Intermediate Species in the Crystallization of Sodium Aluminate Hydroxy Hydrates

Abstract

In highly alkaline "water-in-salt" Na 2 O/Al 2 O 3 /H 2 O solutions where the monomeric Al(OH) 4− anion dominates, isolation of transitional species that seed crystallization of sodium aluminate salt hydrates has been challenging. For example, discrimination of dimeric [for example, Al 2 O(OH) 62− ] species via 27 Al nuclear magnetic resonance (NMR) spectroscopy is limited via fast interconversion through hydrolysis and condensation reactions. Despite this, using magicangle spinning NMR ( 27 Al MAS NMR) at high magnetic field strengths (14.1 and 20.0 T) on crystals of nonasodium bis(hexahydroxyaluminate) trihydroxide hexahydrate [NSA, Na 9 [Al(OH) 6 ] 2 •3(OH)•6(H 2 O)], we observed a 27 Al resonance consistent with Na 2 Al 2 O(OH) 6 . This tetrahedral dimeric species was also identified in the mother liquor by Raman spectroscopy while remaining expectedly unresolved by high-field liquid-state 27 Al NMR. Its substantial abundance in the mother liquor and as either an interstitial or an adsorbate on precipitated solids suggest that it plays a crucial role in NSA crystallization. Our detailed characterization, which includes single-pulse 27 Al MAS NMR, nutation 27 Al MAS NMR, and multiple quantum 27 Al MAS NMR, provides new molecular insights into how aluminate transitions from tetrahedral in solution to octahedral in solid. This understanding could ultimately lead to better predictions of aluminum phase change behavior in caustic sodium hydroxide with wide-ranging technological and environmental implications.

Photon-In/Photon-Out X-ray Free-Electron Laser Studies of Radiolysis

“…Data adapted from Refs. [67][68][69]. The composition of the tank waste promotes formation of some of these species.…”

“…(b) Na 2 O-Al 2 O 3 -H 2 O ternary phase diagram showing solubility region as well as regions saturated with respect to gibbsite, gibbsite/monosodium aluminate hydrate, and monosodium aluminate hydrate/nonasodium bis(hexahydroxyaluminate) trihydroxide hexahydrate/sodium hydroxide. Data adapted from Refs [67][68][69]…”

Speciation of aluminum phases at the Hanford Site

“…Thus, it is important to assess the composition of this component of the waste and the processing requirements to treat this waste. A number of aluminum phases have been identified in Hanford tank waste, including boehmite, [6][7][8] gibbsite, 9,10 sodium aluminate, 11,12 and alumino-silicates such as cancrinite. 13 Figure 2 provides sample Xray diffraction (XRD) patterns for boehmite and gibbsite rich phases in tank waste.…”

Theory-Guided Inelastic Neutron Scattering of Crystalline Alkaline Aluminate Salts Bearing Principal Motifs of Solution-State Species

“…The situation is rather different in concentrated sodium aluminate solutions, which do not yield an equivalent M 2 [Al 2 O(OH) 6 ] salt phase, although trace amounts of a Na 2 [Al 2 O(OH) 6 ] salt were recently detected in sodium aluminate hydrates. 19 At low total [NaOH], gibbsite, its structural polymorph bayerite, and boehmite are the solubilitycontrolling phases, and these minerals have octahedrally coordinated Al in a hydroxy-or oxy-bridged network. However, when total Na + /Al(OH) 4 − ratios are greater than 1 and total [NaOH] is above ∼10 m (molality), gibbsite (or bayerite or boehmite) gives way to solubility-controlling phas es of (1) monosodium alum inate hydrat e (Na 2 [Al 2 O 3 (OH) 2 ]•1.5H 2 O) composed of chains of tetrahedral aluminates each coordinated to three bridging oxygens and one terminal hydroxy group and/or (2) nonasodium bis(hexahydroxy aluminate) trihydroxide hexahydrate (Na 9 [Al-(OH) 6 ] 2 (OH) 3 •6H 2 O), in which aluminum speciates as monomeric aluminate octahedra coordinated to six hydroxides.…”

“…For example, both K 2 [Al 2 O­(OH) 6 ] and Rb 2 [Al 2 O­(OH) 6 ] can be easily precipitated for detailed study before, during, and after crystallization. The situation is rather different in concentrated sodium aluminate solutions, which do not yield an equivalent M 2 [Al 2 O­(OH) 6 ] salt phase, although trace amounts of a Na 2 [Al 2 O­(OH) 6 ] salt were recently detected in sodium aluminate hydrates . At low total [NaOH], gibbsite, its structural polymorph bayerite, and boehmite are the solubility-controlling phases, and these minerals have octahedrally coordinated Al in a hydroxy- or oxy-bridged network.…”