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Bioremediation of radioactive waste

The radioactive waste (U, Np, Am, Sr, Pt and Tc) is released from sources such as nuclear energy generation programs, testing of nuclear weapons and accidental release and it has become a nuisance to environment and living organisms (Lloyd and Gadd 2011). Numerous researches have been initiated since the last two decades to develop an environmentally safe and publicly acceptable method through the use of microbes in bioremediation processes to treat radionuclide-contaminated land and water (Kimber et al. 2011). Microorganisms, including bacterial genera such as Geobacter, Deinococcus, Shewanella, Serratia, Kineococcus radiotolerans, and Hymenobacter metalli can be used to treat the radioactive waste by affecting then- solubility, mobility, and bioavailability (Lloyd and Gadd 2011, Chung 2010).

Uranium as U (VI) and also technetium as Тс (VII) are susceptible to enzymatic reduction by microbes. The oxidized forms of U and Tc are toxic and highly mobile in groundwater because of their high solubility in the aqueous medium, whereas the reduced forms are less toxic, insoluble and precipitable (Istok 2004). Lovley and Phillips (1992) fir st reported that the Fe (Ill)-reducing bacteria like Geobacter metaUireducens and Shewanella oneidensis can conserve energy for anaerobic growth via the U (VI) reduction (Lovley et al. 1991). Other organisms, including a Clostridium sp. (Francis 1994) and Desulfovibrio desulfuricans (Lovley and Phillips 1992) and Desulfovibrio vulgaris (Lovley and Phillips 1992), also reduce uranium but are inefficient to conserve energy for their growth. It means that these bacteria convert the toxic form of U and Tc into less toxic via reduction process.

Neptunium, an alpha-emitting trausurauic radionuclide, is of gr eat concern because of its long half-life (2.14 x Ю6 years), high radio-toxicity, and relatively high solubility as Np (V) under toxic conditions. In comparison, Np (IV) species dominate under-reducing conditions and can be removed from the solution by hydrolysis and a reaction with the surfaces (Dozol and Hagemaim 1993, Kaszuba and Runde 1999). Low Np uptakes (10 mg g~l dry weight) were reported with Pseudomonas aeruginosa, Streptomyces viridochromogenes, Scenedesmus obliquus, and Micrococcus luteus (Strandberg and Arnold 1988). Songkasiri and coworkers suggested that Pseudomonas fluorescens can biosorb significant quantities of Np, removing 85 percent Np (V) from the solution at pH 7 (Songkasiri 2002). Nepfrmyl species (NpO:+) can also be biologically reduced to insoluble Np (IV) under anaerobic conditions (Lloyd et al. 2000, Rittmaim et al. 2002). Shewenellaputrefaciens reduced Np (V) to Np (IV), which was then precipitated from solution as Np (IV) phosphate in the presence of a Citrobacter species with high phosphatase activity. Desulfovibrio desulfuricans have also been reported to reduce Np (V) into Np (IV).

The fission product Strontium-90 (90Sr) is generated in nuclear explosions and Strontium-90 undergoes beta decay, with a half-life of about 29 years. Microccous luteus is capable of Sr-binding on the cell envelope and is sensitive to pretreatment. Bound Sr can be displaced by chelating agents like divalent cations or H+. Sr binding in M. luteus is reversible, via both ion exchange, mediated by acidic cell surface components and intracellular uptake may be involved (Faison et al. 1990).

Plutonium is comparatively far more toxic and challenging than for the other radioactive element (U, Th, Am and Tc) because of its high radiotoxicity and complex redox chemistry. The oxidation state of plutonium is Pu (IV), Pu (III), Pu (V), and Pu (VI); among these, Pu (IV) is most stable under environmental conditions. Pu (VI) and Pu (V) are reduced into Pu (IV) via direct enzymatic reduction by bacterial cell suspension of Shewanella putrefaciens CN32, Shewanella oneidensis MR1, and Geobacter nietallirediicens GS15 reported by Neu et al. (2005). Fe (II), Mil (II), and sulfide are common inorganic reductants that are produced by bacteria and reduce plutonium (Newton 2002).

Americium can stand in oxidation states ranging from II to VII and the environmentally important oxidation state of Am is trivalent oxidation state. E. coli, a marine bacterium, Rhizopusarrhizus, and Candida utilis can play a pivotal role in changing Am (III) solubility and efficient biosorption (Watson and Ellwood 1994, Fisher et al. 1983, Wurtz et al. 1986, Dliami et al. 1998). Manuno et al. demonstrated the new bacterial strains such as Flavobacterium spp.. Pseudomonas gladioli, Chryseobacterium indologenes, and Ochrobactrum anthropi, which are radionuclides-tolerant and also responsible for the degradation processes of organic waste (De Padua Ferreira et al. 2011, Manuno et al. 2008).

Technetium-99 is a long-lived (half-life, 2.13 x Ю5 years), beta-emitting radionuclide and is an important component of radioactive wastes. Under oxic conditions, technetium is present as pertechnetate ion (Тс (VII); TcO4"), which is one of the most mobile radionuclide species and less sorbed (Bondietti and Francis 1979). Bioreduction, a novel phosphor imaging technique, was used to show a reduction of the radionuclide by Shewanella putrefaciens and Geobacter nietallirediicens, with similar activities of Rhodobacter sphaeroides, Paracoccus denitrificans, some Pseudomonas species (Lloyd and Reushaw 2005), Escherichia coli (Lloyd et al. 1997) and a range of sulfate- reducing bacteria (Thiobacillus sp.) (Dliami 1998).

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