Effects of cations on rare earth adsorption and desorption in binding sites of montmorillonite

Abstract

The exchangeability of rare earth (RE) in weathered crust elution-deposited rare earth ores largely depends on its interaction with clay minerals, which may be significantly influenced by various cations. Therefore, the effects of K<sup>+</sup>, Ca<sup>2+</sup> and Al<sup>3+</sup> on RE<sup>3+</sup> adsorption and desorption in binding sites of montmorillonite (MMT) were investigated. Through the pre-saturation, the interlayer ions of MMT had been replaced by K<sup>+</sup>, Ca<sup>2+</sup> or Al<sup>3+</sup>. RE<sup>3+</sup> can adsorb on the interlayer sites of Ca-MMT and K-MMT, but nearly not Al-MMT. The basal spacing of Ca-MMT is larger than K-MMT, which provides a smaller hinder effect of interlayer collapse for the interlayer diffusion of RE<sup>3+</sup>. The adsorption capacity followed the order: Ca-MMT＞K-MMT＞Al-MMT and La<sup>3+</sup>＞Y<sup>3+</sup>＞Eu<sup>3+</sup>. It can predict that the grade of the exchangeable RE in ores abundant in Ca<sup>2+</sup> is the most, followed by the ore rich in K<sup>+</sup> and Al<sup>3+</sup> the least. Clay minerals tend to adsorb light RE and hard to adsorb middle and heavy RE. The reversibility of RE adsorbed in interlayers, especially in collapsed interlayers, is far worse than that on externals. The desorption rates of RE were in the order of RE-Al-MMT＞RE-K-MMT＞RE-Ca-MMT and Eu<sup>3+</sup>＞Y<sup>3+</sup>＞La<sup>3+</sup>. For the desorption of interlayer RE<sup>3+</sup>, NH<sup>4+</sup> is better than Mg<sup>2+</sup> because the larger change of the basal spacings (Δd) provides more minor activation energy barriers (ΔE) for NH<sup>4+</sup> diffusion within interlayers. It can enrich the metallogeny theory of weathered crust elution-deposited rare earth ores and provide a certain theoretical basis for its efficient exploitation.