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Characterization of Hydroxy-Aluminum Solutions by Aluminum-27 Nuclear Magnetic Resonance Spectroscopy¹

ABSTRACT

Laboratory synthesized hydroxy-Al solutions were characterized by nuclear magnetic resonance (NMR) spectroscopy. Solutions ranging in Al concentration from 3.4 x 10^–3 to 1.0 mol L^–1 at OH/Al ratios (ñ) of 0, 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.25, and 2.50 prepared with base injection rates of 0.6 and 1.2 cm³ min^–1 were investigated. For concentrated solutions (0.5 mol L^–1), two polymeric species produced observable resonance peaks, the [AlO₄Al₁₂(OH)₂₄(H₂O)₂]⁷⁺ polynuclear (i.e., "Al₁₃") species with a chemical shift () 63-ppm downfield of the hexaaquaaluminum species, and the dimer Al₂(OH)₂(H₂O)⁴⁺₈, which displayed a broad resonance 3-ppm downfield from the hexaaqua-Al cation. In solutions of much lower Al concentration (3.4 x 10^–2 mol L^–1), the Al₁₃ polynuclear species was observed at ñ>0.25 and its concentration was found to increase linearly as a function of ñ. For ñ = 2.5, the concentration of the Al₁₃ polymer decreased at the 1.2 cm³ min^–1 base injection rate; however, this was not the case at the 0.6 cm³ min^–1 neutralization rate where the concentration continued to increase linearly. The data suggest that the distribution of Al species in partially neutralized solutions are acutely sensitive to Al concentration and neutralization rate.

¹ Journal Article no. 84-3-150 of the Kentucky Agric. Exp. Stn. Preliminary results of this study were presented before Div.S-2, 1 Dec. 1982, at the Soil Science Society of America meeting, Anaheim, CA.

² Former Graduate Research Assistant, now Assistant Professor, Univ. of Georgia, Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29801, and Professors, Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091.

Received for publication December 24, 1984.

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