on, the treatment of eiw containing cr, ni, zn and cn and the optimization of dosage concentration which was reliable for the dissociation of cyanide complex for maximal removal efficiency. we used hydroxide, sulfide and carbonate precipitation from different precipitating agents (naoh, ca (oh)2, caco3 na2s5h2o, nahs and nahso3). sulfide precipitation was a viable option for the treatment of eiw as compared to hydroxide and carbonate precipitation. moreover, cod reduction capacity of sulfide precipitation was higher than others. it was also found that ni and cr made a complex with cyanide that halted the removal efficiency while there was no evidence for zn complexation; otherwise fragile complexation was evidenced.