Time Dependent Leachability and Potential Environmental Implications From Stabilization/Solidification Treatment of Chromium Contaminated Clay

Requirements for sustainable waste management and environmental protection have yielded recent EU regulations, which actively encourage the use of source control remedial approaches such as stabilization/solidification (S/S). S/S has proven particularly viable for heavy metal containment, relying on interactions between introduced reagents and contaminated media to induce containment. However, there is a need to validate long-term performance of the method to prevent accidental releases and subsequent environmental pollution. This requires evaluation of leaching controlling mechanisms for treated contaminated media to address time and equilibrium pH dependent leaching characteristics. The selection of a suitable suite of leaching tests to evaluate leaching is essential to ensure confidence in the applied techniques, and can be used to expedite durable and effective mix design and application. This paper presents findings from leaching evaluations for CEM II (Portland limestone cement) treated chromium contaminated kaolin clay. Leaching tests were undertaken at 28, 150, and 300 days of sample curing, for time dependent, equilibrium, and porewater leaching characteristics. Evaluations were also undertaken for influences of sulfate and organic content (as humic acid) on chromium containment. Chromium was found to be contained effectively within the stabilized contaminated matrix, with leaching and availability comparable to those observed for the stabilized uncontaminated specimen. The primary solubility controlling mineral was found to be chromium hydroxide, and increasing hydration improved chromium containment. Increased organic content resulted in an initial increase in chromium availability at 28 and 150 days of hydration, a trend also observed for an increase in sulfate content. However, after hydration for 300 days, the availabilities for chromium were comparable for all matrices, and comparable to measures observed for leaching from the uncontaminated clay.