Geoscience Reference
In-Depth Information
Reduction and oxidation reactions in the subsurface environment lead to
transformation of organic and inorganic contaminants. We consider chromium
(Cr) as an example of an inorganic toxic chemical for which both oxidation and
reduction processes may transform the valence of this element, in subsurface
aqueous solutions, as a function of the local chemistry.
The most stable oxidation states of chromium in the subsurface environment are
Cr(III) and Cr(VI), the latter being more toxic and more mobile. The oxidation of
Cr(III) in subsurface aqueous solutions is possible in a medium characterized by
the presence of Mn(IV) oxides. Eary and Rai (
1987
), however, state that the extent
of Cr(III) oxidation may be limited by the adsorption of anionic Cr(VI) in acidic
solutions and the adsorption and precipitation of various forms of Cr(OH)
x
. These
authors also report a rapid quantitative stoichiometric reduction of aqueous Cr(VI)
by aqueous Fe(II), in a pH range covering the acidity variability in the subsurface
even in oxygenated solutions.
The effect of the subsurface environment on the kinetics of aqueous Cr(VI)
reduction and oxidation was reported by Kozuh et al. (
2000
), based on a laboratory
experiment where four soils (Table
16.1
) with different chemistry where used as
Cr(VI) incubation media. The concentrations of added Cr(VI) were 1, 10, 25, and
50 lg/g of dry soil. The kinetics of soluble Cr(VI) reduction when added to the
four soils, at a concentration of 1 lg/g of dry soil, are shown in Fig.
16.3
. The
reduction reactions are expressed by nonlinear losses (Fig.
16.3
a), with the highest
loss in the peat and lowest loss in the cambisols. Therefore, the degree of reduction
depends mostly on the organic matter content. The decrease in Cr(VI) concen-
tration is rapid during the first three days of incubation and much slower thereafter.
From the ln(c/c
o
) versus time plots of (Fig.
16.3
b), Kozuh et al. (
2000
) suggest that
Cr(VI) reduction follows a first-order reaction during the first three days of the
experiment but offer no explanation for the subsequent behavior. We see that
reduction takes place rapidly in soils with high organic matter. Additional similar
experiments show, however, that there is no reduction in concentration of soluble
Cr(VI) in the presence of soluble organic matter extracted from peat and clay soil,
even after 10 days of incubation. Based on these findings, the authors suggest that
the reduction of soluble Cr(VI) probably requires the involvement of available
solid organic matter.
Oxidation of Cr(III) occurs when the subsurface environment is characterized
by high Mn(IV, III) oxide content and low organic matter content. Kinetics of
soluble Cr(III) oxidation in four subsurface materials, with different exchangeable
Mn(IV) contents, are reported by Kozuh et al. (
2000
). Results are presented in
Fig.
16.4
for the addition of 100 lg (per g of dry material) of soluble Cr(III), at a
constant moisture and temperature; for this case, the oxidation was measured over
a 10-day period. The results indicate that oxidation of soluble Cr(III) occurs in the
three soils (clay, sand, and cambisols) with low organic matter (\3 %) and high
exchangeable Mn(IV) oxides but does not occur in the ''organic'' peat with low
Mn(IV). The concentrations of oxidized Cr(III) achieved maximum values within
two days and then decreased slightly over time. The decreases in concentration are
a consequence of competition between oxidation and reduction processes of
