Nuclear Waste, Glass, and the Fe2+/Fe3+ Ratio James C. Fanning and R. Todd Hunter Clemson University, Clemson, SC 29634
Chemical problems of considerable interest and importance may occur in many areas of science and technology where chemistry is used. Students need to be made aware of the exciting challenges in applied chemistry. One problem that falls into this category and that is under intense study a t this time is the development of procedures for the determination of the Fe2+/Fe3+ratio in nuclear waste glass. Measurements of the Fe2+/Fe3+ratio of iron-containing rocks, minerals, and glasses have long been made (1) and serve to provide structural (2) and redox (3) information about the melt from which the solid originated. The ratio is a major variable in evaluating the petrogenetic history of magmatic rocks, and it has been calibrated as a function of the melt's bulk composition, temperature, pressure, and oxygen fugacity (4). Dyar (5) has pointed out that iron partitioning between the two oxidation states in glasses is relevant to a number of problems in petrology and material science. Now with plans to immobilize high level nuclear waste in borosilicate glass, new Fe2+/Fe3+determination procedures are required. High-level nuclear waste that has resulted from the production of weapons-grade plutonium is to be encapsulated into borosilicate glass, the radioactive glass sealed into stainless steel canisters, and the canisters placed into an underground, remote repository (6-8). The nation's first full-scale glass plant to convert nuclear waste into glass is located a t the Savannah River Plant near Aiken, South Carolina, and is known as the Defense Waste Processing Facility (DWPF). The cost of the DWPF is about $900 million, and in 1990 it will begin converting to glass the nearly 30 million gallons of radioactive chemical waste now stored in carbon steel tanks. The high-level waste, which is stored in sludge form, has been accumulating for over 30 years and contains considerable amounts of iron (as glass, 7 to 12 wt % Fe). I t is to be mixed with borosilicate frit or powder and heated to 1150 OC to form glass (9,lO). The redox operating conditions of the melter where the glass is formed will be monitored by the ratio value. If the value is too low (too oxidizing), 0.6, precipitates such as sulfides (from sulfates) or noble metals will form, resulting in conductive layers between the melter electrodes and possibly an electrical short. The rapid, precise determination of the Fe2+/Fe3+ratio in silicates has been a challenging problem over the years that has drawn the attention of chemists, geochemists, and ceramists. Unfortunately, there are no silicate standards with known ratio values, so several determination methods have to be compared and checked carefully on a selected set of materials. A satisfactory procedure for use on the radioactive waste glass will require that a minimum number of manipulations be performed since protection of personnel will have to be maintained, possibly using remote control and extensive shielding. The majority of the procedures to determine the Fe2+/ Fe3+ratio in silicates involves dissolving the silicate sample over several hours in an aqueous HF-H2S04 mixture, while producing an acidic fluoride solution that promotes oxidation of Fe2+ to Fe3+. Good technique by the analyst is re888
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The Mossbauer spectrum of a simulated nuclear waste glass from the Savannah River Laboratory, Aiken, SC. The ~ e * + / F e ~ratio + by Mossbauer spectroscopy is 0.43 k 0.01.
quired in order to avoid air oxidation of Fe2+ to Fe3+. Silicates with no easily oxidizable impurities other than Fe2+ have been analyzed by titration methods using an oxidant such as standard permanganate (11). However, titration methods are normally not specific for Fe2+and are unsuitable for use with the nuclear waste glasses. (A sample of such glass may contain over 50 different elements!) Wilson (12) developed an interesting procedure to prevent Fe2+oxidation in solution and avoid the troublesome use of an inert gas and controlled-atmosphere box. He found that addition of ammonium vanadate to the acidic dissolution mixture resulted in no Fe2+loss through the following equilibrium:
Increasing the pH of the solution after dissolution shifts the equilibrium to the left. Incorporating the Wilson procedure with a spectrophotometric measurement of Fe2+has resulted in a relatively easy and exact method of determining Fe2+ in glass (13). Determination of the total amount of iron in a silicate sample may be achieved with chemical procedures or instrumental methods, such as inductively coupled plasma, atomic absorption spectroscopy, electron microprobe, or X-ray fluorescence spectroscopy. With the Fe2+ and total iron contents known, the amount of Fe3+ may be calculated by subtraction and the ratio calculated. Using a spectrophotometric method, the absorbance of an intensely colored Fe2+ complex is measured, sufficient reducing agent (ascorbic acid) is added, and the absorbance is remeasured. Since absorbance is directly proportional to the Fe2+ concentration and the final absorbance is a measure of the total iron content, the ratio may be calculated directly from the absorbance readings. If there is no need for absolute concentration
values and only the ratio value is required, then the spectrophotometric method is a very expeditious method. Mossbauer, or gamma-ray resonance, spectroscopy (14) may also provide a direct evaluation of the ratio, but without having to carry out chemical dissolution (5, 15). A finely powdered sample is placed in a beam of ^Co gamma radiation, and a Mossbauer absorption spectrum of the ^Fe components is obtained over several hours. For a typical glass with significant amounts of both Fe2+ and Fe3+, the spectrum (see figure) normally shows three peaks. (This type of spectrum is obtained when the ratio value is between 0.1 to 1.5, or, in other words, the percent of the Fe2+oxidation state is between 10 and 60% of the iron present.) Each oxidation state of iron is expected to exhibit two peaks. In the glass, the high-velocity peak is due to Fe2+,while the two low-velocity peaks are the result of an overlap of the Fe3+ peaks and one of the Fe2+ peaks. Mossbauer spectra theory (16) tells us that the total area (A) of the two individual peaks for a given iron oxidation state, i, is equal to the following: where k is a proportionality constant, fi, the recoil-free fraction, and Ni the number of atoms of the given iron oxidation state. Assuming that the f values of both Fe2+and Fe3+ are equal, then
intensely colored complex that is detected by its visible wavelength absorbance. A chromatogram results that plots absorbance a t a fixed wavelength vs. time. Through the use of standard solutions the chromatographic peaks may be identified and amounts determined. 1C has just recently been applied to the problem of determining the Fe2+/Fe3+ratio (21). There are some problems, such as preventing some Fe2+oxidation on the column, that need to be eliminated, but the problems do not appear to be formidible. At this time the best determination method for the radioactive glass samples appears to be the spectrophotometric one, employing fiber optic cables and a probe that dips into solution (22). After further study, 1C may be shown to be a suitable method. Both spectrophotometric and 1C methods require chemical dissolution of the sample. Mossbauer spectroscopy works well with nonradioactive simulated waste glass samples; however, measuring a spectrum and carrying out the computer analysis does take several hours. With highly radioactive samples background radiation may prevent a satisfactory spectrum from being obtained. At this time, attempts are being made to eliminate the problems found with Mossbauer spectroscopy (17). Acknowledgment
The author would like to thank Joe F. Allen and Carol M. Jantzen for their critical comments on this paper. Literature Cited
where Al, A2, and As are the peak areas for the lowest to highest velocity peaks. The peak areas are obtained from a computer analysis of the spectrum. The f value is related to the strength of bonding of the Fe atom in the glass matrix. One might expect that Fe3+ would be bound more tightly than Fe2+ and thus have a different, higher, f value than Fe2+,making eq 1 invalid. However, this does not seem to be the case, for when ratios obtained from chemical analysis are compared with those from Mossbauer spectra, they are approximately equal (4, 17, 18). The reasons for this are not clear and some controversy has been generated on this point (19). A third, promising method for determining the ratio is ion chromatography (IC). 1C equipment is now commercially available to measure the amount of Fe2+and Fe3+in solution (20). The sample is dissolved with HF, H2S04,and NH4VOs and the solution injected into the instrument. The eluant, 2, 6-pyridinedicarboxylic acid, is pumped through the system causing the transition metal ions to move along the ion exchange column a t rates determined by their affinity for the column resin. The ions move out of the column as separate bands and into the detection system. A coloring reagent, 4-(2-pyridy1azo)resorcinolmonosodium salt, is added in a postcolumn reactor that allows the metal ions to form an
Hey, M . H. Mineral. Mag. 1982,46,111-118. Mysen, B. 0.;Virgo, D.; Neumann, E-R.; Seifert, F. A. Am. Mineral. 1985,70,317-331. Schreiber, H. D.;Balazs, G. B.; Carpenter, B. E.;Kirkley, J. E.; Minnix, L. M.;Jamison, P. L. Comm. Am. Ceram. Soc. 1984,67, C106-C108. Mysen, B. 0.; Carmichael, I. S. E.; Virgo, D. Contrib. Mineral. Petrol. 1985,90,101106. Dyar, M.D. Am. Mineral. 1985,70,304-316. For a recent general paper on high-level nuclear waste isolation see Hoffman, D. C.; Choppin, G. R. J. Chem. Ed.l986,63,1059-1064. Baxter, R. G. Proc. Symp. Waste Manage. 1986,2,449-454. Plodinec, M. J. J. Non-Cryst. Solids 1986,84,206-214. Bickford, D. F.; Diemer, R. B. J. Non-Cryst. Solids 1986,84,276-284. Goldman, D. S.; Brite, D. W . J. Am. Ceram. Soc. 1986,69,411-413. Jeffrey, P. G. Chemical Methods of Rock Analysis, 2nd ed.; Pergamon: Oxford, 1975; pp 273-298. Wilson, A . D. Analyst 1960,85,823-827; Whipple, E. R. Analyst 1974; 14,223-238. Baumann, E. W.; Coleman, C. J., Karraker, D. G.; Scott, W. H. Abstracts of Papers, 194th National Meeting of the American Chemical Society, New Orleans, LA, 1987; ANYL 175. Herber, R. H. J. Chem. Ed. 1965,42, 180-187; Armstrong, W . H.; Dorflinger, E. E.; Anderson, 0. T.; Willeford, B. R. J. Chem. Ed. 1981,58,515-518. Stevens, J. G. MRS Bull. 1986,11(6), 14-17. Goldanskii, V. I.; Makarov, E. F. In Chemical Applications of Mossbauer Spectroscopy; Goldanskii, V. I.; Herber, R. H., Eds.; Academic: New York, 1968; pp 24-40; Herber, R. H. In Chemical Mossbauer Spectroscopy; Herber, R. H., Ed.; Plenum: New York, 1984; pp 199-216. Goldman, D. S.;Bewley, D. E. J. Am. Ceram. Soc. 1985,68,691-695. Dyar, M. D.;Naney, M. T.; Swanson, S. E. Am. Mineral. 1987,72,792-800. Dyar, M . D. J.Am. Ceram. Soc. 1986,69,C160-162; Goldman, D.S.J.Am Ceram. Soc. 1986,69, C162-C164. Dionex Corp.; 1228 Titan Way; Sunnyvale, CA 94086. Goldman, D. S.Investigation of Potential Analytical Methods for Redox Control of the Vitrification Process", PNL-5581, November, 1985. Guided Wave, Inc.; 5200 Golden Foothill Parkway; El Dorado Hills, CA 95630.
Volume 65
Number 10 October 1988
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