Title: In Situ ²⁷Al NMR Spectroscopy of Aluminate in Sodium Hydroxide Solutions above and below Saturation with Respect to Gibbsite

Abstract

Aluminum hydroxide (Al(OH)3, gibbsite) dissolution and precipitation processes in alkaline environments play a commanding role in aluminum refining and nuclear waste processing, yet mechanistic aspects underlying sluggish kinetics during crystallization have remained obscured due to a lack of in situ probes capable of isolating incipient ion pairs. At a molecular level Al is cycling between tetrahedral (Td) coordination in solution to octahedral (Oh) in the solid. We explored dissolution of Al(OH)3 that was used to produce variably saturated aluminate (Al(OH)4–)-containing solutions under alkaline conditions (pH >13) with in situ 27Al magic angle spinning (MAS)–nuclear magnetic resonance (NMR) spectroscopy, and interrogated the results with ab initio molecular dynamics (AIMD) simulations complemented with chemical shift calculations. The collective results highlight the overall stability of the solvation structure for Td Al in the Al(OH)4– oxyanion as a function of both temperature and Al concentration. The observed chemical shift did not change significantly even when the Al concentration in solution became supersaturated upon cooling and limited precipitation of the octahedral Al(OH)3 phase occurred. However, subtle changes in Al(OH)4– speciation correlated with the dissolution/precipitation reaction were found. AIMD-informed chemical shift calculations indicate that measurable perturbations should begin when the Al(OH)4–···Na+ distance is less than 6more » Å, increasing dramatically at shorter distances, coinciding with appreciable changes to the electrostatic interaction and reorganization of the Al(OH)4– solvation shell. The integrated findings thus suggest that, under conditions incipient to and concurrent with gibbsite crystallization, nominally expected contact ion pairs are insignificant and instead medium-range (4–6 Å) solvent-separated Al(OH)4–···Na+ pairs predominate. Moreover, the fact that these medium-range interactions bear directly on resulting gibbsite characteristics was demonstrated by detailed microscopic and X-ray diffraction analysis and by progressive changes in the fwhm of the Oh resonance, as measured by in situ NMR. Sluggish gibbsite crystallization may arise from the activation energy associated with disrupting this robust medium-range ion pair interaction.« less

Authors:

Graham, Trent R. ^[1]; Dembowski, Mateusz ^[2]; Martinez-Baez, Ernesto ^[3]; Zhang, Xin  ^[2];

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• Search DOE PAGES for ORCID "0000-0003-2000-858X"

Jaegers, Nicholas R.  ^[4];

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Hu, Jianzhi  ^[2];

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Gruszkiewicz, Miroslaw S. ^[5]; Wang, Hsiu-Wen  ^[5];

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Stack, Andrew G.  ^[5];

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Bowden, Mark E. ^[2]; Delegard, Calvin H. ^[6]; Schenter, Gregory K. ^[2]; Clark, Aurora E.  ^[3];

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Clark, Sue B. ^[7]; Felmy, Andrew R. ^[7]; Rosso, Kevin M.  ^[2];

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Pearce, Carolyn I.  ^[2]

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• Search DOE PAGES for ORCID "0000-0003-3098-1615"

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+ Show Author Affiliations

1. Washington State Univ., Pullman, WA (United States). Voiland School of Chemical and Biological Engineering
2. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
3. Washington State Univ., Pullman, WA (United States). Dept. of Chemistry
4. Washington State Univ., Pullman, WA (United States). Voiland School of Chemical and Biological Engineering; Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
5. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
6. TradeWind Services LLC, Richland, WA (United States)
7. Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Washington State Univ., Pullman, WA (United States). Dept. of Chemistry

Publication Date:

Mon Jul 23 00:00:00 EDT 2018

Research Org.:

Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Oak Ridge Leadership Computing Facility (OLCF); Energy Frontier Research Centers (EFRC) (United States). Interfacial Dynamics in Radioactive Environments and Materials (IDREAM); Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1482434

Alternate Identifier(s):

OSTI ID: 1596361

Grant/Contract Number:  

AC05-00OR22725; AC06-76RLO1830; AC06-76RL01830

Resource Type:

Accepted Manuscript

Journal Name:

Inorganic Chemistry

Additional Journal Information:

Journal Volume: 57; Journal Issue: 19; Journal ID: ISSN 0020-1669

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES