Environ. Sci. Technol. 2002, 36, 2451-2458
Raman Study of Aluminum Speciation in Simulated Alkaline Nuclear Waste C L I F F T . J O H N S T O N , * ,† STEPHEN F. AGNEW,‡ JON R. SCHOONOVER,§ J O H N W . K E N N E Y , I I I , |,⊥ B O B B I P A G E , | JILL OSBORN,§ AND ROB CORBIN§ Environmental Sciences and Engineering Institute (ESEI), 376 Potter Engineering Complex, Purdue University, West Lafayette, Indiana 47907-1202, Archimedes Technology Group, Inc., 5405 Oberlin Drive, San Diego, California 92121, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Chemical Physics Laboratory, Department of Physical Sciences, Eastern New Mexico University, Portales, New Mexico 88130
The chemistry of concentrated sodium aluminate solutions stored in many of the large, underground storage tanks containing high-level waste (HLW) at the Hanford and Savannah River Nuclear Reservations is an area of recent research interest. Not only is the presence of aluminate in solution important for continued safe storage of these wastes, the nature of both solid and solution aluminum oxyhydroxides is important for waste pretreatment. Moreover, for many tanks that have leaked high aluminum waste in the past, little is known about the speciation of Al in the soil. In this study, Raman spectroscopy has been used to investigate the speciation of the aqueous species in the Al2O3Na2O-H2O system over a wide range of solution compositions and hydration. A ternary phase diagram has been used to correlate the observed changes in the spectra with the composition of the solution and with dimerization of aluminate that occurs at elevated aluminate concentrations (>1.5 M). Dimerization is evidenced by growth of new Al-O stretching bands at 535 and 695 cm-1 at the expense of the aluminate monomer band at 620 cm-1. The spectrum of water was strongly influenced by the high concentrations of Na+ and OH- (>17 M). Upon increasing the concentration of NaOH in solution, the δ(H-O-H) bending band of water (ν2 mode) increased in frequency to 1663 cm-1, indicating that the water contained in the concentrated caustic solution was more strongly hydrogen bonded at the higher base content. In addition, the sharp, well-resolved band at 3610 cm-1, assigned to the ν(O-H) of free OH-, increased in intensity with increasing NaOH. Analysis of the ν(O-H) bands in the 3800-2600 cm-1 region supported the overall increase in hydrogen bonding as evidenced by the increase in relative intensity * Corresponding author phone: (765) 496-1716; fax: (765) 4962926; e-mail:
[email protected]. † Purdue University. ‡ Archimedes Technology Group, Inc. § Los Alamos National Laboratory. | Eastern New Mexico University. ⊥ Current address: Chemical Physics Laboratory, Concordia University, Irvine, CA 92612-3299. 10.1021/es011226k CCC: $22.00 Published on Web 04/24/2002
2002 American Chemical Society
of a strongly hydrated water band at 3118 cm-1. Taking into consideration the activity of water, the molar concentrations of the monomeric and dimeric aluminate species were estimated using the relative intensities of the Al-O stretching bands from the Raman spectra. A constant apparent log Kdimer value was obtained at aluminate concentrations >1.5 M with a value of 0.97 ( 0.04 at ∼25 °C. This study represents the first spectral-based estimation of a thermodynamic equilibrium constant for the Al2O3-Na2O-H2O system.
Introduction The chemistry of concentrated sodium aluminate solutions has been studied for many years in the context of industrial processing of aluminum ores and the Bayer process (1, 2). More recently, however, a new waste form with similar chemistry is being created in and beneath the large singleshell tanks (SSTs) containing high-level waste (HLW) at the Hanford and Savannah River Nuclear Reservations. Many of these HLW tanks contain high concentrations of aluminate in caustic solutions (3). At the Hanford site, for example, the “tank farm” consists of 177 large underground storage tanks in 18 tank farms. These tanks include 149 single-shell tanks and 28 double-shell tanks that contain a total of 212 million liters (56 million gallons) of HLW liquid, sludge, and salt cake (generally a semisolid crusty material). Of primary concern is the 177 million curies of radiation contained within these tanks and the fact that many of these tanks (68 at last count) are known to leak (4, 5). Sodium aluminate, along with NaOH and high concentrations of nitrate and nitrite, came to be dominant components of HLW from blending and evaporation of HLW supernatants over the past 40 years of plutonium production at Hanford and Savanna River. HLW was generated from the chemical dissolution and extraction of plutonium and uranium from reactor fuel elements that were clad in aluminum. Processing the fuel elements included caustic dissolution of the aluminum cladding and disposal of that solution to the waste tanks. Second, aluminum was added to complex fluoride in the processing waste stream. Finally, aluminum nitrate was added during processing to precipitate certain chemical species and increase ionic strength. Disposal of wastes to the carbon steel tanks required the addition of NaOH to minimize tank corrosion. As tank space was at a premium, successive operations repeatedly blended and concentrated the waste supernatant resulting in highly concentrated alkaline aluminate slurries whose physiochemical properties are often poorly understood. Waste processing steps involving these solutions often result in unwanted precipitation, scale formation, and clogging of transfer lines. This is due, in part, to the saturated sodium ion conditions (>10 M) of the HLW supernatant tank liquors. The concentration of Al as aluminate in HLW supernatants, for example, can exceed 1.5 M. In addition, the pH is often greater than 14, and the individual molar concentrations of NO3- and NO2- are in excess of 2 M. As a consequence of the high molar concentrations of Al3+, Na+, NO2-, and NO3-, both the concentration and the activity of water in these supernatant solutions are greatly decreased (6). The concentration of water is in the range of 35-45 M, which is considerably lower than that of pure water (55 M). More importantly, the activity of water can be as low as 20% of that of pure water (6). On a molecular scale, the low activity of water is caused by hydration requirements of the Na+ and VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Al(OH)4- ions with most of the water residing within the immediate coordination sphere of aqueous Na+, OH-, and Al(OH)4- species, as well as bound water contained in the hydrated solid phases (e.g., sodium aluminate). The solidsolution phase behavior of these water-deficient HLW supernatants is controlled, in part, by the activity of water. Attempts to predict the species of aluminum present in HLW using chemical speciation models based on bulk chemical analysis combined with equilibrium thermodynamics have met with limited success (7). Chemical speciation models adapted to concentrated alkaline nuclear waste solutions have not been able to correctly predict the identity of the solid phase. For example, the Environmental Simulation Program (ESP) predicted that gibbsite is the dominant Al solid phase (7). The presence of gibbsite, however, precludes the formation of sodium aluminate. Sodium aluminate is a dominant solid phase in Hanford caustic nuclear waste (7). The current thermodynamic database does not adequately describe these extreme chemical compositions. As Reynolds (7) stressed, there is an ongoing need to develop an improved thermodynamic database to predict the chemical speciation of the HLW tank liquors. Before this objective can be achieved, however, a more indepth understanding of the chemical species present in these concentrated, caustic solutions and their interaction with water is needed. Raman and Fourier transform infrared (FTIR) methods are well-suited to studying caustic aluminate solutions, in part because they can distinguish between monomeric and dimeric or oligomeric aluminate species. Raman spectra of dilute aluminate (1.5 M). In addition, infrared and 27Al nuclear magnetic resonance (NMR) methods have been used to characterize caustic aluminate solutions. Solution 27Al NMR studies have shown that Al is present in tetrahedral coordination for all of the solution compositions studied (12). The NMR spectra were relatively insensitive to changes with increasing aluminate concentration (12). Infrared spectra of the aluminate solutions are characterized by two bands at 700 and 900 cm-1 that demonstrated little variation in relative intensity with increasing Al concentration (11, 13). Although considerable attention has focused on the vibrational spectra of aqueous aluminate species and Alcontaining solid phases over a wide range of solution composition (11), little connection has been drawn between these spectra and the activity of water, excess hydroxide, and other constituents. This fact is surprising given that the high aluminate concentrations are only possible when water activity is significantly reduced by the presence of excess alkali hydroxide. The removal of water from these mixtures has been and continues to be an important need for both Hanford and Savannah River Sites in the DOE Complex. In addition, there are strong needs at both sites for removal of aluminum from waste solids by high caustic dissolution. Processes termed Enhanced Sludge Wash (ESW) at Hanford (14) and Extended Sludge Processing (ESP) at SRS (15) are critical for these sites to meet their respective goals for minimization of high-level waste glass production vitrification. In this study, we combine Raman spectroscopy with a detailed characterization of the solution chemistry, including relative humidity, to gain insight into a broad range of solution compositions with emphasis on developing a better under2452
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FIGURE 1. Ternary phase diagram of the Na2O-Al2O3-H2O system. The three axes are expressed as the weight fraction of each component. The diagram on the left shows the chemical compositions of the 26 samples. On the right side, four transects along selected data points are referred to in the manuscript and are labeled 22%Na, 30%Na, 1%Al, and 7%Al with percents corresponding to each transect’s approximate weight percent of the oxide label. In addition, isopleths of constant relative humidity are shown plotted in the unsaturated regime as dashed curves corresponding to 60%, 50%, and 40% RH, from left to right. In the inset diagram (upper right), the solid circle represents the water activity and Na2O content of an actual waste concentrate obtained from tank SY-101. standing of the dominant Al species in caustic aqueous solutions. Ultimately, this research seeks to improve the predictive ability of chemical equilibrium models for alkaline aluminate solutions by providing new insights into the identity and concentrations of the actual aluminum species present in these solutions through direct spectroscopic observations.
Materials and Methods Sample Preparation. A total of 26 aqueous sodium aluminate solutions were prepared to effectively sample the solution phase of the alkaline aluminate ternary phase diagram (Figure 1). These aluminate solutions were prepared by combining 0.1-1.8 g of Al wire with 2.2-8.3 g of NaOH and 10 g of water in a 25 mL polymethylpentene reaction vessel of known mass. The cap of the vessel was loosened periodically during the reaction/dissolution process to allow H2(g) to escape. All starting reagents and chemicals were ultrapure grade. The aluminum wire dissolved completely within
