Core-satellite material based metal-doped zeolite LTA for highly efficient iodine sequestration: Adsorption performance and mechanism
Cyrille Ghislain Fotsop, Alexandra Lieb, Franziska Scheffler
Abstract
Efficiently removing radioactive iodine remains a high priority for environmental remediation. The goal of this study was to develop metal-doped zeolite-based adsorbents (Zeo-4A-M, where M = Ag, Zn, or Bi) derived from natural kaolin to remove radioactive iodine (I₂). The metal-doped zeolites were synthesized using an ex-situ ion-exchange-assisted hydrothermal method. They were characterized using XRD, FTIR, FESEM, EDX/EDX mapping, N₂ sorption, solid-state NMR-MAS, and thermogravimetric analysis. SEM and EDX mapping analyses revealed that the metals were primarily concentrated on the zeolite surface and distributed uniformly. The maximum iodine adsorption capacities were 780, 710, 620, and 360 mg/g for Zeo-4A-Ag, Zeo-4A-Zn, Zeo-4A-Bi, and Zeo-4A-Na, respectively. The Langmuir model exhibited a high coefficient of determination (R 2 > 0.998) and small chi-squared test values. Analysis of the isotherms revealed that the Langmuir model was more accurate, indicating that adsorption occurred on a monolayer with progressive coverage and homogeneity of the adsorbent surface. The Freundlich isotherm indicated the presence of chemisorption. Kinetic studies revealed that the pseudo-second-order kinetic model accurately describes the adsorption of iodide ions onto Zeo-4A-M. Additionally, the rate-limiting step in the adsorption reaction of I₂ onto zeolites is a chemical reaction controlled by membrane diffusion. The influence of ionic strength on metal-doped zeolites revealed that the large sizes of the I − and Br − ions block some of the open active sites on the zeolite surface. XRD analysis after adsorption showed that the samples retained their structures. Doped zeolites proved to be better adsorbents for purifying radioactive elements in an aqueous solution or vapor after six adsorption cycles due to the presence of electron-rich active sites. This study of the interaction between metal ions and zeolites is a promising strategy for developing adsorbents for wastewater treatment. • The ex situ ion exchange-assisted hydrothermal method was found to be suitable for synthesizing metal zeolites. • The doped zeolites revealed that the metals were mostly found on the zeolite's surface. • The adsorption occurred on a monolayer with progressive coverage and homogeneity of the adsorbent surface. • The rate-limiting step during the adsorption reaction of I 2 was a chemical reaction controllable by membrane diffusion.