Kinetics of selenomethionine disappearance from reclaimed coal mine soils of Wyoming, U.S.A

Selenomethionine (SeM) is an organic toxicant that is present in seleniferous environments. No kinetic data is yet available regarding SeM reactions in coal mine environments, where selenium (Se) toxicity is a potential concern. A kinetic study was conducted on two reclaimed coal mine soils (Typic Torriorthents) from Wyoming having sandy and clayey textures. Four levels of SeM treatments (0, 50, 100 micromolar, and plant amendment from the mine vegetation) were reacted with the soils for 4, 7, 14, 28, 42, 56, and 84 days to characterize the kinetic behavior of overall SeM disappearance from soil solutions. Detection of SeM in soil solutions at the control level (0 micromolar SeM) indicated occurrence of indigenous SeM in the soils. In the plant-amended soil solutions, much greater concentrations of SeM were observed as compared with the soil-only systems. This indicated the plant material was a more potential source of SeM than the mine soils. A time-dependent loss in solution SeM concentrations was observed for both soils under 0, 50, 100 micromolar SeM treatments. For the soil-plant mixtures, the solution SeM concentration increased initially, reached a maximum after 14 days, and then decreased thereafter. In the plant-amended soil solutions, SeM concentrations at all time intervals were higher for the sandy as compared to the clayey soil. At 50 and 100 micromolar SeM treatments, the solution pH was linearly related to the percentages of SeM disappeared from the solutions; greater percentage of SeM was removed from solutions at comparatively lower pH levels, which was greater than or equal to 90% at pH 7.7 for both soils. Solution SeM concentrations decreased exponentially with time following first-order kinetic reactions. Under all applications (except for the control), C0 (SeM concentration at t = 0) values for the sandy soil were greater than those determined for the clayey soil, indicating higher solution SeM availability for the former and more SeM retention by the latter at t = 0. Comparison of C0 in controls (0 micromolar SeM addition) suggested greater indigenous SeM in the clayey soil. For both soils, C0 values under different treatments followed the order, (soil + 100 micromolar) > (soil + 50 micromolar) > (soil + 0 micromolar). The specific reaction rate constants (Kr) of SeM for both soils were similar (0.031 and 0.029 day-1 for sandy and clayey soils, respectively); low Kr values indicated that SeM loss from our reclaimed coal mine soil solutions would follow rather slow kinetics. The half-life (t(0.5)) of SeM varied from 15 to 55 days depending on treatment level. The knowledge obtained from this study should contribute in developing time-based Se reclamation strategies in coal mine environments.