UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

(tarapacaite) in chromium sludge from a plating facility. They also reported that BaCrO 4 formed
a complete solid solution with BaSO 4. They concluded that these solid solutions can be a major
impediment to the remediation of chromium-contaminated sites by pump-and-treat technologies.
Chromium(VI) is a strong oxidant and is rapidly reduced in the presence of such common electron
donors as aqueous Fe(II), ferrous iron minerals, reduced sulfur, microbes, and organic matter
(Bartlett and Kimble, 1976; Nakayama et al., 1981). Studies indicate that Cr(VI) can be reduced
to Cr(III) by ferrous iron derived from magnetite (Fe 3O 4) and ilmenite (FeTiO 3) (White and
Hochella, 1989), hematite (Fe 2O 3) (Eary and Rai, 1989), 1 and pyrite (FeS 2) (Blowes and Ptacek,
1992).
The reduction of Cr(VI) by Fe(II) is very rapid. The reaction can go to completion in a matter of
minutes (Eary and Rai, 1989). The rate of reduction of Cr(VI) increases with decreasing pH and
increasing initial Cr(VI) and reductant concentrations (Palmer and Puls, 1994). Interestingly, this
reaction does not appear to be slowed by the presence of dissolved oxygen (Eary and Rai, 1989).
When the pH is greater than 4, Cr(III) can precipitate with Fe(III) to form a solid solution with
the general composition Cr xFe 1-x(OH) 3 (Sass and Rai, 1987). The solubility of chromium in this
solid solution decreases as the mole fraction of Fe(III) increases. The oxidation reaction proceeds
much more slowly than the reduction reaction; the former reaction requires months for
completion (Eary and Rai, 1987; Palmer and Puls, 1994). Only 2 constituents in the environment
are known to oxidize Cr(III): dissolved oxygen and manganese-dioxide minerals [e.g., pyrolusite
($-MnO 2)]. Eary and Rai (1987) reported that the rate of Cr(III) oxidation was much greater in
the presence of manganese-dioxide minerals than dissolved oxygen.
5.4.5 Sorption/Desorption
The extent to which Cr(III) sorbs to soils is appreciably greater than that of Cr(VI) because the
former exists in groundwater as a cation, primarily as Cr 3+ (and its complexed species), whereas
2- -
the latter exists as an anion, primarily as CrO4 or HCrO4.
Most information on Cr(VI) adsorption
comes from studies with pure mineral phases (Davis and Leckie, 1980; Griffin et al., 1977; Leckie
et al., 1980). These studies suggest that Cr(VI) adsorbs strongly to gibbsite ("-Al2O3) and
amorphous iron oxide [Fe2O3·H2O(am)] at low to medium pH values (pH 2 to 7) and adsorbs
weakly to silica (SiO2) at all but very low pH values (Davis and Leckie, 1980; Griffin et al., 1977;
Leckie et al., 1980). These results can be explained by considering the isoelectric points (IEP) 2 of
these minerals. When the pH of the system is greater than the isoelectric point, the mineral has a
net negative charge. When the pH is below the isoelectric point, the mineral has a net positive
1 Eary and Rai (1989) attributed the reduction of Cr(VI) to Cr(III) by hematite (Fe2O 3) as
containing having trace quantities of Fe(II).
2 The isoelectric point (IEP) of a mineral is the pH at which it has a net surface charge of zero.
More precisely, it is the pH at which the particle is electrokinetically uncharged.
5.20