Adsorption-desorption And Migration Behavior Of Exogenous Selenate In Different Soils

Selenate, one of the main forms of selenium（Se） absorbed by plants in the field, is greatly influenced by adsorption and desorption by soil for its environmental chemical behavior and bioavailability. Adsorption-desorption effect and migration behavior of exogenous selenate varies in different soils due to their different physical and chemical properties. In this study, 18 kinds of farmland soils with various physicochemical properties in China was used in batch experiments to explore the influence of soil physical and chemical properties on selenate adsorption and desorption. Column experiment was conducted to simulate the bleaching process and explore the change of Se content in different depth of soil after leaching. This study aims to improve current understanding of Se environmental chemical behavior in soil, especially the availability change of Se when exogenous selenate is applied into soil and provide theoretical guidance for Se supplementation in Se-deficient area and soil remediation in Se-polluted area.1. For six different kinds of soil, the start time and peak time of the solute flowing in breakthrough curves（BTCS） shortened with the increase of SeO₄^2- concentration in leaching solution. The peak time of SeO₄^2- was negatively correlated with p H（P < 0.05） and organic matter, carbonate content, amorphous iron and was positively correlated with amorphous aluminum. Every different kinds of soil has different forms of breakthrough curve. Red soil showed asymmetrical breakthrough curves and long peak tails at desorption stage, which was different from aquic soil, lou soil, chestnut soil, that indicate this is a continuous slow process.2. After the process of eluviation, residual Se dominated in Se speciation in all six soils, followed by organic matter-bound Se. Se speciation in moisture soil, lou soil, black soil, chestnut soil, purple soil almost remained the unchanged in different soil depth and SeO₄^2-concentration in leaching solution. As for red soil, soluble, exchangeable and carbonate-bound Se increased with soil profile depth, while other forms of Se also remained homogeneous in soil. In terms of Se fractionation in soil, soluble Se was correlated with soil organic matter and amorphous iron and aluminum content; exchangeable and carbonate-bound Se was correlated with p H, carbonate content, organic matter, amorphous iron and aluminum content; iron and manganese oxides-bound Se was correlated with p H, amorphous iron and aluminum content.3. For the same soil, SeO₄^2- content in leachate and leaching rate increased with the increase of exogenous selenium concentrations. Under the same selenate equilibrium concentration, the order of leaching abilities for different soil was: alluvial soil ≈ lou soi l≈ chestnut soils > black soil ≈ purple soil > red soil. The leaching rate of SeO₄^2- were 50%- 90% in rainfall simulation, indicating that the soil adsorption of SeO₄^2- is reversible and SeO₄^2- is prone to migration. After 30 days of balance after adding selenate, soil Se mainly existed in soluble state（62%- 83.31%）. Iron and manganese oxides-bound state were in least proportion（0.4%- 5.52%）. After the rainfall leaching process, soil Se were mainly dominated by residual form and the availability decreased. The distribution between different fractions varied a lot（Uts values: 2.61- 22.72）, especially in comparison with the soil before leaching. IR values were very high（0.68- 0.85） after leaching and availability was significantly reduced. Thus, the migration of SeO₄^2- is very strong, which indicates a high environmental risk.4. Se adsorption-desorption behavior varied a lot in different soils. The adsorption of SeO₄^2- on 18 soils was a initially fast followed by slow process, where adsorption equilibrium was all achieved after 24 h. Second-order kinetic model（R2 > 0.976） and Freundlich isothermal model（R2 > 0.842） were the fittest for most of the adsorption process. Se O42-adsorption capacity of soil was negatively correlated with soil p H value（P < 0.01） and carbonate content（P < 0.05）, and positively correlated with amorphous iron, aluminum content（P < 0.01） and organic matter content（P < 0.05）. The partition coefficient of solid to liquid（Kd） of SeO₄^2- in adsorption process for the 18 soils were all very low and without big differences（0.99 L·kg-1- 18.18 L·kg-1）. The desorption rates for all tested soils were above 80%, which indicated the reversibility of SeO₄^2- adsorption in soil. The low Kd and high desorption rate reflected the characteristics of selenate that is prone to migrate and leach in soil, which should be emphasized in regional evaluation and regulation of Se level.Physicochemical properties of soil affect the adsorption-desorption and migration behavior of exogenous selenate. Selenate adsorption capacity, peak time of SeO₄^2- leaching were negatively correlated with p H value, carbonate content, and negatively correlated with amorphous iron, aluminum content and organic matter content. After simulate rainfall leaching, the distribution of Se fractions in soils converted to a more stable pattern, significantly reducing the bioavailability of Se. This indicates the character of selenate that prone to migrate and environmentally risky, which should be concerned in regional Se evaluation and regulation.