Laboratory Scale Thermal Plasma Arc Vitrification Studies of Heavy Metal-Laden Waste

Abstract

Plasma processing has been identified as a useful tool for immobilizing heavy metal-contaminated wastes into safe, leach-resistant slag. Although much effort has gone into developing this technology on a pilot scale, not much information has been published on basic research topics. A laboratory-scale plasma arc furnace located at the University of Illinois was operated in cooperation with the U.S. Army Construction Engineering Research Laboratories in an effort to establish an understanding of the chemical and physical processes (such as metal volatilization and resultant gas evolution) that occur during thermal plasma treatment of metal-spiked samples. Experiments were conducted on nickel and chromium using a highly instrumented furnace equipped with a 75 kW transferred arc plasma torch. The volatility of nickel and chromium was examined as a function of varying oxygen partial pressures. Oxidizing conditions reduced the total dust gathered for both the nickel and chromium samples, although each dust sample was found to be metal-enriched. Plasma treating increased the leach-resistance of the slags by at least one order of magnitude when compared to unprocessed specimens. The leach- resistance of the nickel-containing slags increased in the presence of oxygen, whereas chromium samples remained relatively constant.     

Concentration ratios for small mammals collected from the exposed sediments of a 137Cs contaminated reservoir

(137)Cs concentration ratios were computed for small mammals collected from the dried sediments of a partially drained, contaminated reservoir. Soil (137)Cs activity concentrations were heterogeneous on small and large spatial scales and had a geometric mean of 253 (range 23-2110) Bq/kg dry weight. Mean (137)Cs activity concentrations in composite cotton rat Sigmodon hispidus and cotton mouse Peromyscus gossypinus samples averaged 2480 (range 556-6670) and 471 (range 96-1000) Bq/kg whole body dry weight, respectively. About 50% of the variance in cotton rat tissue (137)Cs activity was explained by variation in soil (137)Cs activity. Soil-to-animal dry weight concentration ratios averaged 6.0 for cotton rats and 1.2 for cotton mice and were generally similar to (137)Cs concentration ratios for herbivorous, homeothermic animals from other contaminated ecosystems. In the RESRAD-BIOTA dose model, the default wet-weight concentration ratio for (137)Cs in terrestrial animals is 110 resulting in an estimate of internal and external radiation doses to terrestrial biota that is 44 times more than the dose calculated with the actual measured wet-weight concentration ratio for cotton rats (1.6). These results show that site-specific concentration ratios can significantly affect the estimation of dose.