Adsorption behaviors and mechanisms of lanthanum ions and exchanger cations at halloysite-solution interface: Perspectives from electrical double layer model and spectral analyses

Recovering rare earth elements from dilute aqueous solutions via adsorption methods using clay minerals as adsorbents is an economical strategy, but the recovery efficiencies still need to be improved. Designing more efficient rare earth recovery routes would be valuable, and it is necessary to clarify the adsorption behaviors and mechanisms of rare earth ions and exchanger cations on clay minerals in advance. This study investigated equilibrium adsorption behaviors of La3+ and exchanger cations (Ca2+, Mg2+, and K+) on halloysite. Then, based on the electrical double layer model, the links between the electrokinetic potential of halloysite and the adsorption behaviors of cations on halloysite were analyzed. Finally, by integrating XRD, FT-IR, and XPS data, the study ascertained the adsorption mechanisms of the four cations. The results show that the adsorption behaviors of the four cations on halloysite are classified as monolayer adsorption consistent with the Langmuir model. The analysis based on the electrical double layer model suggests that the four cations are preferentially adsorbed in the Stern layer, followed by a gradual shift to be adsorbed in the diffusion layer. The spectral analyses further demonstrate that the adsorption behaviors of the four cations are synergistically controlled by ion exchange and electrostatic attractions. Notably, La3+, Ca2+, and Mg2+ are predominantly adsorbed in the form of hydrated ionic species as outer-sphere complexes, and K+ dominates the inner-sphere complex. This study reveals the adsorption mechanisms of rare earth ions and several typical exchanger cations at the halloysitesolution interface, which is instructive for developing adsorption strategies and selecting desorption schemes, thereby improving the recovery efficiencies of rare earth elements.