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

Abstract

Abstract. 99Tc and 79 Se, two long-lived radionuclides, are supposed to be highly mobile in soils, because of their anionic forms. Their behaviours in the soil are often considered only from a physico-chemical point of view, although the microorganisms can affect either directly or indirectly their speciation. This study demonstrates the role of the microbial compartment in the retention of Se and Tc in soil by comparing experiments with soils constrained to different microbiological status (sterile / raw / amended). Kd coefficients for Se and Tc were determined in batch experiments, whereas transport of Se and Tc was investigated through column leaching experiments. Kd for Se was enhanced for the raw soil without amendment compared to the value obtained for the sterilised soil. The retention of Se was higher again in the amended soil. Besides, a biofilm, which can directly retain Se, was obtained at the entrance of the amended soil column. This effect was less obvious for Tc in batch experiments, but was revealed by leaching experiments where a high quantity of Tc was retained in the amended soil columns. These results give strong evidence that microorganisms are responsible for a greater retention of Se and Tc in soil.

Radioactive selenium: origin and environmental dispersion scenarios

“…Soils may act as long-term secondary sources of radiation when solid adsorption/complexation of 79 Se as selenite is favoured and/or in waterlogged systems where precipitation of elemental selenium may occur. For example, reduction reactions may be induced abiotically from heterogeneous reactions with Fe(II)-containing minerals (Février & Martin-Garin, 2005). Soils may also present microenvironments where, for example, reduced species can be formed in soil aggregates even if the soils are considered as totally oxic (Eiche et al, 2015).…”

“…Nevertheless, soil systems are highly influenced by biological activities, and Kd transport models alone may not be sufficient to represent migration in soil horizons, particularly in the presence of microorganisms (Février & Martin-Garin, 2005;Natasha et al, 2018). In fact, an active biotic role such as microbial reduction may significantly enhance immobilization of Se compared to abiotic reduction processes (e.g., .85% of environmentally-relevant spikes of 75 Se were incorporated into the low molecular weight organic fraction of forest soils in ,64 h; Darcheville et al, 2008;Gustafsson & Johnsson, 1994).…”