|SREL Reprint #2007|
SODIUM AND CHLORIDE SORPTION BY
CHUNMING SU1, JAMES B. HARSH,2 AND PAUL M. BFRTSCH3
2Department of Crop and Soil Sciences, College of Agriculture and Home Economics
The surface excesses of Na and Cl on synthetic imogolite and allophanes with
varying AUSI molar ratios in 0. IO M and 0.0 I M NaCl solutions were
determined using 22Na and 36 Cl as ion probes. The point of zero net charge
(PZNC) values ranged from 4.1 to 8.4, increasing with the AUSI 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. I and 0.0 I M NaCi
solutions below pH 8.4 and of Na from 0. I M NaCl solutions between pH 5 and
8.4 exceeded the proton charge determined by potentiometric titration. There was
no direct evidence ofpermanent 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 AUSI by 27Al high-resolution solid-state nuclear magnetic resonance
(NMR) spectra of allophanes, and was absent in imogolite. The chemical shifts of
AI(4) and AI(6) were similar in allophanes (63.0-64.7 ppm and 6.1-7.8 ppm,
respectively) and the chemical shift of AI(6) was 9.4 in imogolite.
1Contribution from the College of Agric. and Home Econ. Res. Ctr., Pullman.
Paper No. 9101-56. Project 0694
Su, C., J.B. Harsh, and P.M. Bertsch. 1992. Sodium and chloride sorption by imogolite and allophanes. Clays and Clay Minerals 40:280-286.