Iodine speciation in a silver-amended cementitious system
Silver-impregnated zeolite (AgIZ) has been used for removing radioiodine from contaminated groundwater and nuclear waste streams and the worldwide inventory of such secondary waste is rapidly increasing. The objective of this work was to 1) quantify the effectiveness of two grout waste forms for disposing of the used AgIZ, and 2) determine the I speciation leached from AgIZ encapsulated in grout. A 60-day kinetics batch experiment demonstrated that AgIZ encapsulated in slag-free grout was extremely effective at immobilizing I and Ag, a potential non-radioactive carcinogen. However, AgIZ encapsulated in slag-containing grout, the most common type of grout used for low-level radioactive waste disposal, was entirely ineffective at immobilizing I. While the slagfree grout with AgIZ released only 3.3 μg/L Itotal into the contact solution, the slag-containing grout released 19,269 μg/L Itotal. Based on thermodynamic calculations, the strongly reducing conditions of the slag-containing system (Eh was –392 mV) promoted the reductive dissolution of the AgI, forming Ag0(aq) and releasing iodide (I–) into the aqueous phase. The slag-free grout system was maintained under more oxidizing conditions (Eh was 439 mV) and a minimal amount of I was released from the grout. In both grout systems, the aqueous I, originally added to the AgZ as iodide, was composed primarily of iodide and org-I, and essentially no iodate was detected. More organo-I was detected in the slag-free than the slag-containing grout system because the high redox potential of the former system was more conducive to the formation of oxidized I species, such as I2, which may be intermediates in the covalent bonding of I with organic C in grout. Iodine K-edge XANES analysis suggested that I existed exclusively as silver iodide in both AgIZ-grout samples. Together, these results indicate that subsurface grout disposal of AgIZ waste should be done under oxidizing conditions and that radioiodide released from AgIZ can undergo speciation transformations that have important implications on subsequent mobility and estimated risk.
- Research Organization:
- Univ. of Georgia, Athens, GA (United States); Texas A&M Univ., Galveston, TX (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); USDOE Office of Environmental Management (EM)
- Grant/Contract Number:
- LDRD-2017-00005; EM0004381; FC09-07-SR22506; FC09-07SR22506
- OSTI ID:
- 1547652
- Alternate ID(s):
- OSTI ID: 1547365
- Journal Information:
- Environment International, Journal Name: Environment International Vol. 126 Journal Issue: C; ISSN 0160-4120
- Publisher:
- ElsevierCopyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
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