Coupled production and transport of selenium vapor in unsaturated soil: evaluation by experiments and numerical simulation

Volatilization of selenium (Se) from soil to the atmosphere involves severa l sequential chemical reactions that form volatile Se species, followed by transport of the gaseous Se through the soil. This paper describes a numeri cal model that simulates the chemical and physical processes governing the production and transport of Se vapor in unsaturated soil. The model couples the four Se species involved in the production of Se vapor through chemica l reactions, and allows each to migrate through the soil by advection, liqu id or vapor diffusion depending on its affinity for the dissolved or vapor phase. The coupled transformations and transport of the four Se species, i. e., selenate, selenite, elemental and organic Se, and Se vapor, were calcul ated based on the Crank-Nicolson finite difference method. The model was us ed to analyze fluxes of Se vapor measured from a soil amended with inorgani c Se in the form of selenate and covered with unamended clean soil of vario us thicknesses. Evolution of Se vapor from the soil was very fast, with mea surable amounts of Se detected within 24 h. The peak of Se volatilization, detected at the 6th day, reached 3.31 Se mug/day for the uncovered soil, bu t was reduced to near the detection limit (0.05 mug/day) in the presence of a 8- or 16-cm clean soil cover. With two reaction rate coefficients fitted to the data, the model described Se volatilization very well. The estimate d rate coefficient of Se methylation was unexpectedly high, with a value of 0.167/day. The net volatilization of Se, however, was severely inhibited b y the fast demethylation, i.e., the reverse reaction which converted volati le Se species back into nonvolatile forms. As a result, Se vapor only penet rated a few centimeters in the soil. The demethylation rate coefficient, as sessed by independent transport experiments using dimethyl selenide, was es timated as 186.8/day, corresponding to a half-life of only 5.3 min for Se v apor. Results of this study indicated that rapid demethylation of Se vapor during its diffusive transport through a soil is probably an important limi ting factor in the volatilization of Se under natural conditions. (C) 2001 Elsevier Science B.V. All rights reserved.