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STRUCTURAL CHANGES OF ALLOPHANE DURING PURIFICATION PROCEDURES AS DETERMINED BY SOLID-STATE ²⁷Al AND ²⁹Si NMR

Department of Biological Safety Science, National Institute for Agro-Environmental Sciences (NIAES), 3-1-3 Kan-nondai, Tsukuba, Ibaraki 305-8604, Japan

^* E-mail address of corresponding author: hiradate{at}affrc.go.jp

Allophanes are poorly crystalline and quasi-stable aluminosilicate minerals, the structures of which are sensitive to chemical treatment. In the present study, solid-state ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectra of allophane samples were monitored as they went through several purification procedures. It was confirmed that no significant structural changes were caused by boiling with 6% H₂O₂ to remove organic matter, by size fractionation (sonification), by sedimentation, by precipitation at pH 4.0, or by dithionite-citrate-bicarbonate treatment for the removal of Fe (hydr)oxides. Hot 5% Na₂CO₃ treatment for the removal of reactive silica-alumina gels and adsorbed citrate from allophane samples, however, decreased signal intensity corresponding to imogolite-like Si (Q₃3^VIAl, –78 ppm in ²⁹Si NMR) and increased signal intensities corresponding to ^IVAl (55 ppm in ²⁷Al NMR) and possibly X-ray amorphous aluminosilicates (centered at –85 ppm in ²⁹Si NMR). Cold (room temperature) 5% Na₂CO₃ treatment for 16 h proved to be effective in avoiding these structural changes.

Key Words: ²⁷Al NMR • Imogolite • KiP Pumice • Kitakami Pumice • ²⁹Si NMR • Solid-state MAS NMR • Volcanic Glass

This article has been cited by other articles:

				H. Hashizume and B. K. G. Theng ADENINE, ADENOSINE, RIBOSE AND 5'-AMP ADSORPTION TO ALLOPHANE Clays and Clay Minerals, December 1, 2007; 55(6): 599 - 605. [Abstract] [Full Text] [PDF]