Removal of Heavy Metals from Industrial Wastewater by Chemical Precipitation: Mechanisms and Sludge Characterization

Chemical precipitation using lime (Ca(OH)2), caustic soda (NaOH) and soda ash (Na2CO3) for the removal of simultaneous heavy metals (Cu(II) and Zn(II)) from industrial wastewater of the cable industry was carried out in laboratory by jar tests. For each reagent used, an improvement in copper and zinc removal efficiency was obtained by increasing the precipitating reagent dose (10–400 mg/L). Efficiencies of over 90% can be achieved. Chemical precipitation efficiency is related to the pH of the treatment. At a high final pH level (8 < pH < 10), the removal efficiency of copper for each precipitating agent is slightly higher than that of zinc and the residual metal contents were in conformity with industrial discharge standards. In sludge product, zinc and copper were precipitated as amorphous hydroxides including Zn(OH)2 and Cu(OH)2. Based on XRD analysis, the presence of an amount of other additional phases was noticed for copper. SEM images show that sludges produced are not large in size and are compact in structure. Corresponding EDX (energy-dispersive X-ray spectroscopy) shows that the amount of copper is higher than the amount of zinc in all recovered sludge. Wastewater treatment with soda ash resulted in a lower volume and a large product size of sludge. As a result, drying steps can be less expensive. This is a significant advantage comparably with the other precipitating agents. Soda ash may be considered as cost-effective precipitating agent for Cu(II) and Zn(II) in the industrial wastewater of the cable industry.