Thallium Removal from Aqueous Solutions Using L Zeolite: Structural Modifications, Cation Distribution and Water Network Reorganisation

This study investigates potassium-L zeolite (K-L) as an adsorbent for the removal of thallium (Tl+) from aqueous solutions, focusing on the relationship between cation exchange and framework structural response. X-ray powder diffraction (XRPD), thermal analysis, and Rietveld refinements were employed to monitor structural modifications upon Tl+ uptake, combined with batch adsorption experiments to evaluate the removal performance. At low Tl+ uptake, only minor structural perturbations occur, mainly involving slight shifts in extra-framework cation positions and limited rearrangement of channel water molecules. At higher Tl+ concentrations, a measurable anisotropic expansion of the zeolite framework is observed, consistent with partial substitution of K+ by Tl+ and progressive modification of the hydration environment within the pores. Moreover, the crystallographic distribution of Tl+ differs from that of the original K+ cations, suggesting a specific site preference during the uptake process. Batch experiments reveal rapid uptake kinetics, with equilibrium reached within minutes, and high removal efficiency up to 99.5%. The adsorption behaviour is well described by the Langmuir model, with a maximum adsorption capacity of 631 mg g−1. These findings highlight the coupling between ion exchange and structural flexibility in K-L zeolite and support its potential application for efficient thallium removal from contaminated water.