Decomplexation of copper complexes from synthetic and real electroplating wastewater by alkaline-thermal synergistic activation of persulfate and stabilization of generated sludge by Fe3O4 coating

It is difficult to remove heavy metal complexes from wastewater due to the strong bond between organic ligands and heavy metal ions. In this study, a two-step decomplexation-Fe3O4 coating process has been developed to treat Cu-containing wastewater (Cu(II)-EDTA simulated wastewater and real electroplating wastewater), which could not only remove Cu complexes from wastewater but also stabilize the generated Cu sludge. In the decomplexation step, an alkaline-thermal synergistic activation of persulfate was used for the simultaneous decomplexation of Cu complexes and the generation of Cu sludge by maintaining the pH and reaction temperature at 11 and 60 °C, respectively. The removal efficiency of Cu(II) could reach nearly 100 % after 60 min of treatment of 3.14 mM Cu(II)-EDTA. Reactive oxygen species (ROS), including SO4−, OH, and O2−, generated by alkaline-thermal synergistic activation of persulfate were responsible for the decomplexation of Cu complexes. In the subsequent Fe3O4 coating step, Cu sludge could be stabilized by the formation of Fe3O4 coating (CuO@Fe3O4). With the protection of Fe3O4 coating, the leaching concentration of copper in the TCLP test decreased from 272.0 mg/L (CuO) to 80.1 mg/L (CuO@Fe3O4), which was lower than the leaching concentration limit (<100 mg/L) in the Identification Standards for Hazardous Wastes-Identification for Extraction Toxicity (GB 5085.3–2007) of China. CuO@Fe3O4 could further serve as both the Fe and Cu source for the preparation of Cu/Fe/C micro-electrolysis materials, which thoroughly converted the Cu sludge — the hazardous waste — into valuable materials for wastewater treatment.