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Solidification/stabilization of hazardous wastes containing metals and organic contaminants

The potential of solidification/stabilization (S/S) technology for the safe disposal of hazardous wastes has wide spread recognition. The purpose of this study was to investigate the effectiveness of portland cement-based S/S technology for the safe disposal of hazardous wastes containing toxic metals and organic contaminants. As hazardous wastes, metal enriched mining residue, adsorbable organic halogens (AOX) containing pulp and paper sludge, and polychlorinated biphenyl (PCB) oil-contaminated soil were used. For S/S of waste, portland cement as a binding agent was mixed with wastes at different ratios. For initial waste characterization, contaminant concentrations and some physical waste characteristics such as particle-size distribution, Atterberg limits, specific gravity, and moisture content yielding the maximum compacted dry density were determined. Waste and cement mixtures were cured for 28 days after compacting the desired waste-cement mixtures at their predetermined optimum moisture contents yielding the corresponding maximum dry densities in cylindrical molds having a height of 71 mm and a diameter of 36 mm. At the end of the 28-day cure period, unconfined compressive strength and hydraulic conductivity measurements were conducted on the solidified samples. Subsequently, solidified samples were crushed for fractionation into two different aggregate sizes (between 1–2 and >2 mm) and subjected to the U.S. Environmental Protection Agency Toxicity Characteristic Leaching Procedure (TCLP). The effectiveness of S/S was assessed by comparing the chemical composition of leachates obtained from TCLP tests of untreated and treated, i.e., S/S waste samples, and comparing values of strength and hydraulic conductivity of solidified waste samples with regulatory requirements. For mining waste, effective application of S/S was achieved for most cases. AOX containing sludge yielded acceptable results in terms of strength and hydraulic conductivity but leachate AOX concentrations were above regulatory levels. The effectiveness of S/S for coarse textured-soils contaminated with a PCB oil was not satisfactory, especially at a cement:soil ratio less than 35%.