The removal of thallium (Tl⁺), a highly toxic heavy metal, from aqueous systems is an important environmental priority. In this work, we evaluate the efficiency and structural response of potassium-form L zeolite (K-L) during selective Tl⁺ uptake, combining batch adsorption experiments, X-ray powder diffraction (XRPD), Rietveld refinements, thermal analysis, and ICP-MS. Batch experiments reveal that K-L exhibits strong affinity for Tl⁺ across a wide pH interval, achieving removal efficiencies up to 99% at ≈500 ppm and maintaining significant exchange capacity even at high concentrations (≈0.5 M). Structural analysis shows a concentration-dependent evolution of the zeolite framework. At low Tl⁺ loading, XRPD patterns indicate only minor structural perturbations, with cell parameters remaining close to the original K-L structure (a = b = 18.381 Å, c = 7.529 Å). Rietveld refinements confirm limited substitution of K⁺ by Tl⁺ and minimal changes in T–O distances and angles. Extraframework K⁺ retains its original coordination, while water molecules undergo slight positional adjustments within the channels but maintain a comparable hydration level (~28 H₂O per unit cell). Overall, the channel topology remains largely preserved. In contrast, high Tl⁺ loading induces a marked structural reorganization, reflected in an anisotropic lattice expansion (a = b = 18.443 Å, c = 7.549 Å), mainly in the a/b direction. Refinement data show extensive replacement of K⁺ by Tl⁺, accompanied by a reduction in hydration (~21 H₂O per unit cell). Water molecules redistribute within the channels, forming new hydration pockets and leaving partially dehydrated regions. The larger ionic radius and higher polarizability of Tl⁺ promote its preferential coordination with framework oxygens rather than water, highlighting its strong structure-modifying behavior. Thermal analysis supports this interpretation: samples with low Tl⁺ content display moderate weight losses and well-defined endothermic peaks associated with water removal from K⁺ sites. Conversely, high-loading samples show reduced overall water release, broader thermal features, and higher-temperature dehydration events, consistent with water molecules more tightly bound within rearranged Tl-centered hydration environments. Pore aperture evaluation demonstrates that Tl⁺ uptake systematically enlarges the 12-membered ring channels while preserving the overall framework stability. Overall, the results show that Tl⁺ exchange in K-L zeolite proceeds through selective cation incorporation, framework adjustment, and progressive reorganization of water sites. The zeolite maintains structural integrity while accommodating significant Tl⁺ amounts, confirming its strong potential for environmental remediation and heavy-metal sequestration.
Thallium Removal from Aqueous Solutions Using K-L Zeolite: Structural Adaptation and Water Network Reorganization
Luca Adami
;Maura Mancinelli;Francesco Di Benedetto;Renzo Tassinari;Matteo Alberghini;Giacomo Ferretti;Annalisa Martucci
2025
Abstract
The removal of thallium (Tl⁺), a highly toxic heavy metal, from aqueous systems is an important environmental priority. In this work, we evaluate the efficiency and structural response of potassium-form L zeolite (K-L) during selective Tl⁺ uptake, combining batch adsorption experiments, X-ray powder diffraction (XRPD), Rietveld refinements, thermal analysis, and ICP-MS. Batch experiments reveal that K-L exhibits strong affinity for Tl⁺ across a wide pH interval, achieving removal efficiencies up to 99% at ≈500 ppm and maintaining significant exchange capacity even at high concentrations (≈0.5 M). Structural analysis shows a concentration-dependent evolution of the zeolite framework. At low Tl⁺ loading, XRPD patterns indicate only minor structural perturbations, with cell parameters remaining close to the original K-L structure (a = b = 18.381 Å, c = 7.529 Å). Rietveld refinements confirm limited substitution of K⁺ by Tl⁺ and minimal changes in T–O distances and angles. Extraframework K⁺ retains its original coordination, while water molecules undergo slight positional adjustments within the channels but maintain a comparable hydration level (~28 H₂O per unit cell). Overall, the channel topology remains largely preserved. In contrast, high Tl⁺ loading induces a marked structural reorganization, reflected in an anisotropic lattice expansion (a = b = 18.443 Å, c = 7.549 Å), mainly in the a/b direction. Refinement data show extensive replacement of K⁺ by Tl⁺, accompanied by a reduction in hydration (~21 H₂O per unit cell). Water molecules redistribute within the channels, forming new hydration pockets and leaving partially dehydrated regions. The larger ionic radius and higher polarizability of Tl⁺ promote its preferential coordination with framework oxygens rather than water, highlighting its strong structure-modifying behavior. Thermal analysis supports this interpretation: samples with low Tl⁺ content display moderate weight losses and well-defined endothermic peaks associated with water removal from K⁺ sites. Conversely, high-loading samples show reduced overall water release, broader thermal features, and higher-temperature dehydration events, consistent with water molecules more tightly bound within rearranged Tl-centered hydration environments. Pore aperture evaluation demonstrates that Tl⁺ uptake systematically enlarges the 12-membered ring channels while preserving the overall framework stability. Overall, the results show that Tl⁺ exchange in K-L zeolite proceeds through selective cation incorporation, framework adjustment, and progressive reorganization of water sites. The zeolite maintains structural integrity while accommodating significant Tl⁺ amounts, confirming its strong potential for environmental remediation and heavy-metal sequestration.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
