Chromium Leaching and Immobilization in Treated Soils

Abstract

The purpose of this study is to investigate the effectiveness of a quicklime-based stabilization/solidification (s/s) technique to reduce the leachability of Cr(III) and Cr(VI) in laboratory-prepared soil. The soils were prepared by mixing kaolinite or montmorillonite with fine quartz sand and then subjecting the samples to s/s treatment. Treatment included quicklime, fly ash, and quicklime–fly ash additions. The effectiveness of the treatment was evaluated using the Toxicity Characteristic Leaching Procedure (TCLP) test. To elucidate the controlling Cr(VI) immobilization mechanisms, slurry mixtures were prepared with potassium chromate (K2CrO4), quicklime, and fly ash. X-ray diffraction (XRD) analyses were performed to investigate the reaction products responsible for Cr(VI) immobilization. Experimental results indicate very low Cr(III) leachability upon quicklime treatment. All samples treated with quicklime and quicklime–fly ash show TCLP Cr(III) concentrations below the U.S. EPA TCLP limit of 5 ppm. However, neither quicklime nor fly ash treatment alone was effective in reducing Cr(VI) leachability. An effective reduction in Cr(VI) leachability was achieved only when both quicklime and fly ash were added to the laboratory-prepared soils. Previous research has attributed this reduction in Cr(VI) leachability to the fly ash-induced chemical reduction of Cr(VI) to Cr(III). However, in this study, fly ash was found to be an ineffective reducing agent. XRD analyses results for quicklime–fly ash slurries do not indicate the formation of CaCrO4 or CaCrO4 · 2H2O, which were identified in previous research as the reaction products responsible for Cr(VI) immobilization. A hypothesis is presented in this paper to explain these results and the effectiveness of quicklime-fly ash mixtures in reducing the leachability of Cr(VI). XRD analyses have also identified the pozzolanic compound 3CaOAl2O30.5CaCrO40.5CaSO4 · nH2O as the precipitate most likely responsible for Cr(VI) immobilization.