Ion Selectivity in Three Soil Profiles as Influenced by Mineralogical Characteristics
1972
Murdock, L. W. | Rich, C. I.
Adsorption and release of Rb and Sr by the 2–0.2µ fraction from the horizons of three vermiculitic soils was influenced by mineralogical properties of the 2:1 layer clays present. The lowest Rb/Sr ratio for absorption of these two ions occurred in a clay containing montmorillonite. The Rb/Sr ratio tended to increase as the degree of mica to vermiculite alteration increased. The presence of hydroxy-Al interlayers had little effect on initial selectivity, but these interlayers markedly influenced release of Rb ions. Ammonium acetate released more Rb than did Mg(OAc)₂ from all clay samples from the three soils. Generally, Mg(OAc)₂ released more Sr than did NH₄OAc. The differential exchange of Rb was most pronounced in the clay containing well-developed interlayers. It is proposed that NH₄ ions (and other ions of similar size) have two effects on exchange in vermiculite. NH₄ tends to collapse the interlayer space and thus reduce exchange, but this slightly hydrated ion can also diffuse to wedge zones and exchange Rb, and other cations of similar size, that may be present. The presence of interlayers inhibits the first effect and thus reduces fixation to exchange by NH₄ of ions in wedge zones.
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