Effects of hydrosulfide and pH on iodine release from an alumina matrix solid confining silver iodide

Abstract

ABSTRACTAlumina matrix solidification is a hot isostatic pressing (HIP) technique used to immobilize radioactive iodine (129I) in the form of silver iodide. In the present study, an alumina matrix solidification sample with a porosity of 12.9% was obtained by performing HIP at 175 MPa and 1200°C for 3 hours on a simulated spent silver-sorbent saturated with stable iodine. Material Characterization Centre-1 (MCC-1) leaching tests for the simulated waste form were performed using hydrosulfide (HS-) as a reductant at concentrations ranging from 3 × 10-7 M to 3 × 10-3 M and at pH values ranging from 8.0 to 12.5. Leached iodine concentrations were below the detection limit for ICP-MS measurements at HS- concentrations of 3 × 10-7 M and 3 × 10-5 M. This result was due to the stability of AgI. At an HS- concentration of 3 × 10-3 M, iodine leaching rapidly increased within 10 days. The maximum iodine concentration in the solution was 4.33 × 10-3 M, which corresponds to 85% dissolution of the initial iodine. This value was measured after 552 days under an HS- concentration of 3 × 10-3 M at pH 11. An analysis of specimen cross-sections suggested the following reaction: 2AgI + HS- = Ag2S + 2I- + H+. The pH affected matrix aluminum dissolution but did not significantly affect the iodine leaching behavior. Furthermore, the normalized mass loss of iodine was larger than that of aluminum by a factor greater than 104, which is due to the large porosity and the dissolution of interior AgI of the solid.