Sequestration and Remobilization of Radioiodine (¹²⁹I) by Soil Organic Matter and Possible Consequences of the Remedial Action at Savannah River Site

Abstract

In order to investigate the distributions and speciation of (129)I (and (127)I) in a contaminated F-Area groundwater plume of the Savannah River Site that cannot be explained by simple transport models, soil resuspension experiments simulating surface runoff or stormflow and erosion events were conducted. Results showed that 72-77% of the newly introduced I(-) or IO(3)(-) were irreversibly sequestered into the organic-rich riparian soil, while the rest was transformed by the soil into colloidal and truly dissolved organo-iodine, resulting in (129)I remobilization from the soil greatly exceeding the 1 pCi/L drinking water permit. This contradicts the conventional view that only considers I(-) or IO(3)(-) as the mobile forms. Laboratory iodination experiments indicate that iodine likely covalently binds to aromatic structures of the soil organic matter (SOM). Under very acidic conditions, abiotic iodination of SOM was predominant, whereas under less acidic conditions (pH ≥5), microbial enzymatically assisted iodination of SOM was predominant. The organic-rich soil in the vadose zone of F-Area thus acts primarily as a "sink," but may also behave as a potentially important vector for mobile radioiodine in an on-off carrying mechanism. Generally the riparian zone provides as a natural attenuation zone that greatly reduces radioiodine release.

Biogeochemical Considerations Related To The Remediation Of I-129 Plumes

“…Iodide added to a subsurface sediment with very low OM concentrations (0.01 wt% OC) did not convert to iodate, but trace levels were converted to organo-iodine (Xu et al, 2011a , whereas at concentrations three orders of magnitude greater, iodide traveled along with the water without retardation. Appreciable amounts of iodide were retained in soils during transport due to iodination of OC.…”

“…As part of the SRS groundwater monitoring program, it has been documented that 129 I concentrates in the riparian zone to extraordinarily high levels, >1000 pCi L -1 (unfiltered samples) . Studies were undertaken to identify the process/mechanism responsible for the 129 I concentrated, and in particular to quantify and understand the role of NOM and microbes (Xu et al, 2011a;Xu et al, 2011b;Xu et al, 2012). The F-Area riparian zone is much more vegetative than would be found at the Hanford Site.…”

“…But an organically rich interface exists where aquifers surface with the biosphere, as would be expected for Hanford aquifers along the Columbia River. Laboratory studies using SRS solid and groundwater materials showed that between 72 and 77% of the newly introduced iodide or iodate were irreversibly sequestered into the organic-rich riparian sediment, while the rest was transformed by the sediment into colloidal and truly dissolved organo-iodine (Xu et al, 2011a). Laboratory iodination experiments indicated that iodine likely covalently bond to aromatic structures of the SOM.…”

“…Iodine bonding with SOM is greatly influenced by not only the functionality of the SOM, but also the hydrophilic/hydrophobic forces in the SOM (Xu et al, 2011a;Xu et al, 2011b). Coincident variations in chemical composition, aromaticity, functional groups (e.g., aliphatic), hydrophobicity, and molecular weight indicated that: 1) iodine in different humic acids was bound to a small-size aromatic subunit (~10 kDa); 2) the large-size subunit (~90 kDa), determined the relative mobility of iodine bound to SOM, 3) iodine incorporation into the SOM is via covalent aromatic C-I bond is the key mechanism controlling iodine behavior in the SRS system.…”

Superoxide Production by a Manganese-Oxidizing Bacterium Facilitates Iodide Oxidation

“…Changes in redox potential control the environmental behavior of iodine, including its mobility and biogeochemical cycling (4)(5)(6)(7)(8). Iodide (I Ϫ ) (oxidation state Ϫ1) is considered the main iodine species that is released from nuclear power reprocessing facilities because of the redox properties of the reprocessing wastewater (8).…”