Sodium and Chloride Sorption by Imogolite and Allophanes
The surface excesses of Na and Cl on synthetic imogolite and allophanes with varying Al/Si molar ratios in 0.10 M and 0.01 M NaCl solutions were determined using 22Na and 36Cl as ion probes. The point of zero net charge (PZNC) values ranged from 4.1 to 8.4, increasing with the Al/Si molar ratio for the allophanes, and was highest for imogolite (Al/Si = 2.01). The PZNC values were significantly lower than the point of zero charge (PZC) values previously determined by microelectrophoresis for the same material, indicating that Na resided within the shear plane to a greater extent than Cl. The PZNC values of allophanes were lower than their PZSE values, indicating that permanent charge existed in allophanes, and increased as Al/Si decreased. Conversely, the PZNC of imogolite was higher than its point of zero salt effect (PZSE) determined by potentiometric titration. Adsorption of Cl on imogolite from 0.1 and 0.01 M NaCl solutions below pH 8.4 and of Na from 0.1 M NaCl solutions between pH 5 and 8.4 exceeded the proton charge determined by potentiometric titration. There was no direct evidence of permanent charge in imogolite and excess Cl adsorption could not be entirely explained by simultaneous intercalation of Na and Cl. Isomorphic substitution of Al in tetrahedral sites was shown to increase with decreasing Al/Si by 27Al high-resolution solid-state nuclear magnetic resonance (NMR) spectra of allophanes, and was absent in imogolite. The chemical shifts of Al(4) and Al(6) were similar in allophanes (63.0–64.7 ppm and 6.1–7.8 ppm, respectively) and the chemical shift of Al(6) was 9.4 in imogolite.
Key WordsZero net charge Nuclear magnetic resonance Isomorphic substitution PZC Ion adsorption Permanent charge Salt absorption Intercalation
Unable to display preview. Download preview PDF.
- Egawa T. (1964) A study on coordination number of aluminum in allophane: Clay Sci. 2, 1–7.Google Scholar
- Henmi T. and Wada K. (1976) Morphology and composition of allophane: Amer. Mineral. 61, 379–390.Google Scholar
- Iimura K. (1969) The chemical bonding of atoms in allophane—The “structural formula” of allophane: in Proc. Int. Clay Conf., Tokyo, L. Heller, ed., 1, 161–172.Google Scholar
- Kinsey R. A. (1984) Unpublished Ph.D. Thesis, University of Illinois, 1984, pp 204.Google Scholar
- Kinsey R. A., Kirkpatrick R. J., Hower J., Smith J. A., and Oldfield E. (1985) High resolution aluminum-27 and silicon-29 nuclear magnetic resonance spectroscopic study of layer silicates, including clay minerals: Amer. Mineral. 70, 537–548.Google Scholar
- Okada K., Morikawa S., Iwai S., Ohira Y., and Ossaka J. (1975) A structural model of allophane: Clay Sci. 4, 291–303.Google Scholar
- Sposito G. (1984) The Surface Chemistry of Soils: Oxford University Press, New York, pp 234.Google Scholar
- Udagawa S., Nakada T., and Nakahira M. (1969) Molecular structure of allophane as revealed by its thermal transformation: in Proc. Int. Clay Conf., Tokyo, L. Heller, ed., 1969 1, 151–159.Google Scholar
- Wada K. (1989) Allophane and imogolite: in Minerals in Soil Environments, J. B. Dixon and S. B. Weed, eds., Soil Science Society of America, Madison, Wisconsin, 1051–1087.Google Scholar