Partitioning between aqueous and soil system components for soluble tungsten and lead species.

The establishment of reliable sorptive Kd values for a given metal in a given soil is a critical preliminary step in the determination of metal mobility. The distribution coefficient, Kd, is partially based on assuming the contaminant is a single species, and equilibrium conditions are established between the solid and aqueous phases. However, metals such as tungsten (W) may exist as different chemical species, depending on the aqueous and solid phase chemical environment, and are shown to require an extended period of time for the system to approach equilibrium. Due to the more involved speciation characteristics associated with tungsten, the effect of extended equilibrium periods on W Kd values is examined in this study. The sorption characteristics of tungsten and lead in three natural silty sand (SM) soils are compared. Partition coefficients were obtained both at the traditional 24hour contact time (Kd) and then again after 100 days (Kd). The Kd values were for Pb representative of Kd values for the soils examined. However, in the case of W, dynamic sorptive behavior was observed. The ratios of Day 100 to Day 1 distribution coefficients were 19.6, 6.2, and 2.4 for tungsten in samples SM1, SM2, and SM3, respectively. This suggests that a longer equilibration time may provide a more accurate reflection of W mobility in subsurface environments when using the sorption Kd as a predictive tool.