Comparative Study on the Differential Adsorption Mechanisms of Typical Light/Heavy Rare Earth Ions by Kaolinite and Halloysite

The inevitable toxicity and bioaccumulation of rare earth elements (REEs) have posed potential pollution risks to the environment. In this study, two major clay minerals from weathered ion-adsorption rare earth deposits—tubular halloysite and platy kaolinite—were used as research objects, and a series of batch adsorption experiments were conducted on light rare earth elements (La, Eu) and heavy rare earth elements (Y, Dy) at different concentrations, aiming to clarify the adsorption mechanisms of rare earth ions onto clay minerals. The results showed that under the same conditions, the adsorption capacity of halloysite was higher than that of kaolinite. The unit adsorption capacity of both kaolinite and halloysite for REEs increased with rising pH. The adsorption processes of REEs onto kaolinite and halloysite were better fitted by the pseudo-second-order kinetic model and the Langmuir model, indicating that the adsorption was a homogeneous process dominated by chemisorption, with a fast adsorption rate that was basically completed within the first 5 min. The 1/n values fitted by the Freundlich model were all between 0 and 1, suggesting that the adsorption reaction was favorable. Rare earth ions were adsorbed onto halloysite and kaolinite through outer-sphere complexation (electrostatic attraction) and inner-sphere complexation.