Understanding Partition Coefficient, Kd, Values, Appendix I

APPENDIX I Partition Coefficients For ThoriumAppendix I Partition Coefficients For Thorium I.1.0 BACKGROUND Two generalized, simplifying assumptions were established for the selection of thorium Kd values for the look-up table. These assumptions were based on the findings of the literature review conducted on the geochemical processes affecting thorium sorption. The assumptions are as follows: -9 C Thorium adsorption occurs at concentrations less than 10 M. The extent of thorium adsorption can be estimated by soil pH. -9C Thorium precipitates at concentrations greater than 10 M. This concentration is based on the solubility of Th(OH) at pH 5.5. Although (co)precipitation is usually quantified 4with the solubility construct, a very large K value will be used in the look-up table to dapproximate thorium behavior in systems with high thorium concentrations. These assumptions appear to be reasonable for a wide range of environmental conditions. However, these simplifying assumptions are clearly compromised in systems containing high alkalinity (LaFlamme and Murray, 1987), carbonate (LaFlamme and Murray, 1987), or sulfate (Hunter et al., 1988) concentrations, and low or high pH values (pH values less than 3 or greater than 8) (Hunter et al., 1988; LaFlamme and Murray, 1987; Landa et al., 1995). These assumptions will be discussed in more detail in the following sections. Thorium K values and some important ancillary parameters that influence sorption ...