UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

5.3.6.2.1 Limits of K d Values with Respect to pH
Of the 177 cesium K d values obtained from the literature, 139 of them had associated pH values
for the system under consideration (Appendix D). The average pH of the systems described in the
data set was pH 7.4, ranging from pH 2.4 to 10.2. The correlation coefficient (r) between pH and
cesium K d values was 0.05. This is clearly an insignificant correlation. This poor correlation may
be attributed to the fact that other soil properties having a greater impact on cesium K d values
were not held constant throughout this data set.
5.3.6.2.2 Limits of K d Values with Respect to Potassium, Ammonium, and Aluminum/Iron-
Oxides Concentrations
Potassium, ammonium, and aluminum/iron-oxide mineral concentrations were rarely, if at all,
reported in the experiments used to comprise the cesium K d data set (Appendix D). It was
anticipated that dissolved potassium and ammonium would compete with cesium for adsorption
sites, thereby decreasing K d values. The presence of aluminum and/or iron oxides in the solid
phase was expected to increase cesium K d values.
5.4 Chromium Geochemistry and K d Values
5.4.1 Overview: Important Aqueous- and Solid-Phase Parameters
Controlling Retardation
A plume containing high concentrations of chromium is more likely to be composed of Cr(VI)
than Cr(III) because the former is less likely to adsorb or precipitate to the solid phase.
Chromium(VI) is also appreciably more toxic than Cr(III). It exhibits significant subsurface
mobility in neutral and basic pH environments. In acid environments, Cr(VI) may be moderately
adsorbed by pH-dependent charge minerals, such as iron- and aluminum-oxide minerals. The
reduction of Cr(VI) to Cr(III) by ferrous iron, organic matter, and microbes is generally quite
rapid whereas the oxidation of Cr(III) to Cr(VI) by soil manganese oxides or dissolved oxygen is
kinetically slower. The most important aqueous- and solid-phase parameters controlling
retardation of chromium include redox status, pH, and the concentrations of aluminum- and ironoxide
minerals and organic matter.
5.4.2 General Geochemistry
Chromium is found in the environment primarily in the +3 and +6 oxidation states. The
geochemical behavior and biological toxicity of chromium in these 2 oxidation states are
profoundly different. Chromium(VI) tends to be soluble, forms anionic or neutral dissolved
species, can be very mobile, and is acutely toxic (Nriagu and Nieboer, 1988). In contrast, Cr(III)
tends to precipitate, forms cationic dissolved species, is immobile under moderately alkaline to
slightly acidic conditions, and is relatively nontoxic. The primary human activities leading to the
introduction of chromium into the environment are ore processing, plating operations, and
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