Transitions in Al Coordination during Gibbsite Crystallization Using High-Field ²⁷ Al and ²³ Na MAS NMR Spectroscopy

Journal Article · Thu Nov 30 04:00:00 EST 2017 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/acs.jpcc.7b10424· OSTI ID:1422323

Hu, Jian Zhi ^[1]; Zhang, Xin ^[1]; Jaegers, Nicholas R. ^[2]; Wan, Chuan ^[1]; Graham, Trent R. ^[2]; Hu, Mary ^[1]; Pearce, Carolyn I. ^[1]; Felmy, Andrew R. ^[2]; Clark, Sue B. ^[2]; Rosso, Kevin M. ^[1]

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1. Pacific Northwest National Laboratory, Richland, Washington 99354, United States
2. Pacific Northwest National Laboratory, Richland, Washington 99354, United States; Washington State University, Pullman, Washington 99164, United States

Mechanisms of nucleation and growth of Al hydroxides such as gibbsite from aqueous solution, particularly in highly alkaline conditions, remain poorly understood. In this work, quantitative 27Al and 22Na MAS NMR experiments were conducted on solid samples extracted from the crystallization of gibbsite from an amorphous aluminum hydroxide gel precursor. The use of high magnetic field and fast sample spinning allowed transitional tetrahedral (AlT) and pentahedral (AlP) aluminum species to be observed along with the octahedral aluminum (AlO) that dominates the gibbsite product. Low-coordinated Al species could be detected at concentrations as low as 0.1% of the total Al sites. It is established that (a) AlT and AlP coexist on the surface of growing gibbsites even with a combined percentage over the total Al sites of less than 1%; (b) Different synthesis methods generate gibbsite with varying amounts of low-coordinated Al; (c) the amorphous gel precursor contains a significant amount of low-coordinated Al sites with AO: AlP: AlT ratios of approximately 4:2:1; (d) upon hydration, the external, low-coordinated Al sites become six-fold coordinated by interacting with the oxygen in H2O and the 27Al MAS NMR peak position shifts to that for the AlO sites; (e) gibbsite with increased long range order is synthesized over longer times by gradually incorporating residual AlP and AlT sites into octahedrally-coordinated AlO sites; (f) trace Na is predominantly a surface species on gibbsite particles. These findings provide a basis for understanding the gibbsite crystallization mechanism, along with a general means of characterizing gibbsite surface properties that are of equal importance for understanding related processes such as dissolution behavior.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL); Energy Frontier Research Centers (EFRC) (United States). Interfacial Dynamics in Radioactive Environments and Materials (IDREAM)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1422323
• Report Number(s): PNNL-SA-130259; 49771; KC0307010
• Journal Information: Journal of Physical Chemistry. C, Journal Name: Journal of Physical Chemistry. C Journal Issue: 49 Vol. 121; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

Related Subjects

Environmental Molecular Sciences Laboratory