In view of the stringent nature of the US EPA's newly proposed categorical pretreatment effluent standards for the Metal Products and Machinery industry, metal-sulfide precipitation was evaluated as a potential alternative to the currently used metal-hydroxide precipitation for
the removal of Cu, Ni, Pb, and Zn. There were two aspects of this study: 1) theoretical analysis of both metal-hydroxide and metal-sulfide chemistry with and without ethylenediaminetetraacetic acid (EDTA) and 2) experimental evaluation of commercial sulfide-containing precipitants using wastewater
samples from three Ford plants. The primary findings are: In theory, metal-hydroxide chemistry can achieve the proposed standards in an ideal situation (i.e., no chelating agents and other constituents in water) but not in the presence
of even a small amount of a chelating agent (e.g., as small as 1 mg/L of EDTA). Metal-sulfide precipitation could achieve solubility limits lower than those of metalhydroxide precipitation over a wide range of pH. However, EDTA could still increase
the limits of Ni, Pb, and Zn above the standards. The experiments with wastewater samples showed all sulfur-containing precipitants removed Cu well while Ni and Zn were not well removed. The sample from a transmission plant was the most difficult to treat,
suggesting that it might have had more chelating agents than the other samples. The commercial precipitants used did not perform any better than sodium sulfide. Costs for using the precipitants were estimated to range <1/1000 gal to >5/1000
gal depending on the precipitant.
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