Iodine release from silver iodide under reducing condition with iron-bearing minerals

Iodine release from silver iodide (AgI) to water was evaluated under reducing conditions in the presence of iron-bearing minerals, goethite (FeOOH), magnetite (Fe₃O₄) and Wüstite (FeO). The release tests were performed in a glove box purged with gas mixture (Ar+5%H₂), and the concentrations of dissolved iodine, silver and iron were measured. The apparent equilibrium concentration of dissolved iodine was 4.0 × 10^-8, 4.3×10^-8 mol/l for the tests with FeOOH and Fe₃O₄, respectively. The values were almost the same as in the test without minerals. For the test with FeO, the concentration of dissolved iodine was 5.4×10^-3 mol/l, which is five orders of magnitude higher than for the other tests. Solid phase analyses by using XRD and SEM/EDS indicated that metallic silver precipitated on the surface of the original AgI for the test with FeO and on the surface of Fe₃O₄ for the test with Fe₃O₄, but not for that with FeOOH. These results showed that only FeO could reduce AgI effectively to increase iodine release. The amount of dissolved Fe(II) was a key factor affecting AgI reduction instead of redox potential of solution. Kinetic processes may also play an important role in AgI reduction and associated iodine release under reducing condition.