²⁷Al Mas NMR and Aluminum X-Ray Absorption Near Edge Structure Study of Imogolite and Allophanes

Abstract

Abstract--This paper compares the results of 27A1 nuclear magnetic resonance spectroscopy (NMR) and AI-K-edge X-ray Absorption Near Edge Structure (XANES) of natural imogolite and allophanes and some crystaUine reference minerals. All soil allophanes studied contain 4-coordinated A1 (AlIV). The highest relative proportion of A1TM, 21% of the total AI, was found in Si-rich allophane. This value is close to that found in spring allophanes, which were previously considered to be different from soil allophanes. For a quantitative determination of the AlXV/Alt~ ratio, NMR is more reliable than XANES, because of the sensitivity of the chemical shift to low A1TM concentrations, but XANES may be used even if paramagnetic impurities (mostly Fe) are present. Al-K XANES also yields more information than NMR on the local environment of A1 vi and especially site multiplicity. AI w XANES of imogolite and allophanes are similar regardless of the AI/Si ratio. They yield two well-resolved resonances with maxima near 1568 and 1570 eV, which indicates the presence of a unique A1 vt site by comparison with crystalline references. The presence of only one A1 w site indicates that imogolite and allophanes have an octahedral sheet with a structure similar to 2/1 dioctahedral phyllosilicates but different from gibbsite or kaolinite, previously considered as structural analogues. The 27AltV MAS NMR peak maxima of allophanes are between 58.6 and 59.8 ppm, in the range observed for crystalline and amorphous framework aluminosilicates, and less positive than those of sheet silicates, which are typically in the range 65-75 ppm. 27A1-H 1 CPMAS NMR spectra suggest that both A1TM and A1 vI have A1-O-H linkages.

The Effect of Dissolved Humic Acids on Aluminosilicate Formation and Associated Carbon Sequestration

“…Broad peaks centered at approximately 0.334 nm and 0.222 nm, characteristic of allophane [6,7,27], occur in all samples. Aluminum-rich allophanes (Al 2 O 3 ·xSiO 2 ·yH 2 O (x = 1; y = 2-5)) classified as protoimogolite or imogolite-like allophane [28] can have Al/Si ratios ranging from 2.3 to 2.7 [29], consistent with the observed range of 2.2 to 2.7. The observed XRD reflections are also common to imogolite, although there are additional imogolite reflections [30,31] that are absent in the XRD patterns.…”

“…Both 29 Si and 27 Al nuclear magnetic resonance spectroscopy (NMR) data were collected for generated solids. • pulse (5 μs) and 180 s relaxation delay at a spinning rate of 10 kHz.…”

“…The 29 Si SP MAS/NMR spectra of all the precipitates contain a narrow peak at a chemical shift of δ Si-29 = −78.3 ppm (1.8 ppm full width at half maximum; FWHM) plus a broad signal at more negative chemical shifts that can be represented by two broad peaks, near −82 ppm and −89 ppm, and 9-17 ppm FWHM, in varying proportions ( Figure 5). The narrow peak is characteristic of imogolite-like Si environments, in which a silicate tetrahedron is coordinated to three 6-coordinated Al ions and one hydroxyl ion [35,36].…”

The Surface Chemistry of Imogolite

“…However, Ildefonse et al (1994) showed that the A1-O bond lengths of imogolite are probably much less diverse than those proposed by Cradwick et al (1972), and therefore it seems likely that the tenmolecular unit cell model should be modified.…”

“…Harsh et al (1992) speculated that imogolite carried a weak structural positive charge. It is clear that such a structural charge cannot involve substitutions such as A1 for Si, because nuclear magnetic resonance (NMR) studies have shown imogolite to consist exclusively of hexavalent A1 and unpolymerized, tetravalent Si (Goodman et al, 1985;Ildefonse et al, 1994). In this paper, I argue that differences in A1-O bond valences can explain why the outer walls of imogolite may develop a weak positive charge that is constant over a wide pH range, whereas the inner walls develop a weak negative charge.…”