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Changes in the Physical Properties of Soil Clays Due to Precipitated Aluminum and Iron Hydroxides: I. Swelling and Aggregate Stability After Drying¹

ABSTRACT

Compressed discs of an illite, a kaolinite, and a mixture of the two were formed from freeze-dried dispersed suspensions of the clays, into which one level of Fe(OH)₃ and two levels of Al(OH)₃ had been precipitated. The precipitated Fe was found to consist of particles about 40Å in diameter and amorphous to X-ray, rather than the acicular goethite that precipitates under similar conditions in the absence of clay. It was deduced from N₂ and water vapor sorption measurements that the precipitated Al was present as very thin layers on the surface of the clay particles, which in the case of the illite could bridge some of the particles. Their form was generally indeterminate, in contrast to the well-crystallized bayerite formed in the absence of clay.

All three hydroxy treatments inhibited the double-layer swelling of the Na-illite and Na-kaolinite/illite mixture in dilute NaCl solutions. Both on weight percent and on an equimolar basis, Al was more effective than Fe in reducing the slaking of dry clay discs and in increasing the resistance of clay to dispersion in pyrophosphate solutions. Possible bonding mechanisms between the precipitate and the clays are discussed.

¹ Joint contribution from the University of Hawaii and the Division of Soils, CSIRO. Journal Series no. 1857 of the Hawaii Agric. Exp. Sta. and Contribution no. 660 of the Hawaii Inst. of Geophysics. Work supported in part by Cooperative Agreement no. 12-14-5001-40 with the Agricultural Research Service, USDA, and by the Office of International Programs, National Science Foundation. Presented before Div. S-1, Soil Science Society of America, Chicago, 13 Nov. 1974.

² Professor of Soil Science, Department of Agronomy & Soil Science, Univ. of Hawaii, Honolulu 96822, and Senior Principal Research Scientist, CSIRO, Division of Soils, Adelaide, South Australia.

Received for publication April 23, 1975. Accepted for publication August 13, 1975.