Aqueous Dissolution of Silver Iodide and Associated Iodine Release under Reducing Conditions with FeCl2 Solution

Abstract

An empirical and analytical study was performed on the aqueous dissolution of silver iodide (AgI) to release iodine under reducing conditions with Fe 2þ in order to understand the fundamental chemical and/ or physical behavior of potential radioactive iodine waste forms under geological disposal conditions. Aqueous dissolution tests of AgI powder in FeCl 2 solutions (10 À6 M to 10 À3 M) were performed in a glove box purged with a gas mixture (Ar + 5% H 2 ). The test results showed that AgI dissolves to release iodine at extremely slow rates, being controlled by a diffusion process in any FeCl 2 solution. The comparison with thermodynamic calculations based on redox equilibria suggested that the AgI dissolution proceeds by redox reaction between Ag þ and Fe 2þ ; however, it was far from the thermodynamic equilibrium. These results suggested that the form of AgI itself has a potential to immobilize iodine for a long time even under the disposal conditions. Solid-phase analysis for the reacted AgI by using SEM/EDS showed a certain amount of silver (maybe metallic silver) precipitated at the surface. On the basis of these results and discussion, a potential mechanism for the actual AgI dissolution was proposed as follows. The AgI dissolution proceeds by redox reaction between Ag þ and Fe 2þ to release I À , which results in the precipitation of metallic silver as a reduction product of Ag þ at the AgI surface to form a thin layer covering the AgI surface. The silver layer evolves to be protective against the transport of reactant species, by which the further dissolution to reach the equilibrium is suppressed. Consequently, the dissolution proceeds at extremely slow rates, being controlled by a diffusion process.

Porous sorbents for the capture of radioactive iodine compounds: a review

“…Later, numerous studies were carried out on this topic, especially during the 1970s and 1980s 21,29,34,38,40,47,52,180 and then between 2000 and 2017. 5,8,71,[74][75][76][77][78][79][80][169][170][171][173][174][175][176][177][184][185][186][187][188] These studies focused on the determination of the inuence of various parameters such as the temperature ( C), silver content (wt%), Si/Al ratio, supercial velocity, and pretreatment of the material on the adsorption capacities of silver-exchanged zeolites (ESI, Table 3 †). Other works focused on issues such as aging of the material under different atmospheres (NO x , humidity).…”

“…Studies for a long time mainly focused on zeolites of the FAU and MOR structural types. 5,8,29,31,34,38,40,47,52,71,[169][170][171]173,175,177,178,182,184,185,187,188 The interest in these two types of zeolites can be explained by their large pore openings, which allow easier diffusion of iodine (I 2 , CH 3 I) and non-iodine (NO x , CO x , H 2 O, etc.) compounds inside the zeolites.…”