Comparison of silver zeolites of natural and synthetic origins on their ability to trap irreversibly CH3I in conditions representative of severe nuclear accident

In the present work, a low-cost silver natural zeolite (AgN) with clinoptilolite structure and ≈ 9 wt% Ag was specifically designed and evaluated for the capture of methyl iodide (CH₃I) in a range of conditions representative of severe nuclear accident (T = 80-140°C, high steam contents, presence of CO, γ-irradiation….). The physicochemical and textural characteristics of AgN were first characterized by SEM-EDX, DR-UV-Visible spectroscopy, N₂ porosimetry, elemental analysis and compared to those of well-known synthetic silver faujasite zeolites (AgX and AgY). Common features and differences existing between AgY and AgN in terms of CH₃I trapping mechanism were addressed by FTIR spectroscopy and DRIFTS. CH₃I adsorption capacities were also computed from CH₃I breakthrough curves measured in absence/presence of pre-adsorbed water. Interestingly, the CH₃I trapping efficiencies measured at 140°C were found to strongly depend on the type of silver zeolite and steam content. Up to a water content of 42 wt% in the feed, both AgN sorbent and synthetic exchanged faujasites AgX and AgY performed very well, with decontamination factors in the range 5.10⁵-10³. At higher humidity (45 wt%), the filling of pores by liquid water induced a rather drastic drop in measured DF values, which is less pronounced for AgY. Additional challenging tests (presence of CO, trapping stability under γ-irradiation) have confirmed the potential of tailored silver zeolites for effective CH₃I retention in severe accident scenarios.