Effects of soil type, moisture content, redox potential and methyl bromide fumigation on Kd values of radio-selenium in soil

Understanding the processes that determine the solid–liquid partitioning (Kd value) of Se is of fundamental importance in assessing the risk associated with the disposal of radio-selenium-containing waste. Using a mini-column (rather than batch) approach, 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. The Kd values were generally in the range 50–500 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.