Processing of High Level Waste: Spectroscopic Characterization of Redox Reactions in Supercritical Water - Final Report Page: 4 of 4
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13:46 FURMAN CHEMISTRY 4 1626402882085265548
rates- slower rates at higher degrees of polymerization. The rate of reaction was found to decrease with
increasing concentration of hydroxide ion under our experimental conditions. The rate initially increased with
increasing hydrogen peroxide concentration, but at the higher concentrations of H202 the rate of reaction
decreases. The reaction was carried out over a temperature range from 15 to 25 0C. The Arrhenius plot of
these data gives an activation energy of 63 kJ/mol. The rapid reaction of chromium hydroxide at room
temperature makes this reaction promising as a means of treating Hanford tank wastes which are high in
We have been able to fit the dependence of the effective rate constants for Cr(IU) decay on
concentration of H2O2 and OH by including two reactions. One reaction involves the conjugate base, HOO as
the oxidizing agent, and the other involves neutral hydrogen peroxide as the oxidizing agent. The order of
reaction is complex and depends critically on the acid base equilibrium for formation of HOO- by reaction of
H202 with OH
The second set of experiments has grown out of our observation of the 0-0 stretching vibration in the
Raman spectrum in the solutions prepared for the kinetic studies. In the presence of OH- a new Raman band
appears at 850 cm-1. We identify this band as the 0-0 stretch of the hydroperoxide anion. Because hydrogen
peroxide as a stronger acid than water, HO2 is the dominant form of hydrogen peroxide in basic solution. We
want to determine which species is the actual oxidizing reagent in the reaction with chromium hydroxide, but
have not yet designed experiments to answer this question. There is very little information in the literature on
the vibrational spectroscopy of H02-. We have examined the Raman spectrum of aqueous solutions of HO2
formed by reaction of OH- with H202 and Na202 with H2O and with D20. Evaporation of these solutions
provides crystalline solids, for which we have obtained infrared and Raman spectra.
Extensive ab initio calculations have been carried out to confirm this assignment. Agreement between
the calculated and experimental vibrational frequencies is obtained only when two waters of hydration attached
to HOO- are included in the calculation. For a series of species with 0-0 bonds a good correlation exists
between the calculated charge on oxygen atoms and the observed vibrational wavenumber.
Ginger Denison continued work with the P at LANL. She continued to investigate the kinetics of
oxidation of Cr(1) in basic solution using hydrogen peroxide as the oxidizing agent. She determined the multi-
exponential rate constants for samples with different aging histories covering periods up to several weeks of
aging. Work is continuing on the kinetics of samples of chromium hydroxide that have been aged for more than
one year. These reactions are slower than those of recently prepared samples, but we have found that at
temperatures above 35 oC the reaction does proceed to convert Cr(flI) to Cr(VI). At higher temperatures a
greater fraction of the chromium hydroxide reacts. Mathcad was used to model the dependence of effective rate
constants on the concentrations of hydroxide and hydrogen peroxide. Data collected during the grant period
cover a wide range of concentration conditions. These data are being used to test the general model of a
complex reaction involving at least two different reactions with different oxidizing agents.
Kristen Little worked at Los Alamos with David Monis on the speciation of europium as a surrogate for
uranium in clays using emission spectroscopy. David has had an EMSP grant, and Kristin's project was related
to his interest in remediation of soils contaminated with uranium.
Five students have given talks at the Southeast Regional ACS meeting and the National Conference of
Undergraduate Research. Two posters were presented at the ACS meeting in New Orleans on August 22, 1999.
Ginger Denison presented a talk on her work at the annual meeting of the Beckman Scholars in Newport Beach,
CA in August, 2000. Kristin Little will present her work at the NCUR meeting in Lexington, KY in March,
2001. Senior papers by students who have worked on this project are available from the P. A manuscript on the
vibrational spectrum of H02 is nearly complete and will be submitted to JACS.
N0. 589 D05
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Arrington, C. A., Jr. Processing of High Level Waste: Spectroscopic Characterization of Redox Reactions in Supercritical Water - Final Report, report, November 15, 2000; Greenville, South Carolina. (digital.library.unt.edu/ark:/67531/metadc715181/m1/4/: accessed October 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.