Raman Study of Aluminum Speciation in Simulated Alkaline Nuclear Waste

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 Al2O3−Na2O−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 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 K dimer 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.