Iodine immobilization by silver-impregnated granular activated carbon in cementitious systems

Silver (Ag)-based technologies are amongst the most common approaches to removing radioiodine from aqueous waste streams. As a result, a large worldwide inventory of radioactive AgI waste presently exits, which must be stabilized for final disposition. In this work, the efficacy of silver-impregnated granular activated carbon (Ag-GAC) to remove iodide (I^-), iodate (IO₃^-) and organo-iodine (org-I) from cementitious leachate was examined. In addition, cementitious materials containing I^-, IO₃^-, or org-I loaded Ag-GAC were characterized by iodine K-edge XANES and EXAFS to provide insight into iodine stability and speciation in these waste forms. The Ag-GAC was very effective at removing I^- and org-I, but ineffective at removing IO₃^- from slag-free grout leachate under oxic conditions. I^- or org-I removal was due to the formation of insoluble AgI_(s) or Ag-org-I_(s) on the Ag-GAC. When I^--loaded Ag-GAC material was cured with slag-free and slag grouts, I^- was released from AgI_(s) to form a hydrated I^- species. Conversely, when org-I loaded Ag-GAC material was cured in the two grout formulations, no change was observed in the iodine speciation, indicating the org-I species remained bound to the Ag. Because little IO₃^- was bound to the Ag-GAC, it was not detectable in the grout. Thus, grout formulation and I speciation in the waste stream can significantly influence the effectiveness of the long-term disposal of radioiodine associated with Ag-GAC in grout waste forms.