ASSESSMENT OF A CHEMICAL/BIOLOGICAL PROCESS TO TREAT MIXED WASTES
|M.D. Tucker(1), L.L. Barton(2), and B.M. Thomson(3), (1)Sandia National Laboratories, Albuquerque, NM 87185, and (2)Department of Biology and (3)Department of Civil Engineering, University of New Mexico, Albuquerque, NM 87131||
Mixtures of hazardous and radioactive constituents in waste materials produce a waste form known as mixed waste that is subject to regulation by both the Environmental Protection Agency and the Nuclear Regulatory Commission. The overlapping jurisdictions and differences in their corresponding waste management regulations make mixed wastes very difficult to manage. Many DOE facilities have mixed waste problems which include radionuclides such as uranium (U), plutonium (Pu), technetium (Tc), and strontium (Sr) together with hazardous metals such as arsenic (As), chromium (Cr), and molybdenum (Mo), as well as organic wastes including chelating agents which may increase the mobility of both the metals and the radionuclides.
Current treatment options for addressing mixed waste problems focus on processes which destroy the hazardous organic compounds through some type of oxidation process, thereby producing a radioactive waste that is much easier to manage from a regulatory perspective. Treatment alternatives which have been considered for mixed waste include: 1) thermal destruction of the organics; 2) chemical oxidation by hydrogen peroxide, ozone, or other oxidizing agent; 3) photolytic oxidation in the presence of titanium dioxide or other catalyst; 4) electrochemical degradations, and 5) Fenton reactions. Microbial oxidation of the organic constituents of mixed waste has been the subject of much investigation as well; however, these processes are generally limited by the inability to degrade recalcitrant compounds, such as EDTA, and are further complicated by the production of a radioactive biological sludge that is difficult to manage.
This paper discusses the results of a sequential chemical and biological process which has been used to destroy organic chelating agents, with subsequent precipitation of the metals from solution. A Fenton-based reaction employing ferrous iron (Fe(II)) and hydrogen peroxide (H2O2) generates free radicals to achieve initial degradation of organic compounds. Chemical oxidation is followed by biological oxidation of degradation products. The oxidation of EDTA has been demonstrated through the use of carbon-14 radiolabeled compounds. The effect of pH, temperature, dissolved gases, extraneous organic matter, and redox-active metal ions on this oxidation has been investigated. Assimilatory and dissimilatory degradation of the carbon-14 compounds has been determined, which provides further evidence in support of the efficacy of this process.
Key words: mixed waste, radionuclides, hazardous metals, chelating agents, oxidation
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