Two components of chloride anion exclusion volume in montmorillonitic soils

Exclusion of chloride anions from negatively charged surfaces was examined in different soils such as Udic Argiboroll, Typic Haploboroll, and Aquents at different CaCl₂ concentrations in solutions. The dominant clay mineral in soils is montmorillonite with Ca²⁺ as a major exchangeable cation. The effect of a salt concentration on anion exclusion volumes in the studied soils was quantitatively explained by the two-component model. According to this model, one component of the anion exclusion volume depending on the salt concentration in solution was described by the theory of electric double layer. It included water layers on the external surfaces of soil particles having freely extended electric double layers. A second component involved internal anion exclusion volume in soil tactoids (i.e. water in the interlayer space between plates), and its magnitude was considered not dependent on salt concentration. Based on determined chloride exclusion volumes, the number of plates and separation distances in soil tactoids were calculated using the two-component model. In different soils, the number of plates in soil tactoids varied between 2.1 and 6.5, and separation distances ranged from 5 to 25 Ǻ. At the lower salt concentrations the interlayer space of soil tactoids provides a minor contribution to the chloride exclusion volume of studied soils.