PNNL-13501 - Pacific Northwest National Laboratory

the stepwise reduction can be observed. Figure 1 shows
spectra at various times for the reduction of 0.01M
Mn(VII) to Mn(VI) with 0.005M formate. The Mn(VI)
concentration reaches a maximum around 15 minutes
before it begins decreases as it disproportionates to
Mn(VII) and Mn(V). The Mn(VII) concentration
increases with time as shown in Figure 2 because the
formate had been entirely consumed in the initial
oxidation. The Mn(V) reacts with Mn(VI) producing
more Mn(VII) and Mn(IV) which precipitates from
solution.
Absorbance
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Formate Reduction of Mn(VII) to Mn(VI)
250 450 650
Wavelength (nm)
0.167 min
432 FY 2000 Laboratory Directed Research and Development Annual Report
1 min
2 min
3 min
5 min
9 min
Figure 1. Spectral change on reduction of Mn(VII) to
Mn(VI) with formate
Absorbance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Mn(VI) reacts to give Mn(VII)
0
250 450
Wavelength (nm)
650
85 min
211 min
1303 min
1713 min
2691 min
3137 min
Figure 2. Spectral change as Mn(VI) disproportionates to
Mn(VII)
Using extinction coefficients calculated from Mn(VII)
and Mn(VI) solutions of known concentrations,
concentrations at various reaction times can be
determined and used to examine the kinetics of the
reactions. The reaction kinetics is very complex, with
many concurrent and competing reactions. To compare
the rates of reactions of the various reductants, reactions
were carried out under the same conditions, and the initial
rate of Mn(VII) disappearance was calculated under
psuedo-first-order conditions. The analysis revealed that
formate reacts 40 times faster than glycolate and glycine,
which react at similar rates. Oxalate and nitrite did not
react with permanganate under the conditions studied.
Reactions were allowed to go to completion and the final
products analyzed. Permanganate oxidation of formate
yields carbonate. Both glycolate and glycine yield
oxalate on oxidation, and glycine also yields ammonia.
The permanganate (Mn(VII) is reduced to Mn(IV) which
precipitates from solution. The pH of the solution is
higher then the isoelectric point of manganese dioxide, so
hydroxide is consumed by deprotanation of the
precipitated solids. The dark brown/black solids are
dense and settle to the bottom of the reaction vessel,
resulting in a clear supernatant solution.
Summary and Conclusions
Permanganate reacts rapidly with organic compounds
present in radioactive wastes resulting in a manganese
dioxide precipitate that removes radioactive elements
from solution. The amount of complexant destruction
will be low when significant concentration of formate is
present, since formate reacts much faster. Permanganate
does not react directly with oxalate or nitrite under typical
treatment conditions. A reduction in total organic carbon
will be seen for the oxidation of formate to carbonate, but
glycolate and glycine oxidation yields oxalate, which
would still be included in the total organic carbon
analysis.