Chemistry of Technetium and Rhenium Species during Low-Level

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Chemistry of Technetium and Rhenium Species during Low-Level Radioactive Waste Vitrification John G. Darab* and Peter A. Smith Materials Sciences Department, Pacific Northwest National Laboratory,† Richland, Washington 99352 Received September 7, 1995. Revised Manuscript Received March 5, 1996X

We reviewed the chemistry of radioactive technetium species and their nonradioactive rhenium surrogates in the context of Hanford Site low-level radioactive waste processing and vitrification. Information concerning the hydrolysis, precipitation, phase transformation, volatilization, and redox chemistries of these species during the drying, calcining, and vitrification of aqueous waste slurries is condensed and extrapolated from the literature. The similarities between the chemistry of technetium and rhenium species were highlighted to evaluate the performance of rhenium as a surrogate for technetium in laboratory and engineering-scale low-level radioactive waste vitrification experiments.

Introduction and Background In 1943, the Manhattan Engineer District of the Army Corps of Engineers began construction of the Hanford Engineering Works along the Columbia River in southeastern Washington. The first full-scale nuclear reactors and processing plants, needed for the production and isolation of plutonium-239, were built at the Hanford Site, as it would later be called. Starting in December, 1944, the first irradiated uranium fuel elements from B-Reactor were processed, generating 239Pu bearing solutions.1 This ushered in nearly 30 years of specialty nuclear materials production at Hanford as well as a concomitant generation of vast amounts of solid and liquid radioactive waste. With the end of the cold war, the emphasis at the Hanford Site is now directed at remediation of such radioactive waste. Although the information summarized here will be valuable to those working on nuclear fuel cycles and the remediation of various types of radioactive waste at sites throughout the world, much of what will be discussed in this review will be directly concerned with Hanford Site low-level waste and the proposed method of its remediation. Hanford site single-shell tank (SST) and double-shell tank (DST) wastes are planned to be separated into lowlevel waste (LLW) and high-level waste (HLW) fractions, processed, mixed with glass precursor additives, and then vitrified for long-term storage. The LLW fraction represents the major volume at about 90%. The Hanford LLW inventory has been estimated to amount to approximately 8.0 × 104 kg (88 tons) of solids and 6.8 × 108 L (180 million gallons) of aqueous solutions containing predominantly Na+, K+, Al(OH)4-, Cl-, F-, NO2-, NO3-, OH-, CO32-, and organics, as well as a * To whom correspondence should be addressed. † Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by the Battelle Memorial Institute under contract DE-AC06-76RLO 1830. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Gerber, M. S. The Hanford Site: An Anthology of Early Histories; Westinghouse Hanford Company document WHC-MR-0435 prepared for the U.S. Department of Energy Office of Environmental Restoration and Waste Management: Richland, WA, 1993.

S0897-4756(95)00418-2 CCC: $12.00

plethora of minor ionic species, including radionuclides.2 Some of the more relevant radionuclides found in Hanford LLW include 99Tc, 137Cs, and 90Sr. Because many of the Hanford Site waste tanks are still not fully characterized, a complete compositional description of the various waste streams is not feasible and, furthermore, is beyond the scope of this review article. Aside from causing problems associated with glass formation and chemical durability of the final glass,3,4 many of these LLW components are troublesome due to their volatility, corrosive nature, and in some cases, radiotoxicity. Because the operation of the Hanford Site LLW melter will be a continuous process, the volatilization of LLW components during vitrification does not have to be high to generate significant amounts in the scrubber and offgas systems. Continuous buildup of radioactive/corrosive components downstream from the melter may cause unexpected problems associated with maintaining the equipment and cleaning or disposing of any radiologically contaminated equipment. These volatile components may also pose potential environmental problems resulting from the possible release of radiologically hazardous vapors and aerosols from the offgas system during LLW vitrification. One such troublesome component, which occurs in Hanford LLW, other defense waste, and commercial nuclear fuel cycles, includes compounds of technetium. Compared to other radionuclides occurring in LLW, isotopes of cesium and strontium for example, technetium poses special problems during LLW processing. The half-life of the most common technetium isotope, 99Tc, is more than 4 orders of magnitude greater than that of 137Cs or 90Sr. Although the longer half-life of 99Tc gives it a much lower radioactivity compared to 137Cs or 90Sr, it still exhibits significantly greater radioactivity with respect to natural uranium ore. This contributes to the longer-lived radiotoxicity of 99Tc as compared to 137Cs or 90Sr. For example, even after 1000 (2) Westinghouse Hanford Co. Report WHC-SD-WM-RD-044, p A3. (3) Li, H.; Darab, J. G.; Smith, P. A.; Feng, X.; Peeler, D. K. In INMM 36th Annual Proceedings; Institute for Nuclear Materials Management: Northbrook, IL, 1995; pp 460-465. (4) Li, H.; Darab, J. G.; Smith, P. A.; Schweiger, M. J.; Smith, D. E., Hrma, P. R. In ref 3, pp 466-471..

© 1996 American Chemical Society

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years, when the radiotoxicities of 137Cs and 90Sr have decayed to insignificant levels, that of 99Tc remains virtually unchanged.5 The radionuclides 137Cs, 90Sr, and 99Tc decay via β emission and hence present potential internal radiological hazards upon ingestation or inhalation as well as external hazards to the eyes and skin. However, unlike cesium and strontium, which generally form cationic species in aqueous, nonreducing environments, technetium predominantly forms the pertechnetate anion, TcO4- (see below). Thus, in the event of ground-water contamination by such radionuclides, either through leachates from the waste form or through volatilized products during fuel reprocessing and waste vitrification, TcO4-, will diffuse through soil at different rates than Cs+ and Sr2+.5-9 Cesium (and strontium) cations would be strongly sorbed by the soil at the Hanford burial site, whereas the pertechnetate anion would diffuse relatively unhindered.5-9 The Hanford Site’s current strategy is to stabilize the technetium in LLW purely by vitrification. Thus, the high volatility combined with potentially high mobility in ground water and long-lived radiotoxicity makes accountability of technetium an extremely important issue facing scientists and engineers working on nuclear waste vitrification. Although filter, offgas scrubber, and recycle systems will be an integral part of the final melter design, inefficiencies in these systems may still allow small, yet intolerable, amounts of radiologically hazardous materials to escape into the environment. Thus, researchers must also rely on chemistry to help understand and minimize hazardous-component loss during LLW processing and vitrification. Technetium, unlike the remaining LLW components, is only available in limited quantities as man-made, radioactive isotopes. For laboratory-scale experiments to industrial-scale melter evaluations, scientists and engineers require a more readily available and nonradioactive surrogate for technetium that emulates its chemical and volatility behavior. One very promising candidate for a technetium surrogate is rhenium. Historically, molybdenum and manganese have often been used as surrogates for technetium as well. The proximities of molybdenum, manganese, and rhenium to technetium in the periodic table make the former attractive surrogates for the latter on a first-principles basis. However, because of differences in oxidation states and the effects of the lanthanide contraction, the chemistry of these elements are quite different in many cases. Two early but extensive reviews by Colton10 and Peacock11 provided a considerable source of information and references pertaining to various aspects about the (5) Vida, J. The Chemical Behavior of Technetium during the Treatment of High-Level Radioactive Waste; Dissertation, TH Karlsruhe, 1989; pp 5-9. (6) Lieser, K. H. Radiochim. Acta 1993, 63, 5. (7) Wildung, R. E.; Garland, T. R.; McFadden, K. M.; Cowan, C. E. In Technetium in the Environment; Desmet, G., Myttenaera, C., Eds.; Elsevier Science Publishing Co., Inc.: New York, 1986; pp 115-129. (8) Sheppard, M. I.; Sheppard, S. C. In ref 7; pp 131-141. (9) Kaplan, D. I.; Serne, R. J.; Piepho, M. G. Geochemical Factors Affecting Radionuclide Transport Through Near and Far Fields at a Low-Level Waste Disposal Site; Westinghouse Hanford Co. document prepared for the U.S. Department of Energy Office of Environmental Restoration and Waste Management: Richland, WA, 1994. (10) Colton, R. The Chemistry of Rhenium and Technetium; John Wiley & Sons: New York, 1965; pp 12-98. (11) Peacock, R. D. The Chemistry of Technetium and Rhenium; Elsevier Publishing Co.: New York, 1966; pp 1-78.

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chemistry of technetium and rhenium species: extraction and isolation, compounds involving oxygen, sulfur, and halogens, organometallic compounds, and complexes. Schwochau12 provided a more recent abridged review of similar topics. A report by Rard13,14 summarized in detail the available chemical and thermodynamic data for technetium and some of its inorganic and aqueous species. These reviews provide a basis through which the chemical, environmental, and engineering aspects of processing waste-containing technetium can be extrapolated and addressed. Vida5,15 has provided an excellent report on issues concerning technetium losses as directly related to radioactive waste calcining and vitrification. In this review, we have drawn on the detailed chemical and thermodynamic information of technetium and rhenium compounds previously reported by various researchers working in a wide range of areass from solution redox chemistry to extractive metallurgy. This eclectic collection of information was then molded into a format that helps make sense of the limited information obtained at this and other institutions related to the behavior of technetium and rhenium during LLW processing and vitrification. We also compare the available information between technetium and rhenium to obtain a description of the performance of rhenium as a technetium surrogate in LLW simulants. In some of the sections presented here, there is a lack of, or a complete absence of, chemical information available for either technetium or rhenium. In those instances, a discussion concerning the unavailable chemical information will not be given but could be inferred based on the overall similarity between the chemistry of technetium and rhenium presented in areas where information for both systems is available. Additionally, we highlight some of the reasons for using rhenium over other elements such as molybdenum and manganese as surrogates for technetium. The intent of this review is to expose the problems associated with the remediation of radioactive waste streams containing technetium for what they really areproblems in materials chemistry. It is the hope of the authors that this audience will provide their own insights in helping to solve these problems. Occurrence and Treatment Some of the more long-lived and/or more common isotopes of technetium (along with their half-lives) include 96Tc (4.3 days), 97Tc (2.6 × 106 years), 98Tc (4.2 × 106 years), 99Tc (2.1 × 105 years), 99mTc (6.0 h), and 101-107Tc (21 s to 18.2 min).5,12,16 Technetium-99 is the most abundant long-lived isotope, being produced in nuclear reactors during operation at yields of about 6.1% by the neutron-induced fission of 235U. The shorter-lived isotopes 101-107Tc are also produced during reactor operation at yields ranging from 0.2-5.6%.17 After reactor refuelings, the burned-up fuel elements are dissoluted with nitric acid solutions, and the desired (12) Schwochau, K. Radiochimica Acta 1983, 32, 139. (13) Rard, J. A. Critical Review of Chemistry and Thermodynamics of Technetium and Some of its Inorganic Compounds and Aqueous Species; Lawrence Livermore National Laboratory report UCRL53440: Livermore, CA, 1983; pp 24-67. (14) Rard, J. A. In ref 13; pp 3-24. (15) Vida, J. In ref 5; pp 10-50. (16) Colton, R. In ref 10; pp 1-7. (17) Hill, J. E. In ref 7, pp 1-20.

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material is separated and processed. Technetium occurs in various concentrations in the recycled uranium and plutonium fuels and products as well as in the HLW and LLW components generated during fuel recycling.6,17 It has been estimated that at the Hanford Site, a total of 1990 kg of 99Tc was produced in the various reactors operational between 1943 and 1987.18 Of this original 99Tc inventory, 510 ( 210 kg was shipped off-site, coextracted during fuel element reprocessing into uranium oxide product, and 80 ( 10 kg released to the environment. The remaining 1310 ( 220 kg is estimated to be contained within the Hanford Site tanks. This corresponds to a 99Tc tank inventory of 22 720 ( 3820 Ci and is in good agreement with the estimate of 19 730 Ci by Schmittroth et al.19 The proposed Hanford waste separation, pretreatment, and vitrification processes are schematically illustrated in Figure 1. The anticipated composition of the LLW after pretreatment will thus be different from that of the original tank compositions. Note that for the proposed Hanford Site waste pretreatments, no technetium ion-exchange campaign is envisioned. Such an additional processing step would obviously decrease the concentration of technetium in the LLW stream, but because of the poor efficiency and selectivity of current technetium separation technologies under these conditions, would also generate a considerable amount of HLW in the form of technetium-containing eluant and contaminated ion-exchange material. Table 1 lists the composition of one particular simulated Hanford pretreated wastesthat which is based on the double-shell slurry feed (DSSF). Technetium-99 is the only radioactive isotope used in the DSSF simulant formulation. Of course, a nonradioactive technetium surrogate (such as rhenium) can be substituted for nonradiological work. Because the Hanford waste characterization effort is ongoing and the waste pretreatment details not completely developed, the composition listed in Table 1 is only tentative but has been agreed upon to be used for testing purposes. This composition does not reflect the tank-to-tank variations in chemical composition which, since blending of tank wastes is not foreseen in the process flow sheet, can significantly affect variations in waste processing chemistry. Also, some tank waste species were not included in the simulant inventory for a variety of reasons: their concentrations in the actual tank waste were determined to be extremely low ( 3, precipitation of TcO2‚2H2O. Vida’s summary indicated that precipitates of TcO(OH)2, i.e., TcO2‚2H2O, formed above a pH of 2 under reducing conditions.5 The shaded boxes in Figure 2 represent the presumed working range for solutions of pH equivalent to those expected in Hanford LLW. The lower Eh limiting line of the box has its traditional meaning, representing the reduction of water. A comparison between the stable species in the working range for technetium and rhenium indicates some degree of similarity. In both systems under nonreducing conditions, the predominant species are TcO4- and ReO4-. On the basis of the similarities of the Pourbiax diagrams of technetium and rhenium under nonreducing conditions we conclude that the use of rhenium as a surrogate for technetium in LLW simulants will provide homologous perrhenate, ReO4-, in the simulant. As the conditions are made more reducing, in both systems the formation of a reduced oxide occurs. However, compared to the technetium system, the rhenium system requires a greater reduction potential to obtain equilibrium between the oxo-metal(VII) anion and the reduced metal oxide (-0.361 V SHE for technetium29 vs -0.548 V for rhenium34 at a pH of 13.5 and a concentration of 0.017 mol of MO4-/L). Furthermore, under these conditions in aqueous noncomplexing environments, the stable reduced oxide for technetium is TcO2 (i.e., TcO2‚2H2O5), whereas that for rhenium is Re2O3 (i.e., Re2O3‚xH2O34). Compounds containing technetium with oxidation states less than +4 are generally realized only through stabilization with complexants,39 including those already found in, (e.g., EDTA, oxalic acid, etc.) and those that are being considered as additives to (e.g., citric, glycolic, formic acids, etc.) the LLW stream.39-41 Since the proposed conditions under which Hanford LLW solutions are to be processed will generally not be reducing enough to generate technetium species with oxidation states below +4, such complexes will not be considered here. Coordination compounds with technetium and rhenium of oxidation states greater than +4 have been reviewed,39 but these generally involve complexants and chemical environments atypical of those found in LLW solutions (e.g., N,N-diphenylhydrazine, nonaqueous solvents, etc.). Some special cases that may have some relevance to LLW simulant processing involve the formation of (39) Schwochau, K.; Pleger, U. Radiochim. Acta 1993, 63, 103. (40) Hashimoto, M.; Wada, H.; Omori, T.; Yoshihara, K. Radiochim. Acta 1993, 63, 173. (41) Colton, R. In ref 10; pp 109-112.

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rhenium coordination compounds with oxygen containing ligands such as oxalic, citric, and tartaric acids.39,41 The acid H[Re(OH)3C2O4‚H2O] is formed from the dissolution of solid ReO2‚2H2O in oxalic acid.41 Refluxing ReO2‚2H2O in a solution of potassium oxalate yielded K4[ReO(OH)6(C2O4)2]. The dimeric technetium(IV)-oxalato complex anions, [(C2O4)2Tc(µ-O)2Tc(C2O4)2]4-; are formed by the reaction of TcF62- (see below) with oxalate in aqueous solution.39 Similar reactions might occur between other aqueous Tc(IV) species (e.g., TcO2+, TcO(OH)+, TcO2‚2H2O, etc.) and oxalate or other similar complexants, as modeled by the analogous reactions that occur in the rhenium system. Formation and Stability of Halo and Oxyhalo Species. The reaction of TcO4- with aqueous HCl to form reduced oxychloro and chloro anions is reported to occur at all HCl concentrations.13 It is extensive and fast at HCl concentrations above approximately 5 mol/ L, whereas below this value, the reaction rate drops rapidly with decreasing HCl concentration. Increasing the reaction temperature increases the reaction rate. Although not typically encountered in LLW solutions, it should be noted for reference that HBr reacts similarly and more rapidly with TcO4- compared to HCl and that small amounts of Br- added to HCl solutions also tend to increase the reaction rate. The addition of the LLW component I- to 7 mol/L of HCl solutions was found to cause instantaneous reduction of TcO4- to Tc(IV) species, most likely Tc(OH)Cl52-.13 Initially, TcO4- is reduced by HCl to the Tc(V) oxyhalo anions, TcOCl4- and TcOCl52-, followed by a slower reduction to the Tc(IV) halo anion TcCl62-. At room temperature in concentrated HCl, this later reduction has been reported to require up to 2 years to produce chromatographically pure TcCl62-.13,42 Cifka reported that the rate of TcO4- reduction by HCl in aqueous solution was increased by the presence of absorbic acid and FeCl3.42 In contrast, HI reduces TcO4- directly to TcI62- under most conditions.13 Solid salts of technetium halo anions have been reported to exist,12,14 in particular K+, Rb+, Cs+, and NH4+ salts of TcF62- and TcCl62-. In general, these salts have low solubilities (10-2-10-4 mol/kg) in solutions of their corresponding hydrohalic acids.14 MTcVF6, K2TcIVI6, and K2TcII3 compounds have also been reported to exist. Most of these compounds are unstable in aqueous environments, disproportionating into TcO2‚2H2O and TcO4-.14 The solid oxyhalides, TcO3F and TcOF4, have been prepared by the reduction of TcO4- by anhydrous HF.14,43 Technetium (and rhenium) oxyhalides in general, readily hydrolyze in basic aqueous environments to give pertechnetic acid and hydrohalic acid.11 Additional information relevant to the chemistry of halo and oxyhalo technetium compounds in general are summarized by Rard.13,14 Although the existence of discrete halo and oxyhalo technetium species in typical LLW streams is questionable due to the oxidizing, basic, aqueous solution environments encountered, the halides present in the LLW may play a different role in determining the overall technetium speciation, especially if the redox chemistry of the solution is modified in some way. From the examples described above, it is (42) Cifka, J. Int. J. Appl. Radiat. Isot. 1982, 33, 849. (43) Peacock, R. D. In Comprehensive Inorganic Chemistry; Pergamon Press: Oxford, 1973; pp 877-898.

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conceivable that halides present in solution may act as reductants, providing a means through which a portion of the TcO4- could be converted to Tc(IV) oxo species. For example

TcO4- f TcOCl4-, TcOCl52-, TcCl62-, etc. f TcO2‚2H2O + TcO4- (1) The presence of complexants in the LLW solution may also act to promote and stabilize reduced intermediate and product species suggested in relation 1. Furthermore, during drying, calcining, and vitrification, the system becomes more anhydrous and the electrochemical environment changes. Under these conditions, molten halide salts may be produced and the formation and existence of some of those species described above may become more relevant. Aqueous Solution Chemistry at Elevated Temperatures. In saturated aqueous pertechnetic acid solutions, HTcO4 is described as being easily volatilized, the additional pressure over the solution being about 4 Torr at 50 °C (323 K) and about 60 Torr at 100 °C (373 K).5 The additional vapor pressure, p in Torr, of a saturated solution of pertechnetic acid as a function of absolute temperature was summarized with the equation10,44

log(p) [Torr] ) [-2375/T] + 8.201

(2)

This equation gives vapor pressures of 7 and 68 Torr at 323 and 373 K, respectively, and is in good agreement with those values reported above. For saturated aqueous pertechnetic acid solutions, Smith et al.44 indicated that the additional vapor pressure from HTcO4(aq) is due primarily to H2O(g), which arises from the dissociation of HTcO4 into H2O and Tc2O7. The contribution to the overall vapor pressure of HTcO4(aq) from Tc2O7 is extremely small at temperatures below 100 °C.44,45 Sasahira et al.45 modeled the gas-phase transfer of technetium via Tc2O7 from aqueous nitric acid solutions derived from spent nuclear fuel reprocessing using the following expression:

log(p) [Torr] ) [-4147/T] + 8.6 + log([Tc]2/aH2O) (3)

HTcO4 solutions over concentrated H2SO4.14,46 Additionally, as discussed above, pertechnetic acid can be neutralized by most cations to form pertechnetate salts with various solubilities in aqueous environments. Concentrating solutions containing the pertechnetate anion and appropriate cations is expected to cause precipitation of those pertechnetate salts with the lowest solubility products, e.g., KTcO4, RbTcO4, and CsTcO4. On the basis of similarities in solubility, analogous behavior is expected for perrhenate containing solutions. Precipitation of Tc(IV) species will occur under conditions where the Tc(IV) species can be formed and oxidation to Tc(VII) species can be prevented during drying. For aqueous solutions containing no complexants, the various hydrolyzed forms to Tc(IV), i.e., TcO2+, TcO(OH)+, TcO(OH)20, and [TcO(OH)20]2, are expected to hydrolyze further during solution concentration at elevated temperatures, eventually leading to the precipitation of TcO2‚2H2O. During LLW processing, reducing conditions might be achieved through the action of (complexing) organic acids such as formic, citric, or glycolic at elevated temperatures.23 As stated previously, the presence of halides (and perhaps other species) in the LLW may also provide a pathway through which reduced Tc(IV) oxo species such as TcO2‚2H2O are formed during drying. Furthermore, the presence of complexants such as oxalate and citrate may stabilize these reduced technetium species via the formation of technetium coordination compounds. Implications for the Processing of LLW Solutions. In general, the pretreated Hanford LLW environment can be considered to be nonreducing and caustic. Table 1 indicates that the pH of the pretreated DSSF simulant, which is based on the most current chemical analysis of the appropriate waste tanks and pretreatment modeling, is 13.5. From the Pourbaix diagram illustrated in Figure 2 and in agreement with the statements made above, the most probable form of technetium in the DSSF waste stream, and most likely in the remaining LLW inventory, is TcO4-, the pertechnetate anion. Similarly, ReO4- would also be expected to be the most probable form of rhenium under those conditions.

where [Tc] is the total concentration of all technetium species in solution and aH2O is the activity of water. Their model waste solution started at 2.7 mol/L of HNO3 and 0.02 mol/L fisson products and is concentrated via evaporation to 1.4 mol/L of fission products. During evaporation, they report that the activity of water decreased during solution concentration and eventually leveled out at ≈0.4. For aH2O ) 0.4 and [Tc] ) 0.1 mol/L (i.e., the approximate solubility limit of the relatively abundant and insoluble salt, KTcO4), eq 3 indicates that the vapor pressure of Tc2O7 at 100 °C would be a maximum of only 8 × 10-5 Torr. Red crystals of Tc2O7‚H2O, also referred to as anhydrous HTcO4, ultimately form by concentrating aqueous

The presence of TcO4- in the waste stream could provide a mechanism through which technetium species are lost, viz. relation 3. However, as stated earlier, the vapor pressure of technetium-containing species calculated using eq 3 is extremely small during solution evaporation, making such a mechanism improbable. In agreement with these predictions, using different multicomponent waste stream simulants containing 0.022 mol/L of technetium, Cains et al.47 did not observe loss of technetium species from solution at temperatures of 116-121 °C (389-394 K). At Hanford, however, a recently completed evaporator campaign designed to reduce the waste volume for storage provided evidence for the loss of technetium.19 A final evaporator pass feed containing 3.62 µCi/L of 99Tc (2.13 × 10-6 mol/L) was reduced in volume by a factor of 2 yielding a slurry

(44) Smith, Jr., W. T.; Cobble, J. W.; Boyd, G. E. J. Am. Chem. Soc. 1953, 75, 5773. (45) Sasahira, A.; Hoshikawa, T.; Kamoshida, M.; Kawamura, F. J. Nucl. Sci. Technol. 1994, 31, 1222.

(46) Boyd, G. E.; Cobble, J. W.; Nelson, C. M.; Smith, Jr., W. T. J. Am. Chem. Soc. 1952, 74, 556. (47) Cains, B. P. W.; Yewer, K. C.; Waring, S. Radiochim. Acta 1992, 56, 99.

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containing 6.39 µCi/L of 99Tc.19,48 This represents a 10 ( 3% loss of technetium during the final pass through the evaporator. However, the exact nature of the technetium loss (i.e., volatilation, precipitation, retention and contamination of the evaporator, etc.) was not indicated. In the examples cited above19,47,48 the mechanisms for possible loss of technetium are not clear. In addition to pure volatilization, loss of various chemical species from solution can occur by other mechanisms, such as entrainment of molecules or particles with volatilized solvent and the formation of aerosols. Furthermore, some potential LLW melter designs use an aqueousbased slurry feed. Under these conditions, where the slurry is immediately exposed to melt temperatures (1150-1350 °C), loss of technetium to the offgas stream may be greatly enhanced due to these other mechanisms. Igarashi et al.49 dried and calcined (300-800 °C) simulated HLW solutions containing several radionuclide tracers including technetium. In their experiments, technetium lost by particle entrainment was collected with a sintered metal filter with a 0.5 µm pore size located downstream from the calciner. Technetium species that passed through the filter, which were due to true volatilization products as well as entrained molecular species (and presumably particles that were too small to be trapped by the filter), were collected in a series of scrubber solutions. They found that drying/ calcining the HLW solution at 400 °C caused an 11.3% loss of technetium, based on the original feed composition. Of the total amount of technetium lost, 99.9% of that amount was due to particle entrainment (i.e., material that was trapped by the 0.5 µm filter). Consider the implementation of a separate and controlled nonreducing drying process on the Hanford LLW DSSF solution (Table 1) mixing only with glass precursor additives. On the basis of the discussions in the above sections, it is expected that the most likely form of technetium in typical LLW products will occur in the stable +7 oxidation state as low-solubility pertechnetate salts such as CsTcO4, RbTcO4, KTcO4, and perhaps even Sr(TcO4)2 depending on the type and relative amounts of the various ions involved. After all the waste washes and cesium ion-exchange pretreatment steps, it has been estimated that 0.73% and 82% of the total respective amounts of 137Cs and 99Tc present in the Hanford waste tanks will enter the LLW feed evaporator.19 Current cesium ion-exchange materials and methods are expected to lower the 137Cs transferral to the LLW feed evaporator to less than 0.25%. Assuming that the final evaporator pass reduces the waste volume by half, the concentrations of 99Tc, 137Cs, and 90Sr entering the melter would be approximately 0.1, 0.0002, and 0.0005 mmol/L, respectively. This corresponds to a Cs:Tc molar ratio of about 1 × 10-3. Since the expected Cs:Tc molar ratio in the Hanford LLW stream entering the calcination/vitrification process is extremely small and K+ is relatively (48) Guthrie, M. D. Process Control Plan for 242-A Evaporator Campaign 94-2; Westinghouse Hanford Company document WHC-SDWM-PE-054, prepared for the U.S. Department of Energy Office of Environmental Restoration and Waste Management: Richland, WA, 1995. (49) Igarashi, H.; Kato, K.; Takahashi, T. In Proceedings of the 1991 Joint International Waste Management Conference; Korean Nuclear Society: Seoul, 1991; Vol. 2, pp 143-147.

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abundant and forms the next least soluble pertechnetate, KTcO4 is expected to be the predominant technetium species in the dried Hanford waste. The similarities in both magnitude and trend between the solubilities of various pertechnetates and perrhenates allows us to conclude that the use of rhenium as a surrogate for technetium in LLW simulants will provide homologous perrhenate salts (i.e., KReO4) in the dried simulant. It is not yet clear what role halides (and perhaps other species) in the LLW perform in providing a pathway through which reduced Tc(IV) species form. Nor is it clear if, and how, the presence of complexants such as oxalate and citrate promote and stabilize the reduced technetium species. Reducing conditions might be achieved during LLW drying and calcination through the action of some organic acid such as formic, citric, or glycolic at elevated temperatures. However, what is not indicated in the Eh vs pH diagrams of Figure 2 is that the equilibrium between NO3- and NO2- also occurs within the working range indicated, but well above the equilibrium between technetium/rhenium oxo-metal(VII) anion and the reduced metal oxides. Since NO3- is one of the most predominant ionic species in LLW, the action of any added reductants will be confined primarily to the reduction of NO3- to NO2-. Thus, large quantities of reductant would needed to be added to LLW and LLW simulants to begin affecting the oxidation state of technetium and rhenium (and other metal species as well). A large excess of these types of reductants has profound effects on the glass melt characteristics (see later). Solid-State and Melt Chemistry It is evident that the predominant form of technetium and rhenium in the dried Hanford LLW and LLW simulant will most likely be KTcO4 and KReO4. LLW products from other sites may contain other alkali or alkaline-earth pertechnetates and perrhenates as well. TcO2‚2H2O and ReO2‚2H2O as well as complexed technetium(IV)/rhenium(IV) compounds may also be present to some extent depending on the LLW processing conditions. The thermochemistry and volatility of these species will ultimately be dictated by that of the technetium and rhenium heptoxides, which represent the highest and most stable oxidation state in these systems. We thus begin our discussion at that point and follow it by a review of the thermochemistry and volatility of the dioxides, other oxides, the pertechnetates/perrhenates as well as the halo/oxyhalo compounds of technetium and rhenium. A discussion about the observed vapor species of technetium and rhenium compounds at high temperatures is also included. A graphical summary of the vapor pressures over these technetium and rhenium oxide and pertechnetates/perrhenates species as a function of temperature, which will be discussed in the following sections, is presented in Figure 3. Relavent volatility data for rhenium and methods for controlling its volatility during extraction from molybdenite ore are included as well. Finally, the available information pertaining to the chemistry of technetium and rhenium in melts and in the final waste glass is presented. The information summarized in these separate discussions is then consolidated in terms of implications to the vitrification of LLW and LLW simulants

Reviews

Chem. Mater., Vol. 8, No. 5, 1996 1013 Table 5. Thermodynamic Properties of Tc2O7 and Re2O7 Tc2O7 property

Re2O7

value

ref

value

13 13 10 13 13 13 13 13 13

-1067 (s)

10

-1241 (s)

10, 53

∆Gsub (kJ/mol) ∆Hsub (kJ/mol)

-935.6 (s) -888.5 (g) -1110 (s) -1120 (s) -987.4 (g) 166.0 (s) 449.4 (g) 48.1 132.6

∆Ssub (J/mol K) ∆Hfus (kJ/mol)

283.4 NA

13 a

∆Sfus (J/mol K) ∆Hvap (kJ/mol)

NA NA

a a

∆Svap (J/mol K)

NA

a

207.3 (s) 439.3 (g) NA 138.3 140 244 61.13 66.1 115 75.10 74.1 117

10, 54 10, 54 a 10, 51 10, 52 10, 52 10, 51 10, 52 10, 52 10, 51 10, 52 10, 52

∆Gf (kJ/mol) ∆Hf (kJ/mol) ∆S (J/mol K)

a

ref

Not applicable. NA ) not available.

Table 6. Available Melting Point (Tm), Boiling Point (Tb), and Volatility Data for MoO3 and ReO3a

Figure 3. Literature recommendations (see text) for vapor pressures over the technetium, rhenium, and molybdenum phases indicated. Data obtained from only a single state of matter (i.e., solid or liquid) are so indicated with extrapolations from that data represented by dashed lines. Table 4. Available Melting Point (Tm), Boiling Point (Tb), and Volatility Data for Tc2O7 and Re2O7a compound

state

ref

Tm (°C)

Tb (°C)

A

B

Tc2O7

solid liquid

Re2O7

solid liquid solid liquid

10,44 10,44 13,46 5,50 10,31,51 10,31,51 10,52 10,52

119.5 b 119.5 118.4 297 b 300.3 b

b 310.6 310.8 310.6 b 363 b 360.3

7205 3571 NA NA 7218 3920 7320 3863

18.28 8.999 NA NA 14.84 9.047 15.01 8.98

a The estimated vapor pressure, p (Torr), fitted to the available data is given by log p ) [-A/T] + B. b Not applicable. NA ) not available.

and to the incorporation of technetium and rhenium into the waste glass. Technetium and Rhenium Heptoxide. With a melting point of approximately 119 °C (392 K) and a boiling point of 311 °C (584 K), bright yellow crystalline Tc2O7 is easily volatilized, predominantly as Tc2O7(g) species (see below). Providing technetium with its most stable valence state, +7, Tc2O7 is expected to be the end oxidation product of many technetium compounds. Table 4 summarizes the melting and boiling temperatures as well as the volatility data available for this compound and its rhenium analogue, Re2O7. The available thermodynamic data of these compounds are summarized in Table 5. Technetium and Rhenium Dioxide. As stated above, under reducing conditions, aqueous Tc(IV) species can hydrolyze, causing precipitation of amorphous (50) Muller, O.; White, W. B.; Roy, R. J. Inorg. Nucl. Chem. 1964, 26, 2075. (51) Ogawa, E. Bull. Chem. Soc. Jpn. 1932, 7, 265. (52) Smith, Jr., W. T.; Line, L. E.; Bell, W. A. J. Am. Chem. Soc. 1954, 74, 4965. (53) Boyd, G. E.; Cobble, J. W.; Smith, Jr. W. T. J. Am. Chem. Soc. 1953, 75, 5783. (54) Busey, R. H. J. Am. Chem. Soc. 1956, 78, 3263.

compound

state

ref

Tm (°C)

Tb (°C)

A

B

MoO3

solid liquid b solidd,e solidf

31 31 27 31 10

830c b 795 NA NA

b 1155 NA 747g 614

14570 8200 NA 4966 10882

14.46 8.682 NA 7.745 15.16

ReO3

a The estimated vapor pressure, p (Torr), fitted to the available data is give by log p ) [-A/T] + B. b Not applicable. c Extrapolated from intersection of solid-vapor and liquid-vapor equilibrium pressure curves. d ReO3(s) dissociates into ReO2(s) and Re2O7(g) above ≈440 °C.31 e Data taken over the range 300-480 °C. f Data taken over the range 325-420 °C. g Extrapolated from vapor pressure data. NA ) not available.

hydrated TcO2, i.e., TcO2‚2H2O. The free energy of formation of this compound has been determined to be -836.3 kJ/mol.14 Crystalline anhydrous TcO2 can be prepared by thermal decomposition of the ammonium pertechnetate, NH4TcO4, in an inert atmosphere.14 TcO2‚2H2O can be dehydrated to yield TcO2 under similar conditions. The analogous reactions occur in the rhenium system.10 Both TcO2 and ReO2 are very stable compounds. The only thermodynamic datum available is for the heat of sublimation of the rhenium: 274.6 kJ/mol.10 The vapor pressure of mixed oxide vapor species over ReO2 as a function of absolute temperature has been determined to follow the relation10,31

log(p) [Torr] ) [-4742/T] + 5.345

(4)

TcO2 is reported to sublime at temperatures of 900 °C5,11,12 to 1000 °C10 and decompose above 1100 °C via disproportionation:5,12

TcO2(s) f Tc(s) + Tc2O7(g)

above 1100 °C

(5)

Since above 1100 °C the Tc2O7 generated by the disproportionation of TcO2 is well above the boiling point of the former, it can be assumed that Tc2O7 will exist primarily in the gaseous state, as indicated in relation 5. Rhenium and Molybdenum Trioxides. Table 6 summarizes the melting and boiling temperatures as well as the volatility data available for ReO3 and MoO3. The information pertaining to ReO3 is given as a point of reference since the existence of homologous TcO3 is

1014


questionable.10,11,14 Note that for molybdenum, MoO3 is the highest and most stable molybdenum oxide obtainable. Comparing MoO3 with the highest and most stable oxides of technetium (Tc2O7) and rhenium (Re2O7) at 700 °C, for example, one finds that the vapor pressures of oxide species over Tc2O7 and Re2O7 are about 2500 times greater than that over MoO3 (i.e., 760 Torr vs 0.3 Torr, respectively). Thus, from this volatility comparison we can conclude that rhenium would be a better choice for a technetium surrogate than molybdenum. Volatile Species from Technetium and Rhenium Oxides. Reactions of technetium and rhenium oxides (predominantly TcO2/ReO2) carried out in Knudsen cells have enabled the mass spectroscopic speciation of volatile products up to 1100 °C by Gibson.55 Regardless of the composition of the atmosphere (vacuum, O2, or H2O/O2) over the technetium oxide, the partial pressures of technetium oxide and hydroxide vapors became detectable above about 600 °C and maximized between 900 and 1000 °C. The vapor pressure over technetium oxide heated in vaccum, however, was naturally much lower than those obtained when heated under a more reactive atmosphere (i.e., O2 or H2O/O2). The maximization of the vapor pressure of volatile technetium species over technetium oxide between 900 and 1000 °C corresponds with the reported sublimation temperature of TcO2.10-12 In all cases, major species detected included Tc2O7(g), TcO3(g), TcO3(OH)(g), Tc2O5(g), and TcO2(OH)3(g), the first three also occurring in homologous rhenium. The generation of hydroxide species under vacuum was attributed to residual water associated with the starting oxide. Vapor compositions from the oxides at 900 °C under H2O/O2 include nearly equal concentrations of Tc2O7(g) and Tc2O5(g) for the case of technetium oxide and mostly Re2O7(g) with only 2% Re2O5(g) for that of rhenium oxide.55,56 Although TcO3(g) and ReO3(g) species are reported to volatilize from their respective oxides, TcO3(g) is a much greater contributor to the overall vapor pressure over technetium oxide than ReO3(g) is over its oxide. The generation of highly oxidized technetium and rhenium vapor species when heating the respective oxides under vacuum is believed to be due to a disproportive vaporization mechanism. This mechanism has been demonstrated for the rhenium system, in which ReO2(s)/ReO3(s) disproportionates to a nonvolatile reduced form of rhenium and the volatile fully oxidized Re2O7(g) species.55,57 The most predominant gas-phase species produced during the heating of technetium oxide under an O2/ H2O environment was determined to be TcO3OH(g) by mass spectroscopy.55 This is analogous to the behavior of rhenium where ReO3OH(g) is produced under similar conditions.57 The formation of these monohydroxide species is an important factor which dictates the hightemperature transport of rhenium and technetium. It is believed that the monohydroxide species are generated by hydrolysis of the heptoxides, Tc2O7 and Re2O7, which are produced from lower oxide species such as TcO2 and ReO2.55 Considering that typical LLW feeds (55) Gibson, J. K. Radiochim. Acta 1993, 60, 121. (56) Skinner, H. B.; Searcy, A. W. J. Phys. Chem. 1973, 77, 1578. (57) Battles, J. E.; Gundersen, G. E.; Edwards, R. K. J. Phys. Chem. 1968, 72, 3963.

Reviews Table 7. Availale Melting Point (Tm), Boiling Point (Tb), and Thermodynamic Properties of Pertechnetates and Perrhenates anion ReO4-

TcO4

-

Tm ∆Hm ∆Sm cation (°C) (kJ/mol) (J/mol) Na+ Li+ Fe3+ K+ Rb+ Cs+ Ca2+ ref

378 401

21 23

32.3 34.1

Tm (°C)

Tb (°C)

Tm ∆Hm (°C) (kJ/mol)

≈300

NA

420

33

555

36

620

34

705a

532 581 595

31 31 35

38.5 36.3 40.3

15

NA 550 1095b ≈1040

NA 31

58

a

Value extrapolated from vapor pressure data given in ref 31. Boiling point extrapolated from vapor pressure data given in ref 31 is 1092 °C. The text in ref 31 indicates a boiling point of 1370 °C which is most likely a typographical or translational error. It was assumed that the reported boiling point should be 1370 K (1097 °C). The value indicated in Table 7 is an average of the two values. NA ) not available.

b

contain appreciable amounts of water, the mechanism described above will most likely be an important pathway through which technetium (and rhenium) volatilization could occur. Pertechnetates and Perrhenates and Their Volatile Species. Solid pertechnetate and perrhenates, as defined in this work, include a broad range of compounds involving the pertechnetate or perrhenate anion bound to a cation. The thermodynamic properties available for some of the pertechnetates and perrhenates are summarized in Table 7.15,31,58 Vida15 gives a general discussion concerning the volatility of technetium from the melting of glass frit particles mixed with ammonium, sodium, potassium, and cesium pertechnetates under oxidizing conditions. He reports that for the range of alkali pertechnetates added to borosilicate glass frits, volatilization of technetium from the glass melt was highest for CsTcO4. One pathway for technetium loss during melting proposed by Vida involves pertechnetate dissociation and volatilization as Tc2O7(g):15

MTcO4(s) f MTcO4(l)

pertechnetate melting (6)

MTcO4(l) h M2O(l/g) + Tc2O7(g) pertechnetate dissociation (7) A similar mechanism is expected for alkaline-earth pertechnetates. Since the melting points of all the relevant alkali pertechnetates (Table 7) are above the boiling point of Tc2O7, the Tc2O7 dissociated from the molten alkali pertechnetate will be in a gaseous state, as indicated in relation 7. The degree of dissociation and the volatility of the alkali oxide will thus dictate the volatilization of the alkali pertechnetates. Vida15 as well as Cains et al.47 discuss the highly volatile nature of Cs2Osnearly 3 orders of magnitude higher than other alkali metal oxides under similar conditions (i.e., Cs2O can be considered as gas under these conditions). Hence, although the dissociation constant of CsTcO4(l) has not been reported, even for a small degree of dissociation the extremely volatile dissociation products (58) Lukas, W.; Gaune-Escard, M. J. Chem. Thermodyn. 1982, 14, 593.

Reviews

Chem. Mater., Vol. 8, No. 5, 1996 1015

Table 8. Available Volatility Data for Pertechnetates and Perrhenatesa compound

state

ref

range (°C)

A

B

HTcO4 Fe(ReO4)3 Cu(ReO4)2 KReO4 Ca(ReO4)2

solid liquid solid liquid solid

10, 44 31 31 31 31

20-90 360-700 400-650 740-1000 720-1040

2395 6885.4 4780.9 14188 8080

8.207 9.921 5.667 13.282 7.332

a The estimated vapor pressure, p (Torr), fitted to the data over specified temperature range is given by log p ) [-A/T] + B.

(Cs2O and Tc2O7) will push the equilibrium to the right in relation 7. Thus, technetium (and cesium) volatilization from CsTcO4 is expected and is observed to be extremely high. Vida reports that technetium evolution during the vitrification of CsTcO4 containing feeds begins at 700 °C, is at a maximum at 1000 °C, and that cesium evolution parallels that of technetium.15 Although Vida15 did not specifically address the dissociation of pertechnetates below their melting points most likely via

MTcO4(s) h M2O(s/l/g) + Tc2O7(g)

(8)

one cannot ignore the possibility of such a mechanism. Indeed, as will be discussed below, dissociation of solid homologous perrhenates has been reported.31 One might expect, however, that for a given pertechnetate (or perrhenate) the dissociation constant in the solid state would be smaller than that in the liquid state. Solid pertechnetic acid, HTcO4, has been reported to dissociate to H2O and Tc2O7:10

HTcO4(s) f H2O(g) + Tc2O7(s)

(9)

The volatility of predominantly H2O gas over solid HTcO4 (Tc2O7 has a relatively low vapor pressure at these temperatures compared to H2O) as a function of absolute temperature has been summarized in Table 8. It is interesting to note that the vapor pressure over solid HTcO4 is comparable to that over a saturated solution of pertechnetic acid (relation 2). On the basis of the available thermodynamic information, Migge59 calculated the equilibrium vapor pressures of Tc2O7(g), Cs2O(g), CsO(g), and CsTcO4(g) species over Tc, TcO2, Tc2O7, and CsTcO4 as a function of Cs and O2 activities. These calculations are useful as a first-principles estimate of the volatilization behavior of technetium/rhenium- and cesium-containing species from LLW-based melts. However, the process of melting is a nonequilibrium one and Migge’s results need to be viewed in that perspective.59,60 The technetium-containing gas-phase species predicted by these theoretical results could be brought into alignment with the experimental vitrification results of Lammertz et al.61 if it was assumed that the volatile species included 75% Tc2O7(g) and 25% CsTcO4(g).59 These results suggest that CsTcO4(g) and perhaps alkali pertechnetate (and perrhenate59) vapor species in general, also need to be considered as significant volatilization products via (59) Migge, H. In Scientific Basis for Nuclear Waste Management XIV; Materials Research Society: Pittsburgh, 1990; pp 411-417. (60) Migge, H. In ref 59; pp 205-213. (61) Lammertz, H.; Merz, E.; Halaszovich, S. In Scientific Basis for Nuclear Waste Management VIII; Materials Research Society: Pittsburgh, 1985; p 823.

MTcO4(s/l) h MTcO4(g)

(10)

Vida also discusses the volatilization of MTcO4(g) species (M representing an alkali metal) from molten alkali pertechnetates.15 He reports the formation of CsTcO4 aerosols during vitrification of technetium-containing feeds. Whether this is due to condensation of CsTcO4(g) species or the recombination of Cs2O(g) and Tc2O7(g) species is uncertain. Gibson62 carried out mass spectroscopic speciation of volatile products from a mixture of CsOH and TcO2 as it was gradually heated up to 900 °C in oxygen. From his results it was evident that [CsOH]2(g) dimers were being evolved above ≈350 °C. Even though the cesium and technetium in the sample occurred as discrete compounds (i.e., CsTcO4 was not present in the starting material), ion fragments indicative of CsTcO4(g) were detected beginning at ≈600 °C and increased in concentration with increasing temperature. At 670 °C, it was evident that CsTcO4(g), CsReO4(g) (the rhenium was present in the experimental apparatus as a residual from previous runs), CsOH(g), TcO3OH(g), and ReO3OH(g) were the major volatile products. These results can be interpreted in several ways. The generation of CsTcO4(g) may indicate that the cesium hydroxide vapor species are synergistically extracting technetium. Additionally, the unvolatilized CsOH(s) and TcO2(s) in the sample may react to form CsTcO4(s) which then volatilizes with only partial dissociation, presumably after melting as suggested by Vida.15 The presence of CsOH(g) and TcO3OH(g) supports both cases since these species could originate from the independent volatilization of discrete CsOH(s) and TcO2(s), as well as from partial dissociative volatilization of CsTcO4(s). The coinciding of the temperature at which CsTcO4(g) generation becomes significant (≈600 °C) and the melting point of CsTcO4(s) (595 °C, Table 7) may support the latter case. The volatility data of various alkali, alkaline-earth, and transition-metal perrhenates as well as solid pertechnetic acid, HTcO4, are summarized in Table 8. Like their technetium counterparts, it has been suggested that the perrhenates also dissociate at elevated temperatures via the following relation:31

Mx[ReO4]x h MOx/2 + (x/2)Re2O7

(11)

As was the case for the pertechnetates, the Re2O7 generated from the dissociation of the perrhenates is expected to be in a gaseous state, making the degree of dissociation and the volatility of the other oxide product the dominant factor controlling the volatility of the perrhenates. Potassium pertechnetate and perrhenate, KTcO4 and KReO4, are reported to boil with little or no dissociation.12,31 Thus, it is evident that KTcO4 and KReO4 should provide poor pathways for technetium and rhenium volatilization at elevated temperatures. Indeed, at a temperature of 1000 °C in air the dissociation pressure of pure molten KReO4 is 137.1 Torr.31 Solid Ca(ReO4)2, which may have a smaller dissociation constant than molten KReO4 and whose CaO dissociation product is less volatile than K2O, has a dissociation pressure of only 9.66 Torr under the same conditions. (62) Gibson, J. K. Radiochim. Acta 1993, 62, 127.

1016


Sodium perrhenate is also reported as being nonvolatile, presumably for the same reasons.31 Gibson62 also carried out mass spectroscopic studies of volatile products from samples of KTcO4 and KTcO4/ KReO4 at elevated temperatures. Under vacuum at 620 °C, the major technetium and rhenium species volatilized from KTcO4(s) and KTcO4(s)/KReO4(s) were determined to be KTcO4(g) and KReO4(g). Ion fragments consistent with TcO3OH(g) and ReO3OH(g) (see the previous section) were also detected, but at concentrations much smaller than that of the fragments from KTcO4(g) and KReO4(g). These results agree with the previously reported observation that KReO4 boils with only partial dissociation.31 KTcO4(g) generation from KTcO4 and KTcO4/KReO4 under both vacuum and oxygen atmospheres was observed to begin at 500-550 °C.62 This temperature range coincides well with the reported melting point for KTcO4 (532 °C, Table 7). Ion fragment signals due to KTcO4(g) were maximum at approximately 700 °C. In contrast, the liberation of KReO4(g) was determined to begin at 600-650 °C, approximately 50-100 °C above the reported melting point of KReO4 (550-555 °C, Table 7), and continue to higher temperatures than that of KTcO4(g). Additionally, the ion fragments analyzed from the KTcO4/KReO4 mixture indicated that KReO4(g) occurred at relatively lower concentrations with respect to KTcO4(g). These results indicate that (1) volatilization from KTcO4(s) and KReO4(s) becomes significant only after the compounds melt and (2) volatilization from KTcO4 is slightly greater than that from the surrogate KReO4. The first point may be characteristic of alkali and alkaline-earth pertechnetates and perrhenates in general.15 As mentioned previously, NH4TcO4 and NH4ReO4 decompose upon heating in an inert atmosphere, yielding TcO2 and ReO2.14 During heating under oxidizing conditions mixtures of M(IV) and M(VII) oxide phases will inevitably be produced. As is the case for NH4ReO431 and most likely NH4TcO4 as well, the final decomposition product will become richer in the M(VII) oxide with increasing oxygen partial pressure. Technetium and Rhenium Halides and Oxyhalides. The available volatility data for various technetium and rhenium fluorides and oxyfluorides are summarized in Table 9.10,63,64 It is evident from Table 9 that these species are extremely volatile. Not much information has been published on the analogous chlorides, oxychlorides, iodides, and oxyiodides. What little information and comments on the chloride and oxychloride systems are summarized in Table 10.10 Thermodynamic and volatility data pertaining to halo and oxyhalo technetium and rhenium salts is also extremely limited. However, in glass melts containing metalhalogen compounds, in general, fluorides comprise the least volatile species in relation to the analogous chlorides and iodides.65 Thus, it might be expected that the volatilities of technetium/rhenium chlorides, oxychlorides, iodides, and oxyiodides are similar to if not higher than those of the technetium/rhenium fluorides and oxyfluorides listed in Table 9. Roasting of Molybdenite (MoS2) Ores. Useful information pertaining to rhenium (and technetium) (63) Malm, J. G.; Selig, H. J. Inorg. Nucl. Chem. 1961, 20, 189. (64) Selig, H.; Malm, J. G. J. Inorg. Nucl. Chem. 1962, 24, 641. (65) Volf, M. B. Chemical Approach to Glass; Elsevier: New York, 1984; Vol. 7, pp 562-577.

Reviews Table 9. Available Melting Point (Tm), Boiling Point (Tb), and Volatility Data for Technetium and Rhenium Fluorides and Oxyfluoridesa compound ReF7 TcF6 ReF6 TcF5 ReF5 ReOF5 ReO2F3 TcOF4 ReOF4 TcO3F

state

Tm (°C) Tb (°C)

solid liquid solidb liquid solidc liquid

48.3 73.7 37.4 55.3 18.7

solid liquid solidd liquid solid liquid solid liquid solid liquid

ReO3F

33.8 NA

50 48

221.3 35 55 95 126 165

133 108

171 18.3 147

100 164

A

B

C

2206 13.04 1.470 244.3 -21.6 -9.91 2178 15.33 2.295 2405 24.81 5.804 1765 9.123 0.179 1957 18.21 3.599 NA NA NA NA NA NA 3037 9.024 0.000 1959 8.620 0.000 1679 7.727 0.000 NA NA NA 3437 10.36 0.000 NA NA NA 3888 11.88 0.000 3206 10.09 0.000 3239 12.45 0.000 2065 8.471 0.000 NA NA NA

a The estimated vapor pressure, p (Torr), fitted to the available data is given by log p ) [-A/T] + B -C log T. b There is a solidsolid transformation at -5.3 °C. This datum is for the solid that exists above -5.3 °C. c There is a solid-solid transformation at -3.45 °C. This datum is for the solid that exists above -3.45 °C. d There is a solid-solid transformation at 30 °C. This datum is for the solid that exists above 30 °C. NA ) not available.

Table 10. Available Information for Selected Technetium and Rhenium Chlorides and Oxychlorides compound

statea

TcCl6 ReCl6 ReCl5 TcCl4 ReCl4 ReCl3 TcOCl3 TcOCl4 ReOCl4

solid solid solid solid solid solid solid solid solid

TcO3Cl ReO3Cl

liquid liquid

a

comments melts at 25 °C; quite volatile melts at 25 °C; volatile melts at 220 °C readily hydrolyzed by moisture nonvolatile. nonvolatile. melts at 29-34 °C, boils 223-225 °C; hygroscopic boils at 128-131 °C; hygroscopic

Stable state at room temperature.

volatilization and entrainment in vapors liberated from LLW solids and ways to reduce such losses at elevated temperatures can be obtained by analogy from studying the process of rhenium extraction from molybdenite ore. Naturally occurring rhenium concentrates to the greatest extent in the mineral molybdenite. A typical molybdenite ore contains about 33 wt % MoS2, 23 wt % SiO2, 11 wt % Al2O3, 20 wt % H2O with the balance being CaO, MgO, and compounds of iron, copper, and carbon. Rhenium substitutes for molybdenum in the MoS2 lattice. Rhenium concentrations in the molybdenite ore range from 0.6 to 21 g/ton on a metal basis.31 The oxide components of the ore itself, which amount to an alkaline-earth alumino-silicate glass composition, are in some ways similar to the alkali/alkaline earth boro-alumino-silicate LLW glass systems currently being studied. One extraction method involves roasting the ore in an oxidizing atmosphere at temperatures of 550-700 °C for several (3-12) h. The MoS2 together with the ReS2 are deliberately oxidized to MoO2, MoO3, Mx[MoO4]x/2, Re2O7, and Mx[ReO4]x where Mx in this case equals Ca2+, Cu2+, and Fe3+. The Re2O7 volatilizes to a greater extent than either of the two molybdenum oxides (see above). The formation of less volatile metal

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perrhenates is also possible, however, such reactions are in competition with the formation of analogous metal molybdates. Approximately 50-98% of the rhenium in the ore passes into the offgas stream, predominantly as Re2O7(g). Submicron particles of rhenium oxides are known to condense downstream from the roaster. However, collecting these ultrafine particles on a large scale is extremely difficult. In one case where 84% of the rhenium was volatilized into the gas stream during roasting, only 10% of this amount was recovered as flue dust.31 The gas stream containing the remaining rhenium (as either molecular species or extremely fine particulates) was then passed through an aqueous bubbler which collected 89-96% of the rhenium (predominantly as perrhenate and sludge) from the gas stream. On the basis of the mass balance between the amount of rhenium in the starting molybdenite ore and the collected products, roasting at 550-770 °C produces a 3% loss of rhenium, presumably as a result of emissions to the environment or equipment contamination. Another extraction method involves roasting the molybdenite ore with an excess of additives such as calcium carbonate or mirabilite (Na2SO4‚10H2O). This so-called mirabilite sintering process is designed to limit rhenium volatilization during roasting. The rhenium is then chemically extracted from the resulting sintered product.31 It was found that when approximately equal weights of calcium carbonate or mirabilite are mixed with the molybdenite ore, the loss of rhenium during roasting at 700 °C for 4 h was reduced by at least a factor of 10. The decrease in rhenium loss during roasting with excess mirabilite was due to the formation of the nonvolatile NaReO4(s/l) phase.31 Similarly, the presence of reactive CaCO3 would allow for the formation of nonvolatile Ca[ReO4]2(s). On the basis of the similarities between the chemistries of rhenium and technetium compounds so far, the sodium and potassium salts that occur in dried LLW products may act to reduce technetium volatility during LLW calcination and vitrification by a similar mechanism. Melt and Glass Chemistry. Migge has constructed theoretical oxygen partial pressure (p(O2))-temperature phase diagrams (i.e., log[p(O2)] vs T-1) relevant to LLW/ HLW melts and glasses, in which the regions of predominance for various technetium and rhenium oxide and metal species were delineated.60 In this work, the importance of p(O2) on the vapor pressure of Tc2O7 gas during vitrification rather than the formal oxidation states that the technetium and rhenium exist was emphasized. Within the TcO2(s) and ReO2(s) phase fields, for example, it is clearly evident that although Tc4+ and Re4+ are the predominate stable species, the vapor pressure of Tc2O7(g) at a particular melt temperature (e.g., 800 K) increases with increasing p(O2). However, it is also evident that at this same melt temperature, increasing p(O2) to the point that the melt conditions move from the predominance region of TcO2(s) to that of Tc2O7(s), the vapor pressure of Tc2O7(g) discontinuously jumps to a much greater value (e.g., at 800 K it jumps to atmospheric pressure). These results are supported by the discussion given in the above sections and by Figure 3. The volatility of technetium and rhenium compounds is strongly dependent on the oxidation state of the metal, e.g., TcVII2O7/ReVII2O7 are

Chem. Mater., Vol. 8, No. 5, 1996 1017

far more volatile than TcIVO2/ReIVO2. Thus, the formal oxidation states of the predominant technetium and rhenium species that exist under a given set of melt conditions cannot be ignored and are naturally related to the melt p(O2). Altering the feed chemistry to make the melt more reducing, by adding organic acids or sugar for example, can thus help limit technetium volatilization during vitrification by increasing the amount of reduced species (which are inherently less volatile) in the melt at the expense of Tc2O7 and MTcO4 and by decreasing p(O2) (through the generation of the organic decomposition product, CO2, for example) during some or all of the melt cycle. Freude et al.66 investigated the redox chemistry of technetium and rhenium in borosilicate glass melts by using square-wave voltammetry (SWV) measurements directly in melts derived from glass frit of a HLW glass formulation and a technetium or rhenium salt. The reference electrode used in their work consisted of a yttria-stabilized zirconia rod attached to an alumina tube with a platinum electrode mounted on the inside. The working and auxiliary electrodes were platinum. They determined the 500 Hz SWV redox potentials for reducing melts (i.e., heated in argon) at 1000 °C to be -320 and -180 mV which they assigned to the Tc4+/ Tc0 and Tc7+/Tc4+ couples, respectively. This is consistent with the redox potential of the melt, which they determined to be -250 mV, indicating that Tc4+ was most stable under those conditions. Melts conducted at the same temperature, but under oxidizing conditions (i.e., heated in air) exhibited a 500 Hz SWV redox potential of -50 mV, indicating that Tc7+ was the most stable species.66 Only a single 25 Mz SWV peak at ≈-280 mV was observed in the technetium-spiked reducing melt at 1000 °C and was attributed to the superposition of the Tc4+/Tc0 and Tc7+/Tc4+ couples.66 This superposition was suggested to be due to the formation of the Tc4+ reduction product which precipitated on the electrode as a TcO2 film. This film inhibited further reduction of Tc7+ in the melt. When the reduction potential on the electrode was increased further, reduction of the TcO2 film to Tc0 occurred and further reduction of Tc7+ could then occur. At higher SWV frequencies (i.e. shorter reduction times) or lower melt temperatures (i.e., smaller diffusivities of the redox species), TcO2 film formation was insufficient to inhibit the electrode and both redox couples were observed. For rhenium-spiked reducing melts at various frequencies (25-500 Hz) and melt temperatures (7001200 °C), only a single SWV peak was observed. At 1000 °C, the 25 Hz SWV for this melt system occurred at -520 mV. As was the case for the technetium system, this behavior was attributed to the superposition of the Re4+/Re0 and Re7+/Re4+ couples.66 The more negative 25 Hz SWV potential required for the reduction of the rhenium system at 1000 °C (-520 mV) as compared to that of technetium (-280 mV) is supported by the theoretical predictions of Migge that show for a given melt temperature, the Re2O7/ReO2 equilibrium occurs under more reducing conditions (i.e., lower p(O2)) than that for Tc2O7/TcO2.60 This same trend in the redox chemistries of technetium and rhenium was also (66) Freude, E.; Lutze, W.; Russel, C.; Schaeffer, H. A In ref 59; pp 199-204.

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observed in aqueous solution studies, i.e., TcO4- was easier to reduce than ReO4- (see above). These results are supported by those of Schreiber and co-workers,67 who determined that the electromotive force series for a variety of transition-metal ion redox couples (although not specifically technetium or rhenium) in borosilicate melts was comparable to that in aqueous solutions. The total technetium concentration retained in these glasses after vitrification at 1000 °C were determined using γ-counting. For the melts conducted in argon using TcO2 as a technetium source (i.e., reducing conditions), 75% of the technetium was retained in the final glass, based on the starting feed concentration. For melts conducted in air using NaTcO4 (i.e., oxidizing conditions), only 45% of the technetium was retained.66 These results clearly demonstrate the importance of melt redox chemistry on the incorporation of technetium species into the final glass. Although the reduction potentials of technetium and rhenium species in melts are different, as described by Freud et al.,66 HLW-based melts conducted under argon were sufficiently reducing that black precipitates of presumably TcO2/ReO2 and perhaps further reduced species were detectable in the final glasses. This indicates that the solubilities of the lower valence technetium and rhenium species in the melt and in the final glass are relatively low. However, no indication as to the relative amounts of the reduced species compared to the total amount of technetium/rhenium was given. Glasses prepared from oxidizing melts were colorless with no indication of any reduced precipitates. Antonini et al.68 used X-ray absorption near-edge spectroscopy (XANES) and X-ray absorption fine structure spectroscopy (XAFS) to quantify more accurately the speciation of technetium in borosilicate glasses derived from feeds containing NH4TcO4 and TcO2. Under reducing melt conditions, they found that the technetium existed in the glasses as clusters of both Tc0 and Tc4+ species. These results are in agreement with those of Freude et al.66 Glasses derived from oxidizing melts contained predominantly Tc4+ species, however, which is contrary to the voltammetry predictions that Tc7+ should be most stable under similar conditions.66 Vida performed electron paramagnetic resonance (EPR) on pure and technetium-spiked barium aluminosilicate glass ceramics and sodium borosilicate glasses prepared under reducing conditions.69 A comparison between the EPR spectra of the pure glass ceramic (which had a small, broad impurity/defect resonance near g ) 2.0) and that of the glass ceramic prepared from a feed containing NaTcO4 revealed an isotropic technetium-related resonance at a g value of 1.919. This resonance was assigned to the presence of Tc4+ based on the similar g value of 1.969 obtained from Tc4+-doped SnO2. EPR cannot detect Tc0; however, the presence of metallic deposits of technetium in the glass ceramic were confirmed using X-ray microprobe analysis.69 For undoped sodium borosilicate glass prepared under reducing conditions from a commercially available glass frit, Vida69 identified EPR signals with g values near 4.2 and 2.0, typical of iron impurities as well as one near (67) Schreiber, H. D.; Balazs, G. B.; Carpenter, B. E.; Kirkley, J. E.; Minnix, L. M.; Jamison, P. L. Commun. Amer. Ceramic Soc. 1984, C-106. (68) Antonini, M.; Merlini, E.; Thornley, R. F. J. Non-Cryst. Solids 1985, 71, 219. (69) Vida, J. In ref 5; pp 50-61.

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g ) 2.2, which could be due a number of transition-metal impurities. The EPR spectrum of a similar glass prepared by spiking the frit with NaTcO4 was nearly the same as that of the glass without technetium. However, there appears to be a slight increase in relative intensity of the g ) 2.0 resonance, which could be the result of an additional g ) 1.9-2.0 resonance arising from the presence of Tc4+. If the frit is spiked with TcO2 instead of NaTcO4, the resulting glass exhibits an EPR spectrum which is much different than either of the previously discussed glasses. Observations show a relatively intense resonance near g ) 3.0 and the g ) 4.2 resonance has decreased in intensity dramatically. Although the assignment of the g ) 3.0 resonance is still uncertain, this is a clear example of how variations in the batch chemistry (i.e., TcO2 vs NaTcO4) influence how the technetium is incorporated into the final glass. The manner in which waste species are incorporated into the glass matrix could have profound effects on the properties (e.g., chemical durability, etc.) of the waste form.3 Aside from the determinations of the technetium/rhenium redox chemistries in melts and glasses reviewed above, however, relatively little is known about the remaining chemical and structural issues concerning their incorporation into oxide glasses.68,69 For example, to what extent do Tc4+ and Tc7+ species enter the glass matrix as tetrahedrally coordinated network formers, octahedrally coordinated network intermediates, oligomeric structures, and precipitates? How do feed chemistry and processing conditions influence the distribution of technetium and rhenium in these environments? These issues still need to be addressed. Implications for Waste Vitrification. In evaluating the volatility of technetium and rhenium from dried LLW streams during vitrification, the feed composition, and hence chemistry, becomes an important consideration. For a desired glass composition, the final material can be derived using a variety of feeds. A glass composition having a certain fraction of CaO, for example, can be made from feeds containing CaCO3, Ca(NO3)2, or CaSO4. The composition of the initial liquid and the temperature at which it forms during vitrification will depend on feed variables such as this. This, in turn, will govern how the feed densifies from a collection of liquid-coated glass precursor particles having open porosity, through which volatilized and/or entrained species can readily escape, to a more consolidated viscous melt, where volatility will generally be controlled by diffusion of species through the melt. In general, for a given glass composition, altering the feed chemistry to produce an increase in the point of high volatility (e.g., melting point) of the technetium/rhenium source and a decrease in the consolidation temperature of the melt will shorten the time during which there exists both a high vapor pressure of technetium/rhenium-containing species and an unconsolidated glass precursor material. The worst-case scenario would involve using a highly volatile source for technetium (i.e., Tc2O7 as opposed to KTcO4, etc.) or rhenium together with glass precursors that do not form a liquid phase until high temperatures and melting the batch under oxidizing conditions. Such a scenario was created by the authors in our laboratory by using waste simulants dry-batched entirely from quartz (SiO2), corundum (Al2O3), boric acid (H3BO3),

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Chem. Mater., Vol. 8, No. 5, 1996 1019

Figure 4. Percentage of technetium and rhenium retained (based on the amount of NH4TcO4 or NH4ReO4 added to the initial feed) in the final glass after melting dry-batched feeds at various temperatures for 30 min.

calcium carbonate (CaCO3), sodium carbonate (Na2CO3), and minor amounts of sodium halides, sulfates, and phosphates along with NH4TcO4 or NH4ReO4.70 The amounts of technetium or rhenium retained in the final glass after melting for 30 min at various temperatures are summarized in Figure 4. Three conclusions can be extracted from these results, none of which should be too surprising considering the discussions given in the above sections: under these conditions (1) rhenium closely emulates the behavior of technetium, (2) technetium/rhenium volatility is only slightly dependent of melting temperature in the range 900-1350 °C, and (3) both technetium and rhenium are extremely volatile, even at temperatures as low as 900 °C. In contrast, using a dried simulated DSSF (Table 1)/ glass precursor feed, which most closely resembles the actual Hanford LLW material, the minimum amount of rhenium retained in the final glass is presented in Figure 5.71 For reference the rhenium retention data for a glass produced under similar conditions from a drybatched feed is also included in the figure. The glass prepared from the DSSF-based feed retains 80 times more rhenium than the glass prepared from dry-batched feed (64% vs 0.8%). There are obvious differences between these two feeds which must ultimately be effecting the volatility of the rhenium during vitrification. Although these differences demonstrate the need for the careful designing of tests, they also reflect on possible means of preventing rhenium (and technetium) volatilization during vitrification. (70) SiO2 (56.78 g), H3BO3 (9.12 g), Na2CO3 (32.88 g), CaCO3 (7.15 g), Al2O3 (12.00 g), Bi2O3 (0.014 g), NaCl (0.152 g), Cr2O3 (0.036 g), NaF (0.471 g), Fe2O3 (0.005 g), K2CO3 (0.480 g), MnO (0.007 g), Nd2O3 (0.012 g), NaPO3 (1.705 g), Na2SO4 (0.569 g), ZrO2 (0.005 g), and either NH4TcO4 or NH4ReO4 (0.01 g) were batched together and placed in an Al2O3 crucible partially covered with an Al2O3 lid. The crucible, batch and lid were placed in a Deltech furnace and the temperature ramped at 10 °C/min from ≈25 °C to the desired melt temperature. After a 30 min hold at the melt temperature, the melt was quenced by pouring it onto a stainless steel plate. Tc and Re concentrations in the final glasses were determined by ICP-MS. (71) SiO2 (56.78 g), H3BO3 (9.12 g), CaCO3 (7.15 g), Al2O3 (8.66 g), and 71.2 mL of DSSF simulant (described in Table 1) spiked with 0.017 M NH4ReO4 (in place of NH4TcO4) were mixed together to form a slurry and placed in an Al2O3 crucible. The crucible and slurry were placed in a drying oven set at 100 °C. After 24 h, the crucible along with the dried DSSF/glass precursor slurry were partially covered with an Al2O3 lid and placed in Deltech furnace. The temperature was ramped at 10 °C/min from ≈25 °C to the desired melt temperature. After a 30 min hold at the melt temperature, the melt was quenced by pouring it onto a stainless steel plate. Re concentrations in the final glasses were determined by ICP-MS and XRF.

Figure 5. Percentage of rhenium retained (based on the amount of NH4ReO4 added to the initial feed) in the final glass after melting various feeds at 1150-1200 °C for 30 min.

First, the dry-batched feed contains NH4ReO4, whose Re2O7 oxidative-decomposition product becomes highly volatile above ≈300 °C (i.e., above its melting point). The dried simulated DSSF-based feed most likely contains CsReO4 and to some extent, KReO4 (see above), which do not become significantly volatile until they melt at >550 °C. Second, NaNO3 melts at 320 °C72 and would form the bulk of the liquid phase at low temperatures during vitrification of the DSSF-based feed. In the dry-batched feed, however the Na2CO3 would not melt until 850 °C. Although lower temperature eutectics can form between Na2CO3 and a host of other components in the batch,72 significant liquid-phase formation will generally occur at higher temperatures for the dry-batched feed compared to that for the DSSF-based feed. Thus, the densification of the feed will occur at lower temperatures for the DSSF-based feed compared to that of the drybatched feed. Furthermore, analogous to what occurs in the mirabilite sintering process (see above), the reactive molten NaNO3 and KNO3 salts that form in the DSSF-based feed are capable of reacting with the decomposition products Re2O7 and ReO3(OH) and perhaps even with any of the starting CsReO4, forming nonvolatile NaReO4 and KReO4 species at the earliest stages of vitrification. Last, the DSSF-based feed contains organicss specifically, EDTA. Using TGA/DTA/MS, we have found that the molten nitrate salts that form above ≈320 °C react with the EDTA in a strongly exothermic manner, liberating CO2, NOx, CH4, and in some cases H2. A second and larger CO2 generation takes place at temperatures above ≈700 °C and is due to the decomposition of CaCO3 and/or its reaction with quartz. In the dry-batched feed, only this later reaction takes place. Thus, the DSSF-based feed creates a non-oxidizing (i.e., lower p(O2)), and hence less volatilizing, environment (72) Hrma, P. In Chemistry of Glass, 2nd ed.; Paul, A., Ed.; Chapman & Hall: New York, 1982; pp 157-178.

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at much lower temperatures during vitrification than does the dry-batched feed. As with the work performed by Freude et al.66 the more reducing conditions of the DSSF-based melt over that of the dry-batched melt will allow for a greater retention of rhenium (and technetium) in the glass derived from the former as opposed to the latter. Vida15 summarized laboratory work performed on the vitrification of HLW nitric acid solutions and calcines.73 For melt temperatures of 1000-1150 °C under nonoxidizing conditions (either melting under gases such as NOx generated from solution-based feed or under a stream of argon for the case of calcine feeds), technetium retention in the final glasses ranged from 47 to 70% based on the original feed composition. Thus, under similar melt conditions using similar feeds, the technetium retention during vitrification summarized by Vida15 and by Freude et al.66 brackets that of rhenium determined in our laboratory. Vida15 also discusses the denitration, calcination, and vitrification (

Chemistry of Technetium and Rhenium Species during Low-Level

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