UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

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competing anions such as SO4 , and CO3 /HCO3 . The adsorption data developed by these
investigators was used to calculate the Kd values (Appendix E). These Kd values were used as the
basis to develop the look-up Table 5.7.
5.4.6.2.1 Limits of K d Values with Respect to pH
Natural soil pH typically ranges from about 4 to 11 (Richards, 1954). The 2 most common
methods of measuring soil pH are either using a soil paste or a saturation extract. The standard
procedure for obtaining saturation extracts from soils has been described by Rhoades (1996). The
saturation extracts are obtained by saturating and equilibrating the soil with distilled water
followed by collection using vacuum filtration. Saturation extracts are usually used to determine
the pH, the electrical conductivity, and dissolved salts in soils.
The narrow pH ranges in the look-up table (Table 5.7) were selected from the observed rate of
change of K d with pH. The K d values for all 4 soils were observed to decline with increasing pH
and at pH values beyond about 9, K d values for Cr(VI) are #1 ml/g (see Appendix E).
5.4.6.2.2 Limits of K d Values with Respect to Extractable Iron Content
The soil characterization data provided by Rai et al. (1988) indicate the soils with DCB
extractable iron contents above ~0.3 mmol/g can reduce Cr(VI) to Cr(III). Therefore the
measured K d values for such soils reflect both redox-mediated precipitation and adsorption
phenomena. The data also show that soils with DCB extractable iron contents of about
0.25 mmol/g or less do not appear to reduce Cr(VI). Therefore, 3 ranges of DCB extractable iron
contents were selected which represent the categories of soils that definitely reduce ($0.3
mmol/g), probably reduce (0.26 - 0.29 mmol/g), and do not reduce (#0.25 mmol/g) Cr(VI) to
Cr(III) form.
5.4.6.2.3 Limits of K d Values with Respect to Competing Anion Concentrations
The adsorption data (Rai et al., 1988) show that when total sulfate concentration in solution is
about 2 x 10 -3 M (191.5 mg/l), the chromium K d values are reduced by about an order of
magnitude as compared to a noncompetitive condition. Therefore, a sulfate concentration of
about 2 x 10 -3 M (191.5 mg/l) has been used as a limit at which an order of magnitude reduction
in K d values are expected. Four ranges of soluble sulfate concentrations (0 - 1.9, 2 -18.9, 19 -
189, and $190 mg/l) have been used to develop the look-up table. The soluble sulfate
concentrations in soils can be assessed from saturation extracts (Richards, 1954).
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