Complexation and redox interactions between aqueous plutonium and manganese oxide interfaces Page: 3 of 4
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Pu(V) were also reduced to Pu(IV) after 24 hours of contact.
The best fits were achieved by fitting the residual plutonium
in each case as Pu(V). There was something wrong with
the absolute energy calibration in Fig. 3(b) such that all of
the plutonium standards needed to be shifted 0.92 eV to
correctly line up the references of the plutonium standards to
the reference from the data. In all four spectra, however,
the characteristic high-energy plutonyl feature is missing.
This indicates that the plutonyl cation (either Pu(VI) or
Pu(V)) is mostly reduced and is instead sorbed to the
mineral surfaces as Pu(IV). Extended x-ray absorption fine
structure (EXAFS) data, which will be presented in a future
publication, also give evidence for the reduction to
Pu(IV).15) Fits to the EXAFS show that the nearest
oxygen neighbors have bond lengths resembling what would
be expected for Pu(IV).2.
1.1.
'~0.
So* Data
Total fit
- ----- Pu(IV) standard
0 -------- Pu(V) standard
-Pu(VI) + MnOOH
5
0-
5 - - ---) 180
2.0
~0
1.5
N
E 1.0-
0.540
18040
18050 18060 18070
18050 18060
Energy (eV)18070
into solution immediately after formation. The Mn(II) in
solution could reduce the plutonium in solution to Pu(IV),
which would then sorb to the mineral surfaces.
IV. Conclusions
Manganite and hausmannite, which represent typical
Mn(III) minerals found in vadose zone conditions under the
Hanford tank farm, have been shown to sorb Pu(VI) and
Pu(V) at pH 5. The hausmannite sorbs approximately 2-3
times the amount of plutonium than the manganite due to its
larger surface area. The comparison of XANES spectra
taken at the Pu Lm11 edge to standard solutions containing a
.Data
-Total fit
-- --- Pu(IV) standard
2.0 -(a) .....--- Pu(V> standard
Pu(VI) + Mn304
1.5-
1 -
0.5 -a 18
0 2.0-
.0
1.5-
E
C) 1.0-
Z18080
0.5-
040
18040
18080
Fig. 2 XANES spectra of (a) Pu(VI) and (b) Pu(V) solutions
in contact with MnOOH at pH 5 for 24 hours. The
plutonium standards used to fit the spectra are also
shown in their relative amounts.
The exact mechanism behind the reduction of plutonium
is currently unknown, however one possibility is that
Mn(III) (or Mn(II) in the case of hausmannite) is directly
reducing the plutonium (and therefore becoming oxidized in
the process) on the mineral surface or when the plutonium in
solution is in close contact with the surface. Another
possibility is that the Mn(III) in these minerals is undergoing
disproportionation after contact with the solution into Mn(II)
and Mn(IV). Mn(II) (which is already present in
hausmannite) is highly soluble and would likely dissolve18050 18060 18070 18080
18050 18060
Energy (eV)18070
18080
Fig. 3 XANES spectra of (a) Pu(VI) and (b) Pu(V) solutions
in contact with Mn304 at pH 5 for 24 hours. The
plutonium standards used to fit the spectra are also
shown in their relative amounts. The absolute
energy calibration was unreliable for the data set in
(b), and an overall energy shift of 0.92 eV was
allowed for the plutonium standards in the fit.
single plutonium oxidation state shows that both manganite
and hausmannite reduce Pu(VI) and Pu(V) to Pu(IV) after
24 hours of contact. The mechanism responsible for the
plutonium reduction could be attributed to direct reduction
by Mn(III) or Mn(II) on the mineral surfaces or possibly
from Mn(II) in solution, which would be formed as the
result of disproportionation of Mn(III). Regardless of the
exact mechanism governing this redox interaction between
manganese and plutonium, the reduction to Pu(IV) has
important environmental ramifications. For instance,
Pu(IV) forms insoluble colloids, which carry a positive
surface charge and have a tendency to adsorb on various(b)
Pu(V) + Mn304(b)
Pu(V) + MnOOH .
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Shaughnessy, Dawn A.; Nitsche, Heino; Booth, Corwin H.; Shuh, David K.; Waychunas, Glenn A.; Wilson, Richard E. et al. Complexation and redox interactions between aqueous plutonium and manganese oxide interfaces, article, November 1, 2001; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc715410/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.