Characterizing kinetics of sequential selenium transformation in soil

The speciation of selenium (Se) in a Hanford sandy loam was measured as a f unction of time at two soil water contents under aerobic and anaerobic cond itions to evaluate the kinetics of selenium transformation. The stepwise re action coefficients describing sequential selenium transformation were dete rmined using an inverse method based on a five-compartment model that inclu ded selenate, selenite, elemental Se, organic Se, and gaseous Se as the maj or Se species (or fractions). Transformation of Se was less affected by soi l water content at greater than or equal to 21.5%, but was distinctively fa ster under anaerobic conditions where O-2 was displaced with N-2 gas. Durin g the incubation period of 49 to 52 d, between 27.9 and 74.9% of the total selenium, initially present as selenate, was reduced to elemental or organi c Se species. Although volatilization of Se required a reduced precursor, t he rate of volatilization was not correlated to the rate of reduction. No v olatilization of Se was detected under anaerobic conditions, suggesting tha t O-2 was a critical factor controlling the methylation of Se, the reaction that converts nonvolatile Se species into gaseous forms.