Metal Immobilization by Solidification of Hydroxide and Xanthate Sludges

Abstract

The U.S. Department of the Army (DA) is responsible for the treatment and disposal of heavy metal wastes generated at DA facilities. Heavy metal wastewaters are gen erally treated by hydroxide precipitation. An alternative technique uses xanthates to precipitate the heavy metals. Both of these processes produce a sludge defined as hazardous by the Resource Conservation and Recovery Act (RCRA). One method proposed for treating such materials is solidification/stabilization (S/S). S/S renders a waste less toxic by chemically immobilizing the hazardous constitutes in a solid matrix. This study evaluated the ability of cellulose and starch xanthate to immobilize heavy metals and compares the results with the metal immobilization capability of hydroxide precipitation. Solidified and unsolidified sludges resulting from starch xanthate, cellulose xanthate, and hydroxide precipitation processes were tested for their physical and chemical characteristics. Sludges were prepared using either cellulose xanthate, starch xanthate, or calcium hydroxide to treat a synthetic waste solution containing four heavy metals (cadmium, chromium, nickel, and mercury) in a laboratory-scale study. Sludges were solidified with portland Type I cement. Physical tests performed on the solidified and unsolidified xanthate and hydroxide sludges included unconfined compressive strength and the cone penetration tests. Chemical leaching characteristics of the unsolidified sludges were determined using the U.S. Environmental Protection Agency's extraction procedure test and a serial graded batch extraction procedure. Results from this study indicated that the solidified materials reduced the concentration of metals in the leachate when compared to the unsolidified sludges. The solidified sludges also developed unconfined compressive strengths as high as 2000 kPa.