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Fixation of Hydroxy-Aluminum Polymers by Vermiculite¹

ABSTRACT

The addition of vermiculite to hydroxy-aluminum (OH-Al) solutions prepared by the controlled addition of NaOH to AlCl₃ results in the fixation of OH-Al polymers by the clay. With solutions of NaOH/Al molar ratios = 0.3 to 2.1, the polymers held by the clay are of nearly constant composition, very close to the formula [Al₆(OH)₁₂]⁶⁺ (single ring) or [Al₁₀(OH)₂₂]⁸⁺ (double ring). The polymers are present in a single layer in the interlayer space. All specimens give a strong 14Å. X-ray diffraction peak of similar intensity on K⁺ saturation even though the proportion of cation-exchange positions occupied by the OH-Al polymers varies from 24% to 61% on an equivalent basis.

With solutions of NaOH/Al = 2.25 to 2.70, the polymers held by clay gradually increase in size and decrease in net positive charge per Al atom with increasing NaOH/Al ratio. The polymers are also present in a single layer in the interlayer space, but it takes more Al atoms and a longer time for the polymers in this group to produce a 14Å. peak of an intensity corresponding to that obtained when the Al is in the form of single or double ring units. In those specimens in which the amount of Al fixed is not sufficient to hold the vermiculite layer apart effectively, the Al polymer-vermiculite layer is randomly interstratified with K-vermiculite, and the resultant X-ray diffraction patterns vary widely, some appearing as though X-ray amorphous components were present.

For all of the above mentioned specimens, no noticeable change has been found in the composition of the polymers held by clay minerals during aging up to 6 months. No crystalline Al(OH)₃ was detected in any of these specimens. It is concluded that vermiculite functions as an anion which has a strong affinity for OH-Al polymers and which inhibits the formation of gibbsite in soils.

At NaOH/Al = 3, no OH-Al polymer is found in the interlayer space but crystalline Al(OH)₃ forms outside the clay lattice within two weeks.

¹ Contribution No. 63–71 from the College of Mineral Industries, The Pennsylvania State University. Presented before Div. S-2, Soil Sci. Soc. Am., Denver, Colo., Nov. 19, 1963. The authors express their sincere thanks to Dr. L. J. Johnson, The Pennsylvania State University, for his review and comments in the preparation of this report. The research is supported by a grant from the Earth Science Division of the National Science Foundation.

² Research Associate and Professor of Mineralogy, respectively.

Received for publication March 20, 1964. Accepted for publication May 15, 1964.