Swelling of lanthanide (La, Nd, Gd) montmorillonites heated in air and under vacuum

Abstract

A sodium-montmorilion ite from Almeria (Spain), with the structural formula (Si7.644+ Al0.363+) (Al3.093+ Fe0.283+ Mg0.6092+) O20 (OH)4 has been used as starting material for the preparation of samples saturated with lanthanum, neodymium and gadolinium. By application of the Greene-Kelly Li+- test the inability of the sample to re-expand with glycerol, has been observed, in agreement with the existence of a predominant charge in the octahedral sheet. The effect of heating at 300° C, in air and under vacuum, on the swelling capacity of the lanthanide montmorillonites has been studied. Prolonged re-expansion in water after thermal treatment in air is necessary to reproduce the initial basal spacing of the samples. From Fourier transform infrared spectra it is concluded that the interlamellar hydrated lanthanide cation concentration decreases on heating, which agrees with the decrease in the cation exchange capacity claimed in the literature. The mechanism of exchangeable ion migration into octahedral vacant sites with thermal treatments, advocated in the case of lithium-saturated montmorillonite, does not seem to occur in the case of lanthanide-saturated samples. The partial hydrolysis of the interlamellar hydrated cations to polyhydroxycations is proposed as the only mechanism for explaining the loss or exchange able 4f ions.