An examination of selenium interactions between soil and water at environmentally relevant concentrations

Selenate is considered the most mobile form of selenium in the soil environment. Selenium concentrations in the ppb range are considered toxic to animals; however, research focusing on selenate interactions with the soil solid-phase at these concentrations is limited. Nearly all macrochemical adsorption experiments with selenate have shown it does not adsorb to soil. In contrast, spectroscopic studies indicate it can form inner-sphere complexes to minerals. I hypothesize that at low solution concentrations there is a significant adsorption of selenate on soils. Experiments were conducted at low selenate concentrations using three different soils to test for adsorption. The Reyes soil adsorbed 20% of the initial selenate treatment (50 mug Se kg-1 solution). The Ciervo and Panoche soils adsorbed 19 and 14% of the same treatment, respectively. Increasing the pH of the Reyes soil to 7.5 using a HCO3 -/CO32- buffer resulted in a 75% reduction of adsorption. Selenate adsorption was also related to extracted Fe and Mn, with higher selenate adsorption measured on soils with more extractable Fe and Mn. When the ionic strength of the system was increased 5-fold, the Reyes soil had a maximum selenate adsorption of 150 mug Se kg-1 soil at initial treatments as low as 15 mug Se kg-1 solution (compared to a maximum of 404 mug Se kg-1 soil at the lower ionic strength). The Reyes soil (pH 5.0) had approximately equal percent of the total selenium lost from solution in the soluble and adsorbed fractions of the soil. Selenate lost from solution in the Ciervo (pH 8.2) and Panoche (pH 7.9) soils was predominantly in the soluble fraction, with a small percentage found in the adsorbed and residual fractions. The difference between Se fractionation of the soils and variation in adsorption due to changes in ionic strength supports a mixed inner-outer sphere adsorption model with adsorption being outer-sphere at high pH and transitioning to a mixture of inner and outer-sphere as pH decreases. This is some of the first macrochemical evidence from adsorption isotherms and sequential extraction of non-native selenium from soils that has been presented supporting the mixed inner/outer-sphere adsorption profile in soils.