840 Experientia 46 (1990), Birkh~iuser Verlag, CH-4010 Basel/Switzerland Reviews 34 Trevors, J. T., Stratton, G. W., and Gadd, G. M., Cadmium transport, 38 Tsezos, M., and Volesky, B., The mechanism of uranium biosorption resistance and toxicity in bacteria, algae and fungi. Can. J. Microbiol. by Rhizopus arrhizus. Biotechnol. Bioeng. 24 (1982) 385-401. 32 (1986) 447-464. 39 Wainwright, M., and Grayston, S. J., Accumulation and oxidation of 35 Tsezos, M., Recovery of uranium from biological adsorbents-desorp- metal sulphides by fungi, in: Metal-Microbe Interactions, pp. 119- tion equilibrium. Biotechnol. Bioeng. 26 (1984) 973-981. 130. IRL Press, Oxford 1989. 36 Tsezos, M., Absorption by microbial biomass as a process for removal of ions from process or waste solutions, in: Immobilization of Ions by Bio-sorption, pp. 201-218. Ellis Horwood, Chichester 1986. 37 Tsezos, M., and Volesky, B., Biosorption of uranium and thorium. 0014-4754/90/080834-0751.50 + 0.20/0 Biotechnol. Bioeng. 22 (1981) 583-604. Birkhfiuser Verlag Basel, 1990 Microbial dissolution and stabilization of toxic metals and radionucfides in mixed wastes A. J. Francis Department of Applied Science, Brookhaven National Laboratory, Upton (New York 11973, USA) Summary. Microbial activity in mixed wastes can have an appreciable effect on the dissolution or precipitation of toxic metals and radionuclides. Fundamental information on microbial dissolution and stabilization (immobilization) of toxic metals and radionuclides, in particular actinides and fission products, in nuclear wastes under various microbial process conditions, e.g., aerobic, denitrifying, iron-reducing, fermentative, sulfate-reducing, and methanogenic conditions is very limited. Microbial transformations of typical waste components such as metal oxides, metal coprecipitates, naturally occurring minerals, and metal organic complexes are reviewed. Such informa- tion can be useful in the development of 1) predictive models on the fate and long-term transport of toxic metals and radionuclides from waste disposal sites, and 2) biotechnological applications of waste treatment leading to volume reduction and stabilization as well as recovery and recycling of radionuclides and toxic metals. Key words. Toxic metals; radionuclides; natural radioactive mineral deposits; metal oxides; carbonate complexes; organic complexes; coprecipitates; uranium; plutonium; low-level radioactive wastes; transuranic wastes; coal wastes. Introduction Microorganisms, which are ubiquitous throughout na- ture, have long been recognized for their ability to bring about transformations of organic and inorganic com- pounds. Such microbial processes have not been fully exploited in the treatment and management of nuclear and fossil-energy wastes, particularly in regulating the mobility of toxic metals and radionuclides or in the biodegradation of organic constituents to innocuous products. The contaminants in wastes may be present initially as soluble forms or they may be formed after disposal by chemical or microbiological processes. The organic compounds and inorganic elements (major and minor metals and radionuclides), depending upon their chemical forms, may react with each other to varying Figure 1. Interactions of organics, inorganics, radionuclides, and mi- degrees in the waste. These include organic-inorganic crobes in mixed wastes. complex formation, precipitation reactions, coprecipita- tion of metals and radionuclides with Fe- and Mn-ox- ides, and formation of minerals. In figure 1, the interac- tions between the organics, inorganics, radionuclides and tions of such mixed wastes. There is a paucity of microbes are depicted. Because of the complexity of the information on the fate and long-term transport of toxic system, it is often difficult to elucidate clearly the exact metals and radionuclides present in energy-related wastes mechanisms involved in the transformation of mixed disposed of in subsurface environments. Of particular wastes. Nevertheless, studies with pure or model com- concern is the lack of information on specific microbial pounds under defined conditions should provide infor- processes and the biochemical mechanisms involved in mation on the mechanisms involved in the transfolrna- the dissolution (mobilization) or stabilization (immobi- Reviews Experientia 46 (1990), Birkhfiuser Verlag, CH-4010 Basel/Switzerland 841 lization) of toxic metals and radionuclides in mixed 1. Microbial processes wastes. Mobilization and immobilization of radionuclides and The microbial effects on low-level radioactive wastes toxic metals under aerobic conditions, by the activities of disposed of in shallow-land burial grounds 43'75 and autotrophs in the case of inorganic wastes, and hetero- the potential effects in the deep geological forma- trophs in mixed wastes containing organics, could be tions 119,12o have been previously reviewed. A wide vari- significant. Similarly, under anaerobic conditions hetero- ety of microorganisms have been identified in coal wastes 4s'55'56'85'9~ and in low-level radioactive trophic microbial activity which is influenced by the pres- ence and type of electron donors and acceptors in the wastes45, 46 which can affect the integrity and the long- waste can affect the mobility of radionuclides and toxic term stability of wastes. Leachate generation is a com- metals. Several of the key microbial processes which af- plex process involving physical, chemical, and biological fect mobilization or immobilization of toxic metals and actions. Microbial transformations of the waste at the radionuclides under aerobic and anaerobic conditions disposal site may significantly influence the composition are summarized in figure 2. Microorganisms under ap- of leachates. Comprehensive information of the transfor- propriate conditions bring about metal dissolution and mations of wastes under different microbial process con- mobilization or immobilization by one or more of the ditions will be useful in developing predictive models on following mechanisms: 1) oxidation-reduction of metals the fate and transport of toxic metals and radionuclides which affect solubility; 2) changes in pH and Eh (which in the environment 52' sa, 76. It will also provide useful affect the valence or ionic state of the metals and enhance information for biotechnological applications in waste or retard their mobility in the subsurface environment); treatment such as recovery and recycling of elements, 3) solubilization and leaching of certain elements by mi- waste volume reduction and minimization, and waste crobial metabolites or decomposition products, alkyl- stabilization as well as better waste management meth- ation, chelation or production of specific sequestering ods. The purpose of this paper is to review the relevant agents; and 4) immobilization leading to formation of information available on the microbial processes in- volved in mobilization and immobilization of toxic stable minerals or biosorpfion by cells and biopolymers and, 5) release of biosorbed metals due to remineraliza- metals and radionuclides present in mixed wastes. tion elsewhere in the environment. Wastes I Microbial characteristics Chemical characteristics Aerobic, anaerobic, Inorganic mesophilic, thermophilic, and autotrophic and organic heterotrophic groups I Microbial activities I I I Autotrophic Heterotrophic ] I I I I [ [ Sulfur and Iron oxidation Sulfate reduction o Changes in pH which determine solubility sulfide oxidation Fe 2 + ~ Fe 3 + under anaerobic o Oxidation-reduction reactions and Eh conditions which affect valence state and solubility I [ o Chelation, solubilization, and leaching HzSO 4 Oxidation of Precipitation and of elements by microbial metabolites Solubilization and uranite to metal sulfide and decomposition products leaching of metals soluble uranium formation (insoluble) o Bioaccumulation by cells and exo- [ polymers o Biomethylation, and production of Leaching volatile and/or toxic alkylated metal compounds o Biodegradation of organic complexes Mobilization I l Mo lizat on 1 I ,mmobil z t oo of radionuclides and metals I Mobilization and Figure 2. Microbial transformations of toxic metals and radionuclides. immobilization 842 Experientia 46 (1990), Birkh~user Verlag, CH-4010 Basel/Switzerland Reviews The form in which the metal occurs (e.g., elemental, ox- level radioactive waste is a major concern because they ide, sulfide, ionic, inorganic complex, organic complex, are responsible for most of the environmental contami- or organometallics), the availability of electron donors nation problems encountered in and around the disposal and nutrients (carbon, nitrogen, phosphorus), the pres- areas. Many of the organic compounds are capable of ence of alternate electron acceptors (Fe 3 +, Mn 4+, No~, forming stable complexes with heavy metals and ra- SO 2- organic compounds), and the environmental fac- dionuclides or increasing their solubilization and leach- tors (pH, Eh, temperature, moisture) will affect the types, ing 42'43. Likewise, microbial metabolites and waste de- rates, and extent of microbial activity and microbial gradation products or intermediates may be an impor- transformation of metals. The oxidizing and reducing tant source of agents that affect the long-term solubility conditions which prevail at the disposal sites to a large characteristics of heavy metals and radionuclides. extent influence the mobilization and immobilization of There are a few reported studies on microbial activity radionuclides and metals. Eh levels