Accumulation of Thorium and Uranium by Microbes – the Effect of pH, Concentration of Metals, and Time Course on the Accumulation of Both Elements Using Streptomyces levoris

The removal of radionuclides, such as uranium (U) and thorium (Th) from aqueous solutions, especially from contaminated sources, seems to be a significantly useful subject for exploiting unused or abandoned energy resources. In this regard, efforts have been especially concentrated on studying the accumulation of U using microorganisms, such as the actinomycetes, bacteria, fungi, and yeasts. 14 Thorium accumulation has also been investigated by some researchers. Tzezos and Volesky 15 reported the biosorption of U and Th by some microorganisms and the mechanism of Th biosorption by Rhizopus arrhizus. White and Gadds reported the biosorption of Th by fungal biomass. Andres et al reported the adsorption of Th and U by Mycobacterium smegmatis. Recently, various species and strains of actinomycetes, bacteria, fungi, and yeasts were screened for their abilities to accumulate U or Th from a solution containing only each metal at pH 3.5. 17 Among the microorganisms, a high Th accumulating ability was exhibited by the gram-positive bacterial strains, especially Arthrobacter nicotianae IAM12342, Bacilles subtilis IAM1026, and Micrococcus luteus IAM1056. The amount of Th accumulated by the gram-positive bacteria was greater than those by actinomycetes, gram-negative bacteria, fungi, and yeasts. On the other hand, high ability to accumulate U was found in some actinomycetes strains, such as S. albus HUT6047 and S. levoris HUT6156. The amount of U accumulated by actinomycetes was higher than those by most of the other gram-positive bacteria, gram-negative bacteria, fungi, and yeasts. The effect of Th (or U) addition on the U (or Th) accumulation rate and the amount of accumulated U (or Th) using S. levoris cells was also investigated. 17 Thorium strongly affected the U accumulation, whereas U had only a very slight affect on the Th accumulation. Additionally, the UTh exchange reaction occurred in the solution containing an excess amount of U. In this study, various species and strains of actinomycetes, bacteria, fungi, and yeasts were screened for their ability to accumulate Th and U from a solution containing the same amount of both elements at pH 3.5. The ability to accumulate Th and U, the effect of pH on the accumulation, and the time course of the Th and U accumulation were also investigated by S levoris cells, which accumulated the largest amount of U from an aqueous solution containing only U at pH 3.5. Furthermore, an attempt was made to evaluate these effects by numerical formulas.