Selenate Transport in Steady‐State, Water‐Saturated Soil Columns

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Abstract
Transport of selenate (Se6+) in steady‐state, water‐saturated soil columns was evaluated under different pore water velocities, ionic compositions, and two soil bulk densities. The soil was Panoche loam (Typic Torriorthents) saturated with 0.005 M CaSO4, 0.005 M CaCl2, or 0.05 M CaCl2. To examine the movement of selenate in soil relative to water and SO2−4 (a biogeochemically similar anion), breakthrough curves of 75SeO2−4, along witH+3H2O or 35SO2−4, were observed. Selenate appeared before elution of one pore volume and always ahead of the 3H2O and SO2−4 except in 0.05 M CaCl2, indicating exclusion of selenate by the soil matrix. On the other hand, soil analysis of selenate following the leaching indicated that selenate was retained by the soil, suggesting simultaneous exclusion and adsorption of selenate. Increasing pore water velocity and soil bulk density did not shift the position of the breakthrough curves significantly. Assuming equilibrium linear adsorption‐desorption of selenate by soil, the values of the apparent diffusion coefficient (D) and the retardation factor (R) were obtained by fitting the solution of the solute transport equation without sink and source terms to the measured breakthrough curves of selenate. The values of D showed an increasing trend with pore water velocity and the values of R were influenced by ionic composition of the soil solution.

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