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Diffusion of Iron in Montmorillonite as Determined by X-ray Emission¹

ABSTRACT

The experimental distribution of Fe²⁺ and Fe³⁺ diffusing in thin, water-saturated montmorillonite clay films (43.5% H₂O on oven dry basis) were obtained using a miniature-probe attachment for an x-ray emission spectrometer; the montmorillonite clay was initially Ca-saturated. The ionic concentrations were measured in either 0.5- or 1.0-mm intervals from the origin of the diffusing salt giving detailed, experimental distributions of the diffusing ions. The method is sensitive to slight changes of iron concentration resulting in precise determination of experimental distributions. The method should work equally well for diffusion studies involving other ions with atomic numbers greater than Na.

The diffusion coefficient of Fe³⁺ increased from 1.6 x 10^-7 cm²sec^-1 at a concentration of 0.1 meq Fe³⁺ per g clay to 4.35 x 10^-7 cm²sec^-1 at a concentration of 1.6 meq Fe³⁺ per g clay. Diffusion coefficients of Fe²⁺ were measured at concentrations up to 0.6 meq per g clay. These values were two to five times larger than for Fe³⁺.

¹ Contribution from the Department of Agronomy, Kentucky Agr. Exp. Sta. Lexington, 40506. This paper (no. 69-3-138) is published with the approval of the Director of the Kentucky Agr. Exp. Sta.

² Graduate Student, Professor, and Assistant Professor, respectively. Dept. of Agronomy, University of Kentucky.

Received for publication October 24, 1969. Accepted for publication April 3, 1970.