Modeling Selenium Transport in Steady‐State, Unsaturated Soil Columns

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Abstract
This study was conducted to characterize batch adsorption of selenate (SeO2−4), selenite (SeO2−3), and selenomethionine (Se‐CH2CH2CHNH2COOH)2 and transformations of Se in the presence of microbial activity in Punoche loam (fine‐loamy, mixed, calcareous, thermic Typic Torriorthents). The effects of sterilization, enhanced microbial activity, and aerobic vs. anaerobic conditions on the transport of pulse‐fed Se in soil columns were also investigated. Selenate was rapidly leached in the soil columns compared with selenomethionine and selenite both of which invaded the columns to only a limited extent. Adsorption was highest for selenite and least for selenate. Selenate was transformed to reduced and less mobile forms when soil was C‐enriched, particularly with smaller water fluxes and under anaerobic conditions. Systems of partial differential equations (PDEs) involving equilibrium adsorption of Se are set up to simulate transport of selenate, selenite, and selenomethionine under sterilized conditions. The transport model is solved by an iterative procedure using a finite difference scheme. Experimentally determined adsorption parameters of selenite and selenomethionine did not produce a satisfactory fit when used in simulation of these Se species in column studies. The Se transport model adequately simulated Se concentrations in soil solution under sterilized conditions when the adsorption coefficients derived from batch experiments were adjusted. No transport modeling was considered for nonsterilized conditions.

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