Understanding Variation in Partition Coefficient, Kd, Values Volume II

5.2.6.2 Look-Up Tables
One cadmium K d look-up table was created. The table requires knowledge of the pH of the system
(Table 5.4). The pH was selected as the key independent variable because it had a highly significant (P
< 0.001) correlation with cadmium K d, a correlation coefficient value of 0.75. A detailed explanation
of the approach used in selecting the K d values used in the table is presented in Appendix C. Briefly, it
involved conducting a regression analysis between pH and K d values). The subsequent regression
equation was used to provide central estimates. Minimum and maximum values were estimated by
plotting the data and estimating where the limits of the data existed.
There is an unusually wide range of possible cadmium K d values for each of the 3 pH categories. The
cause for this is likely that there are several other soil parameters influencing the K d in addition to pH.
Unfortunately, the correlations between the cadmium K d values and the other soil parameters in this
data set were not significant (Appendix C).
5.2.6.2.1 Limits of K d Values With Respect to Aluminum/Iron-Oxide Concentrations
The effect of iron-oxide concentrations on cadmium K d values was evaluated using the data presented
in Appendix C. Of the 174 cadmium K d values in the data set presented in Appendix C, only 16
values had associated iron oxide concentration data. In each case iron, and not aluminum, oxide
concentration data were measured. The correlation coefficient describing the linear relationship
between cadmium K d values and iron oxide concentration was 0.18, which is nonsignificant at the 5
percent level of probability. It was anticipated that there would be a positive correlation between iron
or aluminum oxide concentrations and cadmium K d values because oxide minerals provide adsorption
(surface complexation) sites.
Table 5.4. Estimated range of K d values for cadmium as a function of pH.
[Tabulated values pertain to systems consisting of natural soils (as
opposed to pure mineral phases), low ionic strength (< 0.1 M),
low humic material concentrations (