SREL Reprint #3495

Iodine speciation in a silver-amended cementitious system

Daniel I. Kaplan1, Kimberly A. Price2, Chen Xu3, Dien Li1, Peng Lin3, Wei Xing3, Ralph Nichols1, Kathleen Schwehr3, John C. Seaman2, Toshihiko Ohnuki4, Ning Chen5, and Peter H. Santschi3

1Savannah River National Laboratory, Aiken, SC 29808, United States
2Savannah River Ecology Laboratory, University of Georgia, Aiken, SC 29802, United States
3Department of Marine Science, Texas A & M University at Galveston, Galveston, TX 77553, United States
4Laboratory for Advance Nuclear Energy, Insitute of Innovative Research,
Tokyo Institute of Technology, Tokyo, Japan
5Canadian Light Source, Saskatoon, SK S7N 0X4, Canada

Abstract: 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 study 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 slag-free 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 indicated 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.

Keywords: Iodine speciation; Cement; Slag; Immobilization; Redox; Silver-impregnated zeolite

SREL Reprint #3495

Kaplan, D. I., K. A. Price, C. Xu, D. Li, P. Lin, W. Xing, R. Nichols, K. A. Schwehr, J. C. Seaman, T. Ohnuki, N. Chen, and P. H. Santschi. 2019. Iodine speciation in a silver-amended cementitious system. Environment International 126(2019): 576-584.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).