Removal of Heavy Metals from Automotive Wastewater by Sulfide Precipitation

The United States Environmental Protection Agency has proposed new categorical pretreatment effluent standards for the Metal Products and Machinery Industry, which are more stringent than current discharge limits in the automotive industry. Therefore, this study was conducted to evaluate metal-sulfide precipitation chemistry as an alternative to metal-hydroxide precipitation chemistry for removing Cu, Ni, Pb, and Zn. There were three aspects of this study: (1) theoretical analysis of both metal–hydroxide and metal–sulfide chemistry; (2) experimental evaluation of commercially available sulfur-containing precipitants using deionized water; and (3) experimental evaluation of the precipitants using wastewater samples from three automotive manufacturing plants (transmission, engine, and assembly plants). The primary findings are: (1) In theory, metal–hydroxide chemistry can achieve the proposed standards when no chelating agents are present. This is not true when as small as 1 mg/L of ethylenediaminetetra-acetic acid (EDTA) is present. (2) Metal–sulfide precipitation chemistry could achieve solubility limits lower than those of metal–hydroxide chemistry over a wide range of pH. However, EDTA could still inhibit precipitation of Ni, Pb, and Zn to concentrations above the proposed standards. (3) The experiments with wastewater samples showed all precipitants removed Cu well while Ni and Zn were not well removed. The sample from transmission and engine plants were more difficult to treat than from an assembly plant, suggesting that it might have had more chelating agents. The commercially available precipitants did not perform any better than sodium sulfide. (4) Costs for using the precipitants were estimated to range from <$1/1,000 gal to >$5/1,000 gal depending on the precipitant.