Geoscience Reference
In-Depth Information
Fig. 16.7 Plutonium L
3
-edge XANES spectra plotted with respect to the relative XANES edge
energy for sorbed Pu on Yucca Mountain Tuff at 6 months and 2 years. All spectra taken after
two years indicated an average oxidation state of Pu(IV); those taken after 6 months had average
oxidation states predominantly of Pu(V) and Pu(VI). Reprinted with permission from Powell
et al. (
2006
). Copyright 2006 American Chemical Society
C
6
Cl
5
NO
2
in reactions and the change in sorbed Fe(II), [Fe(II)]
s
as a function of
pH are shown in Fig.
16.8
. The primary degradation route for C
6
Cl
5
NO
2
occurs
through a surface-mediated reaction with Fe(II); the final product is C
6
Cl
5
NH
2
,
with an intermediate product believed to be phenylhydroxylamine (C
6
Cl
5
NHOH).
Comparing degradation kinetics for reactions with different goethite contents
(Fig.
16.9
), Klupinski et al. (
2004
) found that [Fe(II)]
s
changes linearly with
goethite content. The rate constant (k
A
) decreased by more than a factor of 10 as
the goethite content was adjusted from 100 to 10 mg/L, and it decreased by an
even larger factor as the goethite content was further lowered to 1 mg/L. In an
additional experiment, it was observed that the rate constant is affected by
decreasing [Fe(II)]
s
while increasing the amount of goethite, which reveals that the
change in the first-order rate constant for C
6
Cl
5
NO
2
reduction, k
A
, is proportionally
greater than the change in [Fe(II)]
s
.
Klupinski et al. (
2004
) conclude that the reduction of nitroaromatic compounds
is a surface-mediated process and suggest that, with lack of an iron mineral,
reductive transformation induced only by Fe(II) does not occur. However, when
C
6
Cl
5
NO
2
degradation was investigated in reaction media containing Fe(II) with
no mineral phase added, a slow reductive transformation of the contaminant was
observed. Because the loss of C
6
Cl
5
NO
2
in this case was not described by a first-
order kinetic model, as in the case of high concentration of Fe(II), but better by a
