Physico-chemical treatment techniques for wastewater laden with heavy metals

This article reviews the technical applicability of various physico-chemical treatments for the removal of heavy metals such as Cd(II), Cr(III), Cr(VI), Cu(II), Ni(II) and Zn(II) from contaminated wastewater. A particular focus is given to chemical precipitation, coagulation-flocculation, flotation, ion exchange and membrane filtration. Their advantages and limitations in application are evaluated. Their operating conditions such as pH, dose required, initial metal concentration and treatment performance are presented. About 124 published studies (1980-2006) are reviewed. It is evident from the survey that ion exchange and membrane filtration are the most frequently studied and widely applied for the treatment of metal-contaminated wastewater. Ion exchange has achieved a complete removal of Cd(II), Cr(III), Cu(II), Ni(II) and Zn(II) with an initial concentration of 100 mg/L, respectively. The results are comparable to that of reverse osmosis (99% of Cd(II) rejection with an initial concentration of 200 mg/L). Lime precipitation has been found as one of the most effective means to treat inorganic effluent with a metal concentration of higher than 1000 mg/L. It is important to note that the overall treatment cost of metal-contaminated water varies, depending on the process employed and the local conditions. In general, the technical applicability, plant simplicity and cost-effectiveness are the key factors in selecting the most suitable treatment for inorganic effluent.