Bioaccumulation and Chemical Modification of Tc by Soil Bacteria

Abstract

Bioaccumulation and chemical modification of pertechnetate (TcO4-) by aerobically and anaerobically grown soil bacteria and by pure cultures of sulfate-reducing bacteria (Desulfovibrio sp.) were studied to gain insight on the possible mechanisms by which bacteria can affect the solubility of Tc in soil. Aerobically grown bacteria had no apparent effect on TcO4-; they did not accumulate Tc nor modify its chemical form. Anaerobically grown bacteria exhibited high bioaccumulation and reduced TcO4-, enabling its association with organics of the growth medium. Reduction was a metabolic process and not merely the result of reducing conditions in the growth medium. Association of Tc with bacterial polysaccharides was observed only in cultures of anaerobic bacteria. Sulfate-reducing bacteria efficiently removed Tc from solution and promoted its association with organics. Up to 70% of the total Tc in the growth medium was bioaccumulated and/or precipitated. The remaining Tc in soluble form was entirely associated with organics. Pertechnetate was not reduced by the same mechanism as dissimilatory sulfate reduction, but rather by some reducing agent released in the growth medium. A calculation of the amount of Tc that could be associated with the bacterial biomass present in soil demonstrates that high concentration ratios in cultures do not necessarily imply that bioaccumulation is an important mechanism for long-term retention of Tc in soil.

Biogeochemical behaviour of anionic radionuclides in soil: Evidence for biotic interactions

“…Yet to correlate this result to a direct or an indirect effect of the microbial activity of the soil was difficult. Contrary to Se, microbially-mediated reduction of Tc(VII) was proved only in anaerobic condition [7,8]. But abiotic reduction of Tc(VII) should not be excluded [4].…”

“…These reduction reactions can be achieved abiotically by heterogeneous reactions with FeII-containing minerals for example [3,4]. But they have also been proved to be microbially mediated, resulting either from direct interaction between Se / Tc and some microorganisms [5,6,7,8] or from physico-chemical changes due to the microbial activity [4,9].…”

Soil availability, plant uptake and soil to plant transfer of 99Tc—A review

“…Complexation of TcO Ϫ 4 with humic acid and other dissolved organic carbon is only possible when the Eh-value becomes negative (E 0 =Ϫ0.50 V for Tc(VII)/Tc(IV)), (Rößler et al, 2000). As noted by Lieser and Bauscher (1987) (Henrot, 1989). Literature reports therefore suggest that neither chemical nor biological factors have much impact on the generally high availability of 99 Tc in aerobic soils.…”

“…It seems likely that they might also play a role in progressive immobilization of 99 Tc. A number of authors have previously suggested that anaerobic microsites in soils might be important in TcO Ϫ 4 reduction (Hoffman et al, 1982;Henrot, 1989;Echevaria et al, 1997) but the potential scale of the process may only now be emerging with recent research into denitrification.…”

Comportement de radionucléides (Cs, I, Sr, Se, Tc) dans le sol : proposition de valeurs de Kd par défaut

“…La sorption du technétium est corrélée principalement aux teneurs en carbone organique, en oxydes de fer et d'aluminium et en argile des sols (Gu et Schulz, 1991). Les formes réduites du technétium forment des complexes très stables avec la matière organique, en particulier avec les acides humiques (Sekine et al, 1997 ;Henrot, 1989 ;Stalmans et al, 1996). Le technétium est ainsi majoritairement présent dans les horizons organiques du sol.…”