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[en] Highlights: • Ag-granular activated carbon (Ag-GAC) removed I− and organic I, but not IO3−. • I− was immobilized in AgI, but organic I remained unchanged, binding with Ag. • AgI in Ag-GAC was dissolved as it was mixed/cured with slag-free and slag grouts. • Organic I in Ag-GAC remained unchanged as it was mixed/cured with both grouts. - Abstract: 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.