Development of a process to remediate soils contaminated with metal wastes

Abstract

The contamination of the environment has led to a need to develop innovative processes to remediate the contaminated sites. This research developed two processes for remediating soils contaminated with metal wastes. The four primary metal contaminants were chromium, nickel, cadmium and copper. Only chromium exceeded the EPA required action levels. Both processes utilized mineral processing techniques to make a size separation since the majority of the contamination existed within the fines or minus 20 micron material. This size separation reduced the amount of soil requiring further treatment. The first process was focused at recovering the metal contaminants from the soil for reuse in other industries. Chromium required recovery mainly because it exceeded the EPA action levels but it also had a potential reuse in the steel industry. Copper was evaluated for recovery strictly for its potential resale value. A mixed metals precipitate would also be recovered for potential use at a recycling facility. Sulfuric, oxalic, and hydrochloric acid were tested for their ability to leach the contaminants from the soil. The sulfuric acid leach was then optimized for pH and leach time. Consequently, a conceptual recovery process utilizing cementation to recover the copper, pH adjustment to precipitate a ferro-chrome, and further pH adjustment to precipitate a mixed metals precipitate was developed. Due to the amount of by-product produced from this process and the large quantity of reagents required, a second process was developed. The second process was focused at only removing and recovering the chromium since it was the only contaminant that exceeded EPA action levels. Following the size separation, the soil was subjected to a hypochlorous acid/hypochlorite leach. This leach was performed at slightly acidic conditions to use hypochlorous acid as the oxidizing agent. This leach oxidized the chromium from the trivalent to the hexavalent state thereby increasing its solubility. Once the chromium, as HCrO₄⁻, had gone into solution, the liquid was separated and sent to reduction processing. Using sulfur dioxide, the hexavalent chromium was reduced back to the trivalent state and with the addition of sodium hydroxide, the chromium precipitated.