PNNL-13501 - Pacific Northwest National Laboratory

Permanganate Treatment for the Decontamination of Radioactive Contaminated Wastes
Study Control Number: PN00073/1480
Richard T. Hallen, Michael A. Lilga, Amber M. Gauger
The Department of Energy has radioactive waste and contaminated equipment and facilities that must be treated as part of
the nuclear defense legacy cleanup. High-level radioactive wastes, such as those stored at Hanford, Idaho, Oak Ridge,
and Savannah River, present one of the major challenges because of the complex mixture of chemicals that have been
used and disposed of to storage tanks. Fundamental unit operations such as solid/liquid separations are a challenge for
processing radioactive wastes stored at Hanford. Pretreatment will likely be required for many of the wastes prior to
solidification and final disposal.
Project Description
High-level wastes stored at Hanford and other DOE sites
contain a large variety of organic compounds. At
Hanford, the majority of the organic compounds disposed
of to the tanks include glycolate, citrate, EDTA, HEDTA,
and a few other organic complexants. These original
compounds have degraded over time to lower molecular
weight organics such as formate, oxalate, acetate, and
various amine-containing compounds. The complexant
concentrate wastes contain the highest concentration of
organic compounds and present additional challenges to
treatment and disposal. The organic complexants increase
the solubility of radioactive strontium and transuranic
wastes, and prevent the treatment by more conventional
separation processes. Permanganate, a chemical oxidant,
can destroy organic compounds which complex
radioactive ions, allowing them to precipitate from
solution. Permanganate reduction in solution under basic
conditions results in formation of dense, manganese
dioxide solids which co-precipitate/flocculate transuranics
and act as an aid to solid/liquid separations. Adding
manganese dioxide as a solid does not provide the
benefits of permanganate addition. So the permanganate
reduction chemistry and in situ solids formation are very
important.
The potential use of permanganate to decontaminate tank
waste was first demonstrated by Orth et al. (1995).
Actual waste from tank SY-101 was treated with
permanganate, and decontamination factors for strontium
were greater than 140 and for plutonium were 12 to 140.
More conventional treatment schemes did not work
because this waste contained significant concentrations of
organic complexants. Permanganate was found to be a
selective oxidant, solubilizing chromium first, then
reducing the total organic carbon concentration, and
lastly, nitrite oxidation to nitrate. However,
permanganate was never seriously considered for
pretreatment of tank wastes because it had been used in
quite high concentration (>0.1M) and increased the
volume of high activity glass produced.
Results and Accomplishments
The reaction of permanganate with various organic
compounds, formate, oxalate, glycolate, and glycine, was
studied under basic conditions. Formate and oxalate are
the major aging products from the complexants and are
present in relatively high concentrations in tank wastes.
Glycolate was added to the tanks and is also similar to
citrate in structure. Glycine was studied as a model
compound for the amine-based complexants, EDTA and
HEDTA, as well as the aging products from these
compounds. Reactions were conducted at various
concentrations of permanganate (oxidant) and organic
(reductant). Permanganate reaction with nitrite was also
studied at one concentration for comparison to the
reactions of the organic compounds.
Upon reaction with a reductant, permanganate undergoes
a series of reactions and oxidation changes from Mn(VII)
to Mn(VI), Mn(V), and Mn(IV) which precipitates in
hydroxide solution. Permanganate, Mn(VII), and
manganate, Mn(VI), are highly colored allowing the use
of UV-VIS spectroscopy to monitor the reduction
chemistry of manganese. Mn(VII) is characterized by
strong absorption at 546, 526, and 311 nm. Mn(VI) is
characterized by strong absorption at 606, 439, 347, and
299 nm. At treatment concentrations of 0.03 to 0.05M
permanganate and with an excess of reductant such as
formate, complete reduction of Mn(VII), purple solution,
to Mn(IV), clear solution and dark brown/black solids,
occurs in less than 5 minutes. The characteristic green
color of Mn(VI) is observed for a few minutes while the
reaction is occurring. Conducting the experiments at
lower concentrations of permanganate and stiochiometric
or lower levels of reductant, the reactions slow down so
Separations and Conversions 431