Aqueous dissolution of silver iodide and associated iodine release under reducing conditions with FeCl₂ Solution

An empirical and analytical study was performed on the aqueous dissolution of silver iodide (AgI) to release iodine under reducing conditions with Fe²⁺ 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 FeCl₂ solutions (10⁻⁶ M to 10⁻³ M) were performed in a glove box purged with a gas mixture (Ar + 5% H₂). The test results showed that AgI dissolves to release iodine at extremely slow rates, being controlled by a diffusion process in any FeCl₂ solution. The comparison with thermodynamic calculations based on redox equilibria suggested that the AgI dissolution proceeds by redox reaction between Ag⁺ and Fe²⁺; 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⁺ and Fe²⁺ to release I⁻, which results in the precipitation of metallic silver as a reduction product of Ag⁺ 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.