Separating selenium species by diffusion in Brazilian bentonite: a mathematical modeling approach

Bentonite was applied in diffusive studies for selenium, an emerging contaminant. Apparent and effective diffusion coefficients were determined by the planar source method involving the mobility of selenium species. A double Gaussian function described the results. Different diffusion coefficients were associated with different mobilities, and consequently, the coexistence of two selenium species: Se(IV) and Se(VI). Da results were higher for Se(VI), ≈10−10 m2 s−1, while for Se(IV) were ≈10−12 m2 s−1. Results from sequential extraction and distribution coefficient (Kd) justified the greater mobility of Se(VI) compared to Se(IV). While the increase in redox potential from 448 to 511 mV may be associated with Se(IV) oxidation in an interconversion process, the diffusion in bentonite demonstrates that applications in geological barriers deserve attention regarding the mobilization of selenium species. Interconversions can mobilize selenium, as most reduced ones can shift to most oxidized species and be mobilized, enhancing the environmental contamination. Keywords: Selenium, diffusion, speciation, mathematical modeling, bentonite