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Aluminous Chlorite Origin of pH-Dependent Cation Exchange Capacity Variations¹

ABSTRACT

Cation exchange capacity (CEC) and K-fixing power of a micaceous vermiculite were markedly reduced following treatment with solutions of polymeric hydroxy alumina. The partially chloritized product exhibited linear increase in CEC (measured with KCl) from 48 to 95 meq/100 g as the pH of the NaOAc buffer and then KCl washings increased from pH 4 to 8.5; the CEC increased further to 117 meq/100 g upon brief exposure ₂ alkali (2% Na₂CO₃, pH 11) at 25C. The pH-dependent CEC appeared to arise from isomorphous substitutional charge (permanent negative charge), blocked by hydroxy Al charged positively by edge — AlOH₂ groups, and restored by deprotonation of the latter on addition of base. The present findings with chloritized vermiculite further establish the mechanism of pH-dependent CEC of soils since the behavior found is essentially identical to that reported earlier for soil clays containing aluminous, pedogenically formed chlorite, the constituent responsible for pH-functional variations in CEC in crystalline clays.

Key Words: hydroxy Al interlayering • CEC hysteresis • depotassication of mica • electron microscope replicas • anion exchange • vermiculite • acid soils

¹ Contribution from the Department of Soil Science, University of Wisconsin, Madison. Supported in part by a study leave grant to the first author from the Department of Agricultural Technical Services and the University of Natal, Republic of South Africa, and in part by US Atomic Energy Commission Contract AT(11-1)-1515-Jackson. Presented before Div. S-2 and S-5, Soil Science Society of America, Nov. 1, 1965, Columbus, Ohio.

² Senior Lecturer, University of Natal, Pietermartizburg, South Africa (formerly Visiting Associate Professor at Wisconsin) and Professor of Soil and Water Sciences, respectively.

Received for publication January 9, 1967. Accepted for publication June 22, 1967.