Adsorption mechanisms of Eu3+ onto high-charge micas and smectites: An isothermal, kinetic and optical study

The adsorption behavior of trivalent europium cations (Eu 3+ ) onto 2:1 trioctahedral clay minerals, including high-charge micas, was investigated to determine the mechanisms governing their retention in aqueous media. Eu 3+ adsorption onto two additional smectites -a natural montmorillonite and a synthetic saponite- was also evaluated. Eu 3+ incorporation occurs predominantly through interlayer cation exchange, producing a significant expansion of the basal spacing in montmorillonite and Na-2-mica, due to the strong hydration energy of the trivalent ion. In contrast, the highly attractive forces between the layers in Na-4-mica prevented interlayer expansion. Trioctahedral saponite also exhibited a secondary adsorption process, complexation of Eu 3+ with the edge hydroxyl groups, consistent with its high surface area. Equilibrium in Eu 3+ –clay systems was reached within 1 h for both di- and trioctahedral clay minerals, and adsorption is well described by a pseudo-second-order kinetic model, indicating chemisorption as the rate-limiting step. At higher concentrations (> 180 mg·L −1 ) and longer contact times, a desorption stage was observed in montmorillonite, associated with cation redistribution toward lower-energy sites or Eu 3+ complexation. Adsorption isotherms for all Eu 3+ –clay systems were adequately fitted by the Langmuir model, confirming the adsorption dominance of equally-energy sites. The maximum adsorption capacities ( q m ) followed the order Na-4-mica > Na-2-mica > saponite > montmorillonite, reflecting their abundance of accessible negatively charged sites. Photoluminescence spectroscopy confirms that Eu 3+ occupies well-defined local environments, adopting a predominant configuration of outer-sphere complexes in the dioctahedral clay mineral, and of inner-sphere complexes in the high-charge micas. Correspondingly, Eu 3+ lifetimes followed the decreasing order τ saponite > τ Na-2-mica > τ Na-4-mica > τ montmorillonite at maximum adsorption capacities. These findings demonstrated that the faster and strongly bonding adsorption of Eu 3+ onto 2:1 clay minerals provides a promising approach for the retention and recovery of trivalent lanthanides from aqueous rare earth elements (REE) sources, without requiring prior pretreatment. The presented insights exhibit the high potential of dioctahedral and trioctahedral clay minerals, particularly Na- n -micas, as effective materials for REE recovery from secondary resources.