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Iron Adsorption by Montmorillonite Systems: II. Determination of Adsorbed Iron¹

ABSTRACT

Comparison of methods for determination of Fe adsorption capacity of montmorillonite and a qualitative method for differentiation of adsorbed and precipitated Fe are presented.

Separate H-montmorillonite samples (Wyoming Clay Spur, < 1_µ diameter) were exchange-saturated by washing with FeCl₃, Fe₂(SO₄)₃ and Fe(NO₃)₃ solutions. Excess ions were removed with either distilled H₂O, absolute methanol, 95% ethanol, or 0.003M HCl. Fe retained by the systems after washing was removed with acidified 1N NH₄OAc.

Fe(III) removed after 0.003M HCl washing was essentially the same in all systems (between 1.17 and 1.25 me. per g.) but was much higher (up to 2.2 me. per g.) and quite variable from H₂O or alcohol-washed samples. The 0.003M HCl washing was found to remove negligible amounts of adsorbed iron. Base titration curves of all the samples were essentially the same, indicating that each system retained the same amounts of adsorbed Fe.

The data indicate that excess Fe was precipitated in the systems where H₂O and alcohols were employed for removal of the excess ions. Data are presented which show that adsorbed Fe may be qualitatively differentiated from precipitated Fe in the systems studied by using NH₄F solution as an extracting reagent.

¹ Contribution from the Department of Soils and Plant Nutrition, University of California, Davis. Presented before Div. II, Soil Science Society of America, Dec. 5, 1960, at Chicago, Ill.

² Research Assistant and Assistant Professor of Soil Science, respectively. Former now Assistant Professor, Department of Soils and Plant Nutrition, University of California, Riverside.

Received for publication August 8, 1960. Accepted for publication March 21, 1961.