Geoscience Reference
In-Depth Information
Two well-defined trends in the As(III) oxidation reactions can be distinguished: (1)
the extent of As(III) oxidation decreases with increasing pH from 4.5 to 6.0 and (2)
oxidation on a percent basis is suppressed with increasing initial As(III) concen-
tration from 100 to 300 lM. The pH effects on As(III) oxidation may have been
influenced by competitive adsorption reactions between As(III) and reaction
products (e.g., Mn(II)) and were not influenced by arsenic solution speciation. The
suppressed As(III) oxidation rate constant may be a result of differences in the
amount of Mn(II) release, which compete with dissolved As(III) species for
unreacted Mn(IV) surface sites, and of Mn(II) adsorption, which inhibit the
reaction between As(III) and Mn(IV) surface sites.
The extent and rate of As(III) oxidation on birnessite surfaces are affected
strongly by sorbed or competitive metal ligands in solution. Figure
16.6
shows
As(III) oxidation when Zn is pre-adsorbed or applied in solution. The abbrevia-
tions shown in the figure denote specific reaction conditions used. For example,
''PAs100ph45'' refers to 100 lM of Zn pre-sorbed prior to the 100 lM As(III)
addition at pH 4.5, and ''SAs100ph6'' refers to the simultaneous 100 lM Zn/
100 lM As(III) addition at pH 6.0. Even though adsorbed Zn was present in the
system, As(III) readily oxidized over time. However, Power et al. (
2005
) suggest
that Zn is likely to form inner-sphere complexes on birnessite surfaces and
chemisorbed Zn ions inhibit electron transfer reactions. When Zn was present,
As(III) oxidation was further suppressed by nonadsorbed and pre-adsorbed Zn,
compared with the control system, but the pre-adsorbed system was more effective
in interfering with electron transfer reactions.
Disposal of spent nuclear fuel and other radioactive wastes in the subsurface
and assessment of the hazards associated with the potential release of these con-
taminants into the environment require knowledge of radionuclide geochemistry.
Plutonium (Pu), for example, exhibits complex environmental chemistry; under-
standing the mechanism of Pu oxidation and subsequent reduction, particularly by
Mn-bearing minerals, is of major importance for predicting the fate of Pu in the
subsurface.
Plutonium may exist simultaneously in several oxidation states. Choppin (
2003
)
shows that, in oxic natural groundwaters, Pu may exist as Pu(IV), Pu(V), and
Pu(VI); the most common form is believed to be Pu(IV), found in the environment
as PuO
2(s)
. While Pu(IV) is found adsorbed to solid particles or associated with
suspended particulates in natural waters, Pu(V) is the predominant form in the
natural aqueous phase (Penrose et al.
1987
).
An example of plutonium transformation in the environment is given by con-
sidering the oxidation states of Pu adsorbed on a natural tuff, originating from
Yucca Mountain (Powell et al.
2006
). This tuff contains trace quantities of Mn
oxides and more abundant Fe oxide phases. After adding aqueous Pu(V) to the tuff,
elemental maps generated with micro-XRF (X-ray fluorescence) imaging, Pu was
observed to preferentially associate with Mn oxides. Figure
16.7
shows L
3
-edge
X-ray absorption near-edge structure (XANES) spectra versus the relative XANES
edge energy for sorbed Pu on Yucca Mountain tuff, for measurements made six
months and two years after Pu(V) application. Six months after application, Pu(V)
