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Salt Displacement and Titration of AlCl₃-Treated Trioctahedral Vermiculites¹

ABSTRACT

Freshly prepared and 45-day-aged Al-saturated trioctahedral vermiculites from Libby, Montana, and northeastern Transvaal, South Africa, were: (i) leached with 1N and 0.05N solutions of NaCl, KCl, and CaCl₂, (ii) subjected to repeated cycles of 1N NaCl leaching and Al saturation, and (iii) potentiometrically titrated in H₂O or N NaCl. In the leaching experiments, the exchange acidity, exchangeable H, Al, and Mg, and the CEC after leaching were determined.

Salt solutions displaced only part of the adsorbed Al, with less from South African than from Libby samples. Also displaced were H₃O and small amounts of Mg ions. The exchange acidity (H₃O + Al), the ratio of H₃O to Al, the displaced Mg, and the apparent CEC after Al saturation and displacement varied with the kind and concentration of the displacing solution, the source of vermiculite, the aging period, and the number of cycles of Al saturation and displacement.

With freshly prepared Libby samples, the exchange acidity varied from about 60 to 80% of the CEC, with Al constituting 60 to 90%. The corresponding ranges for South African samples were 35 to 50% and 30 to 85%. Of the three salts, KCl solutions gave the smallest exchange acidity and the lowest values for Al/exchange acidity; CaCl₂ effected the highest values, with NaCl in between. Dilution of displacing solutions generally increased the H₃O ion component of exchange actidity. Repeated Al saturation and displacement resulted in decreased exchange acidity.

Potentiometric titration curves of Al-vermiculites in H₂O did not suggest the presence of appreciable amounts of exchangeable H₃O. Titration curves in NaCl had buffer ranges interpreted as suggesting the presence of H₃O, Al, and another component presumed to be mostly monohydroxy-Al ions.

The two Al-vermiculites manifested by X-ray analysis different collapsibility on K saturation and heat treatment. The X-ray data support the conclusion that Al hydrolysis occurs between vermiculite plates, with general correspondence between the amount of nonexchangeable Al and the failure of the minerals to collapse to 10A upon K saturation and drying.

¹ Paper No. 1774, Univ. of California, CRC-AES, Riverside, Calif. Paper presented before Div. S-2, Soil Sci. Soc. Amer., Nov. 3, 1965, at Columbus, Ohio. Work supported by a grant from the National Science Foundation (NSF-GP-323-Coleman).

² Postdoctoral Fellow and Professor of Soils and Plant Nutrition, Univ. of California, Riverside.

Received for publication September 19, 1966. Accepted for publication January 16, 1967.