Aluminous Chlorite Origin of pH‐Dependent Cation Exchange Capacity Variations

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% Na 2 CO 3 , 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 — AlOH 2 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.