The effects of ten common waste constituents that have the potential to interfere with the strength and contaminant immobilization properties of solidified/stabilized metal finishing wastes were studied. A synthetic, metal finishing waste treatment sludge containing high levels of cadmium, chromium, mercury, and nickel was prepared. Interference materials, which were individually added to the sludge during treatment, included: three heavy metals (copper, lead, and zinc as nitrate salts), sodium hydroxide (NaOH), sodium sulfate (Na2SO4), and five organic substances (grease, oil, trichloroethene, hexachlorobenzene, and phenol). Solidification/ stabilization was accomplished using one of three binder systems: portland cement (CEM), portland cement plus fly ash (CFA), and a lime/fly ash (LFA). Specimens were tested in duplicate using the U.S. Environmental Protection Agency (EPA) extraction procedure.
Leachate concentrations of the three metals in the sludge (cadmium, chromium, and nickel) and the three metal interference materials (copper, lead, and zinc) were primarily related to the final pH of the extraction solution. Of the interference materials tested, the metal nitrates reduced the leachate pH values to the greatest extent and thus had the greatest effect on the metal concentrations in the leachate. The addition of NaOH as an interfering material increased the final pH of the leachate and thus tended to suppress leachate metal concentrations, especially in the less buffered CFA specimens. On the other hand, the addition of Na2SO4 reduced leachate pH only slightly, again mainly in the CFA specimens, and thus tended to slightly increase the metal concentration in the leachates. The addition of up to 8% by weight of the organic interference materials had no significant effect on metal concentrations in the leachates, nor on the leachate pH. Phenol, the most polar of the organic interference materials used in this study, was itself extracted at appreciable levels from all three binder systems. However, phenol addition had little effect on metal leaching. Leachate mercury concentrations were not affected by the addition of the interference materials, except perhaps by phenol. Nearly equal mercury concentrations were found in leachates from the raw sludge and all treated samples.
In general, the effects of the added interference materials on the short-term leachate metal concentrations could be explained by differences in the leachate final pH. None of the interfering materials appeared to act as carriers or to change the solubility of the metal contaminates other than through their effects on the leachate pH. This research confirms the need for waste-binder specific studies before selecting a chemical stabilization/solidification process for the treatment of hazardous waste.