Geoscience Reference
In-Depth Information
Subsurface contamination by uranium wastes and contaminant speciation during
transport from a wastewater pond (originating from a plutonium production plant)
to groundwater were studied by Catalano et al. (
2006
). Land disposal of basic
sodium aluminates and acidic U(VI)-Cu(II) and their redistribution in the vadose
zone resulted in development of a groundwater uranium plume. The solid-phase
speciation of uranium from the base of the pond, through the subsurface, to the
groundwater region was investigated by analyzing a depth sequence of sediments.
The mineralogy of the sediments was relatively uniform (quartz, plagioclase
feldspar with minor muscovite, chlorite, hornblende, and smectite) except for the
calcite content, which was greater in the upper sediment below the pond and
decreased with depth. The uranium concentration is positively correlated with the
calcite content. Near-surface sediments contain uranium coprecipitated with calcite
and formed due to overneutralization of the waste pond with NaOH. At interme-
diate depths in the vadose zone, meta-torbernite Cu(UO
2
PO
4
)
2
8H
2
O was detected,
which presumably precipitated during pond operation. In the deeper zones and the
groundwater, uranium was predominantly sorbed onto phyllosilicates.
Based on EXAFS (extended X-ray adsorption fine structure) spectra, Catalano
et al. (
2006
) suggest that the chemical speciation of uranium changed systemati-
cally with increasing depth, even though XANES (X-ray adsorption near-edge
structure) spectroscopy indicates uranium presence throughout the subsurface,
primarily as U(VI). The phases present in the sample, as evaluated by PCA
(principle component analysis) of the EXAFS spectra, include two primary
components. In small concentrations, additional phases also may be found.
Because adsorbed U(VI) is the dominant uranium species in the contaminated
sediment, Catalano et al. (
2006
) suggest that adsorption-desorption processes
dominate the future fate and transport of uranium in the site. Ca
2+
and CO
3
2-
in
the local groundwater, at concentrations close to saturation with respect to calcite,
complex with U(VI), thus inhibiting its reaction with the solid phase and favoring
contaminant transport to the groundwater.
Natural organic matter, in general, and humic substances, in particular, may
affect the fate and transport of heavy metals in the subsurface by speciation of
toxic chemicals. Binding constants of divalent mercury (Hg
2+
) by subsurface
organic matter were studied by Khwaja et al. (
2006
). The formation constants of
mercury in humic substances extracted from peat (PHA) and from a soil with high
soil organic matter (SOM) content were reported. The authors used a competitive
ligand exchange method with DL-penicillamine, a synthetic thiol amino acid that
has Hg
2+
binding abilities, to determine distribution coefficients (K
oc
) for Hg
2+
.
The formation constants for PHA and SOM were calculated assuming that Hg
2+
bonds to two thiol groups of bidentate sites. Khwaja et al. (
2006
) show a linear
increase in log K
oc
in the pH range 1.9-5.8 and the slope of pH versus log K
oc
was
2.68. This indicates that two or more protons are released when each Hg
2+
is
bound to two thiol groups.
The effect of solution chemistry on the speciation of the organic contaminant
1-naphthol (1-hydroxynaphthalene) and its complexatiom with humic acid is
reported by Karthikeyan and Chorover (
2000
). The complexation of 1-naphthol
