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Reactions Affecting Cation Exchange Kinetics in Vermiculite¹

ABSTRACT

Strong retention of cations, commonly referred to as fixation, was studied using Cs and Sr exchange reactions with Montana vermiculite. Fixation was defined as the property of an exchanger to retain cations in a state such that they are not available for rapid isotopic exchange in a well-stirred system. Efforts were directed toward qualitatively determining some of the conditions under which fixation occurs.

Equations were derived to calculate fixation using isotopic equilibrium values attained with separate aliquots of carrier-free isotope. Cesium fixation occurred in all size fractions of vermiculite ranging from 40–20 mesh through < 0.2 µ upon addition of 1 symmetry stable CsCl. The amount of ¹³⁴Cs fixed at this Cs level depended on the sequence of adding separate aliquots of isotope and the corresponding stable salt, and appeared to be caused by lattice collapse.

The amount of Cs fixed increased with increasing additions of CsCl. However, when Cs saturation of the vermiculite exchange capacity exceeded 4.5%, the fraction of sorbed Cs that became fixed was relatively constant. A plot of Cs fixation vs. percent Cs saturation of the exchange capacity indicated that fixation was independent of time over an interval of 4 months.

In contrast to the Cs results, Sr was always readily exchangeable, with no evidence of lattice collapse.

¹ This manuscript is a contribution from the Agr. Res. Lab. of The Univ. of Tennessee, Oak Ridge, which is operated by the Tennessee Agr. Exp. Sta. for the US Atomic Energy Comm. under Contract no AT-40-1-GEN-242; published with the permission of the Director of the Univ. of Tennessee Agr. Exp. Sta., Knoxville. Presented before Div. S-2, Amer. Soc. Agron. Meetings, Stillwater, Okla., Aug. 24, 1966.

² Assistant Professor, Associate Professor, respectively, Soil Chemistry.

Received for publication April 17, 1967. Accepted for publication July 28, 1967.