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 several sequential chemical reactions that form volatile Se species, followed by transport of the gaseous Se through the soil. This paper describes a numerical 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 chemical reactions, and allows each to migrate through the soil by advection, liquid 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 calculated based on the Crank-Nicolson finite difference method. The model was used to analyze fluxes of Se vapor measured from a soil amended with inorganic Se in the form of selenate and covered with unamended clean soil of various thicknesses. Evolution of Se vapor from the soil was very fast, with measurable amounts of Se detected within 24 h. The peak of Se volatilization, detected at the 6th day, reached 3.31 Se microgram/day for the uncovered soil, but was reduced to near the detection limit (0.05 microgram/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 estimated rate coefficient of Se methylation was unexpectedly high, with a value of 0.167/day. The net volatilization of Se, however, was severely inhibited by the fast demethylation, i.e., the reverse reaction which converted volatile Se species back into nonvolatile forms. As a result, Se vapor only penetrated a few centimeters in the soil. The demethylation rate coefficient, assessed by independent transport experiments using dimethyl selenide, was estimated as 186.8/day, corresponding to a half-life of only 5.3 min for Se vapor. Results of this study indicated that rapid demethylation of Se vapor during its diffusive transport through a soil is probably an important limiting factor in the volatilization of Se under natural conditions.

Accelerated volatilization rates of selenium from different soils.

Selenium (Se), an element found naturally in a variety of soils, can accumulate in drainage water of lands under intensive irrigation, even reaching levels that are toxic to mammals and birds. Volatilization of Se by soil microorganisms into dimethylselenide (DMSe) can be enhanced by certain soil amendments and, thus, be used as a soil remediation process. In an 8-wk laboratory study, five soils from California and one from Germany were spiked with 75SeO3(2-) (22.3 mg/kg Se). Two amino acids (DL-homocysteine and L-methionine), a carbohydrate (pectin), and a protein (zein) were tested as soil amendments. Gaseous 75Se emissions were trapped with activated carbon and measured in a gamma counter. Depending on soil type, the cumulative volatilization from the control flasks varied between 1.2% and 9.0% of applied 75Se. Both zein and L-methionine strongly increased volatilization (max. 43% of 75Se applied), whereas DL-homocysteine had a much smaller stimulating effect. Pectin showed a moderate effect, but enhanced Se volatilization rates were sustained much longer when compared to the zein amendment. Volatilization rates of Se followed a simple first-order reaction. Gaseous Se emission in the soils treated with L-methionine yielded an S-shaped curve, which fit a growth-modified first-order rate model. Although zein and L-methionine were the most favorable treatments enhancing Se volatilization, all six soils responded differently to the soil amendments.

Role of the air-water-solid interface in bacteriophage sorption experiments.

Batch sorption experiments were carried out with the bacteriophages MS2 and phi X174. Two types of reactor vessels, polypropylene and glass, were used. Consistently lower concentrations of MS2 were found in the liquid phase in the absence of soil (control blanks) than in the presence of soil after mixing. High levels of MS2 inactivation (approximately 99.9%) were observed in control tubes made of polypropylene (PP), with comparatively little loss of virus seen in PP tubes when soil was present. Minimal inactivation of MS2 was observed when the air-water interface was completely eliminated from PP control blanks during mixing. All batch experiments performed with reactor tubes made of glass demonstrated no substantial inactivation of MS2. In similar experiments, bacteriophage phi X174 did not undergo inactivation in either PP or glass control blanks, implying that this virus is not affected by the same factors which led to inactivation of MS2 in the PP control tubes. When possible, phage adsorption to soil was calculated by the Freundlich isotherm. Our data suggest that forces associated with the air-water-solid interface (where the solid is a hydrophobic surface) are responsible for inactivation of MS2 in the PP control tubes. The influence of air-water interfacial forces should be carefully considered when batch sorption experiments are conducted with certain viruses.