Most radioactive species in the actinide family are released in the waste streams as oxyions of the elements. The oxyionic forms are seen when the high oxidation states are present. Metals in the pentavalent and hexavalent states (V to VI, respectively) have high affinity for oxygen. The waste streams can also contain the oxyanions of the transition metals such as chromium and cobalt. The most common of these is hexavalent chromium Cr(VI) in the oxygen combined form of chromate (CrO42-) and dichromate (Cr2O72-).

15.1.2 Biological reduction of actinides

Recent studies have shown that certain species of bacteria are capable of reducing the toxic oxyions of the actinide and transition metals to less toxic and less mobile forms. For example, U(VI) can be reduced to U(IV) by sulfate reducing bacteria such as Desulfovibrio vulgaris (ATCC 29579), Desulfovibrio desulfuricans (ATCC 29577), and Geobacter metallireducens (Lovley et al., 1991; Lovley et al, 1993; Payne et al., 2004). U(VI) reduction in these species was demonstrated to be dissimilatory respiratory with the bacteria deriving metabolic energy through the U(VI) reduction pathway (Lovley et al., 1993). Notably, U(VI) reduction by the above species of bacteria required incubation of the cultures under strictly anaerobic condi­tions. This is because the Desulfovibrio species favour reducing environ­ments [ORP = -300 to -0.400 V] (Boonchayaanant et al., 2007).

Lately, U(VI) reduction by bacteria under micro-aerobic conditions has been demonstrated (Chabalala and Chirwa, 2010a). In this case, a non- purified consortium from the mine soil was used to reduce U(VI) [UO22+] to the less toxic and less mobile tertravalent state [U(IV)]. Three pure isolates were used, i. e., Pantoea sp, Pseudomonas sp. and Enterobacter sp., to reduce U(VI) under a pH range of 5 to 6 (Chabalala and Chirwa, 2010b). Figure 15.3 shows an example of the action of U(VI) reduction by the three species of bacteria while acting as pure cultures.

The significance of the above results is that the new cultures reduced uranium under conditions supporting the facultative range of bacteria. Such a culture could be less expensive to maintain. Additionally, by avoiding the formation of toxic sulfur precipitates, the culture could achieve more sus­tainable long-term operation and the separation of the product for reuse could be less expensive.