Application of microwave energy to remediation of soils contaminated with chromium

A process based upon microwave treatment has been developed to render soil non-hazardous after that soil had been classified as hazardous due to heavy metal contamination. It has been applied to chromium-contaminated soil. The object of the present regulations is to prevent damage to water supplies as a result of the leaching of heavy metal ions from the soils. This object is expressed in the standard test for leaching, namely, the EPA EP Leachate test. This process can be used to immobilize metal ions sufficiently to meet the aforementioned standard. The present standard is based upon acid leaching, and not upon bioavailability. Thus it is necessary to effect immobilization. Immobilization can be effected before reaching the vitrification state, therefore, the operating cost is much less. In addition, the object of the remediation in superfund sites is site-specific and can be based upon the elimination of the bioavailability of the metal ions. It is possible using this process to eliminate the bioavailability of the metal ions by effecting vitrification of the soil. Soil constituents do not absorb microwave radiation strongly at ambient temperatures because their rotational relaxation times are too long even if they are polar; additives decrease the rotational relaxation times. The additives absorb microwave energy strongly and get heated to high temperatures; the heat transferred to the soil constituents decreases their relaxation times and thus they become strong microwave absorbers. The additives chosen for this study were iron and granulated activated charcoal (GAC), respectively. The iron was chosen because scrap iron could be used and that would add little to the cost of the process; the GAC, because spent hazardous waste-contaminated GAC could be used, and in this way two different hazardous waste streams could be destroyed at the same time. In addition to establishing that the above processes are realizable, studies were undertaken to determine the cost-efffectiveness of the process. These were bench-scale studies so the applicability of the calculations to scale-up is tenuous. However, the reactor designed and the parameters defined can be used in pilot plant studies. Such studies which will conmmence shortly in the future will provide a more realistic assessment of the cost-effectiveness of the process. The reactor is designed to simulate the exposure of a core of the soil either on the surface or in the bulk to an ascertainable microwave power. That quantity of power can be dertermined either by a power meter or the absorption of the power by water. The latter method was used in this study. The cores were sampled over a range of depths to determine the degree of immobilization of the chromium. Studies were also undertaken to develop design parameters which could determine the depth of penetration of the incident microwave radiation. These were compared to depths of penetration as calculated by standard methods