Effects of Moisture Content and Redox Potential on in situ Kd Values for Radiodine in Soil

Authors: Ashworth, Daniel; SHAW, G - UNIV OF NOTTINGHAM

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2005
Publication Date: 1/2/2006
Citation: Ashworth, D.J., Shaw, G. 2006. Effects of Moisture Content and Redox Potential on in situ Kd Values for Radiodine in Soil. Science of the Total Environment. Vol 359:244-254.

Interpretive Summary: Radioselenium is an important component of radioactive waste from nuclear reactor operation. Following deep geological disposal of such waste, there is a potential for component radionuclides to migrate towards the biosphere in a soluble form. Understanding the processes that determine the solid-liquid partitioning (or Kd value) of Se are therefore of fundamental importance in assessing the risk associated with the disposal of radioselenium-containing waste. The main finding of this work was that saturation of the soil with water can increase the Kd value of radioselenium, i.e. inhibit its partitioning into the liquid phase. This was thought to be due to the onset of anoxic conditions within the soil and a corresponding change in selenium speciation to a less soluble selenium form. These findings, coupled with the fact that methyl bromide fumigation had no discernible effect on 75Se Kd value, suggest that geochemical, rather than microbial, processes controlled 75Se partitioning. The inter-relations between soil moisture content, oxic/anoxic status and Se speciation should be considered in the modelling and assessment of radioactive Se fate and transport in the environment.

Technical Abstract: Understanding the processes that determine the solid-liquid partitioning (Kd value) of Se are of fundamental importance in assessing the risk associated with the disposal of radioselenium-containing waste. Using a mini-column approach, in-situ Kd values for 75Se were determined over time in relation to soil moisture content (field capacity or saturated), redox potential and methyl bromide fumigation (used to disrupt the soil microbial population) in three contrasting soil types: clay loam, organic and sandy loam. Kd values were generally in the range 50 to 500 L kg-1, with mean soil Kd increasing with increasing organic matter content. Saturation with water lowered the measured redox potentials in the soils. However, only in the sandy loam soil did redox potential become negative, and this led to an increase in 75Se Kd value in this soil. Comparison of the data with the Eh-pH stability diagram for Se suggested that such strong reduction may have been consistent with the formation of the insoluble Se species, selenide. These findings, coupled with the fact that methyl bromide fumigation had no discernible effect on 75Se Kd value in the sandy loam soil, suggest that geochemical, rather than microbial, processes controlled 75Se partitioning. The inter-relations between soil moisture content, redox potential and Se speciation should be considered in the modelling and assessment of radioactive Se fate and transport in the environment.