Zeolite-Based Removal of Thallium from Contaminated Waters: Structural Insights from XRPD and XAS

Thallium (Tl) is a highly toxic element increasingly recognized as a relevant contaminant in polluted water systems. This study investigates the adsorption and structural incorporation of Tl in zeolites to evaluate their potential for water remediation under both laboratory and environmentally relevant conditions. Particular attention was devoted to the crystallographic localization of Tl in K-LTL zeolite and to the influence of concentration and competing ions on the adsorption mechanism. A comparative experimental approach was adopted using synthetic solutions with decreasing Tl concentrations and real contaminated waters containing ~17 ppb Tl and complex ionic matrices. Batch adsorption experiments coupled with ICP–MS analyses confirmed efficient Tl removal even at trace levels (>90%). XRPD combined with Rietveld refinement on Tl-saturated samples showed that Tl(I) is distributed over several cationic sites in the LTL framework, inducing a reorganization of the hydration network and behaving as a structure-breaking ion. At low Tl concentrations, however, the limited sensitivity of diffraction techniques prevented a reliable localization of the adsorbed species. To overcome this limitation, X-ray Absorption Spectroscopy (XAS) was employed. XANES spectra showed that Tl is systematically present as Tl(I), independently of concentration and water composition. Similar spectral features observed for concentrated synthetic systems and real contaminated waters indicate that Tl occupies the same preferential adsorption sites even at trace levels and in the presence of competing cations. EXAFS analysis further confirmed persistent Tl–framework interactions in all samples, with Tl–Si/Al second-shell contributions detectable even at ppb concentrations. The results demonstrate that the adsorption mechanism identified under saturation conditions remains valid under environmentally relevant conditions, supporting zeolites as effective materials for Tl remediation in natural waters.