SREL Reprint #3287

Model of radioiodine speciation and partitioning in organic-rich and organic-poor soils from the Savannah River Site

Hyun-shik Chang1, Chen Xu2, Kathy A. Schwehr2, Saijin Zhang2, Daniel I. Kaplan3, John C. Seaman1,
Chris Yeager4, and Peter H. Santschi2

1Savannah River Ecology Laboratory, P.O. Drawer E, Aiken, SC 29802, United States
2Laboratory for Environmental and Oceanographic Research, Department of Marine Sciences,
Texas A&M University, Building 3029, Galveston, TX 77551, United States
3Savannah River National Laboratory, Aiken, SC 29808, United States
4Los Alamos National Laboratory, Los Alamos, NM 87544, United States

Abstract: Radioiodine biogeochemistry was investigated by developing an integrated comprehensive model describing multiple physiochemical and enzyme catalyzed reactions based on detailed iodine speciation data obtained previously from laboratory experiments using organic-rich and organic-poor soils from the Savannah River Site, South Carolina. The model accounted for iodine speciation, inter-conversion kinetics (I-, IO3-, organo-I, and colloidal-I), reversible partitioning to soil organic matter (SOM) and mineral surfaces, irreversible covalent bonding to SOM, and abiotic and biotic (enzymatic / catalyst-type) reactions. Modeling results strongly supported the assertion that iodine–SOM interactions dominate iodine geochemistry; the iodine uptake coefficient for SOM was an order-of-magnitude greater than that for mineral surface. The proposed model simulated well the iodine partitioning among the soil, colloid, and solution phases. The previously proposed process of soil reduction of IO3- to I- was strongly supported through model simulations. The model revealed that during the first 14 days of contact most iodine in soil was comprised of I- or IO3- associated with mineral surfaces and reversibly bound to SOM. After 14 days, the continued uptake of iodine by soil was attributed primarily to the irreversible bonding of organo-I to SOM. Finally, the model was successfully validated using an independent experimental data set. This biogeochemical modeling study underscores the importance of capturing the dynamic nature of iodine speciation transformations and the importance of treating SOM as a sink (irreversible covalent bonding) and a source (colloidal- and organo-iodine mobile species) for subsurface iodine.

Keywords: Iodine, Speciation, Modeling, Soil organic matter, Colloids, Kinetics

SREL Reprint #3287

Chang, H., C. Xu, K. A. Schwehr, S. Zhang, D. I. Kaplan, J. C. Seaman, C. Yeager, and P. H. Santschi. 2014. Model of radioiodine speciation and partitioning in organic-rich and organic-poor soils from the Savannah River Site. Journal of Environmental Chemical Engineering 2(2014): 1321-1330.

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