UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

5.2.6.2.2 Limits of K d Values with Respect to CEC
The effect of CEC 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 22 values had
associated CEC data. The correlation coefficient describing the linear relationship between
cadmium K d values and CEC was 0.40, which is nonsignificant at the 5 percent level of
probability. It was anticipated that there would be a positive correlation between CEC and
cadmium K d values because cadmium can adsorb to minerals via cation exchange.
5.2.6.2.3 Limits of K d Values with Respect to Clay Content
The effect of clay content 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, 64 values
had associated clay content data. The correlation coefficient describing the linear relationship
between cadmium K d values and clay content was -0.04, which is nonsignificant at the 5 percent
level of probability. It was anticipated that there would be a positive correlation between clay
content and cadmium K d values, because clay content is often highly correlated to CEC, which in
turn may be correlated to the number of sites available for cadmium adsorption.
5.2.6.2.4 Limits of K d Values with Respect to Concentration of Organic Matter
The effect of organic matter concentration, as approximated by total organic carbon, 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, 63 values had associated total
organic carbon concentration data. The correlation coefficient describing the linear relationship
between cadmium K d values and total organic carbon concentration was 0.20, which is
nonsignificant at the 5 percent level of probability. It was anticipated that there would be a
positive correlation between total organic carbon concentration and cadmium K d values because
soil organic carbon can have extremely high CEC values, providing additional sorption sites for
dissolved cadmium.
5.2.6.2.5 Limits of K d Values with Respect to Dissolved Calcium, Magnesium, and Sulfide
Concentrations, and Redox Conditions
Calcium, magnesium, and sulfide solution concentrations were rarely, if at all, reported in the
experiments used to comprise the cadmium data set. It was anticipated that dissolved calcium and
magnesium would compete with cadmium for adsorption sites, thereby decreasing K d values. It
was anticipated that sulfides would induce cadmium precipitation, thereby increasing cadmium K d
values. Similarly, low redox status was expected to provide an indirect measure of sulfide
concentrations, which would in turn induce cadmium precipitation. Sulfides only exist in low
redox environments; in high redox environments, the sulfides oxidize to sulfates that are less
prone to form cadmium precipitates.
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