Aqueous Dissolution of Silver Iodide and Associated Iodine Release under Reducing Conditions with Sulfide
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0985-NN12-12
Aqueous Dissolution of Silver Iodide and Associated Iodine Release under Reducing Conditions with Sulfide Yaohiro Inagaki1, Toshitaka Imamura1, Kazuya Idemitsu1, Tatsumi Arima1, Osamu Kato2, Hidekazu Asano3, and Tsutomu Nishimura3 1 Kyushu University, Fukuoka, 819-0395, Japan 2 Kobe Steel Inc., Kobe, 657-0845, Japan 3 RWMC, Tokyo, 105-0001, Japan
ABSTRACT Aqueous dissolution tests of silver iodide (AgI) were performed in Na2S solutions in order to evaluate, empirically, dissolution of AgI to release iodine under reducing conditions with sulfide. The results indicated that AgI dissolves to release iodine being controlled by mainly precipitation of Ag2S. However, the dissolution of AgI can be depressed to proceed, and the thermodynamic equilibrium cannot be attained easily. Solid phase analysis for the reacted AgI suggested that a thin layer of solid silver forming at AgI surface may evolve to be protective against transportation of reactant species, which can lead to the depression in the dissolution of AgI. INTRODUCTION During reprocessing of spent nuclear fuels in Japan, radioactive iodine (129I, T1/2=1.6x107 y) is collected as a form of silver iodide (AgI) in adsorbents to be one of the major reprocessing wastes [1]. The form of AgI will be confined into more durable forms, such as crystallized Al2O3 and SiO2, for geological disposal. Evaluation of the long-term iodine release from the waste forms is essential for safety assessment of the disposal because of its long half-life and weak adsorption on geological media [2,3]. Several techniques have been applied to fabrication of the potential waste forms to study their performance [1,4]. However, only a few studies have concerned the fundamental physical and/or chemical behavior of AgI dissolution itself. As commonly known, AgI is one of the most stable iodides with a low solubility products of -16 10 [4-6], which can result in a limited iodine release under oxidizing conditions. Under reducing conditions expected for the potential geological disposal, however, redox potential calculations predict that Ag+ can be reduced to solid silver (Ag(s)) as follows, AgI + e- = Ag(s) + I-, which can cause a significant increase in the iodine release. On the other hand, the recent studies pointed out that the actual dissolution of AgI under reducing conditions with iron bearing minerals is much less than that predicted by the calculations [7]. The disagreement between them suggests that the valid prediction of iodine release requires a sound understanding of the dissolution mechanism under the disposal conditions where redox conditions may be controlled by redox couples such as Fe(II)/Fe(III) and S(-II)/S(VI). However, there have been only a few empirical and/or analytical studies on the actual dissolution of AgI under reducing conditions. In the present study, aqueous dissolution tests of AgI were performed in Na2S solutions in order to evaluate, empirically, dissolution of AgI to release iodine under reducing conditions with sulfide. On the basis of the results
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