ARTICLE pubs.acs.org/est
Sequestration and Remobilization of Radioiodine (129I) by Soil Organic Matter and Possible Consequences of the Remedial Action at Savannah River Site Chen Xu,†,* Eric J Miller,† Saijin Zhang,† Hsiu-Ping Li,† Yi-Fang Ho,† Kathleen A Schwehr,† Daniel I. Kaplan,‡ Shigeyoshi Otosaka,§ Kimberly A Roberts,‡ Robin Brinkmeyer,† Chris M. Yeager,‡ and Peter H. Santschi† †
Laboratory for Environmental and Oceanographic Research, Department of Marine Sciences, Texas A&M University, Building 3029, Galveston, Texas 77551, United States ‡ Savannah River National Laboratory, Aiken, South Carolina 29808, United States § Research Group for Environmental Science, Japan Atomic Energy Agency, Tokai-Mura, Ibaraki 319-1195, Japan
bS Supporting Information ABSTRACT: In order to investigate the distributions and speciation of 129I (and 127I) 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 7277% of the newly introduced I or IO3 were irreversibly sequestered into the organicrich riparian soil, while the rest was transformed by the soil into colloidal and truly dissolved organo-iodine, resulting in 129I remobilization from the soil greatly exceeding the 1 pCi/L drinking water permit. This contradicts the conventional view that only considers I or IO3 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 g5), 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 onoff carrying mechanism. Generally the riparian zone provides as a natural attenuation zone that greatly reduces radioiodine release.
’ INTRODUCTION 129 I has been recognized as a high risk radionuclide in groundwater at most low-level and high-level radionuclide disposal sites, including locations at Savannah River (SRS), Hanford and Idaho.1 This high risk stems mainly from its long half-life (16 M years), relatively high mobility, the large inventory in these sites, and its biophilic properties to highly concentrate in human thyroid. Some of these factors have contributed to 129I having the lowest maximum contaminant level of 1 pCi L1 in groundwater among all radionuclides. The F-Area at the SRS was a radionuclide separation facility for the production of nuclear weapons. During production at F-Area between 1955 and 1988, large amounts of radionuclides were disposed in an acidic aqueous form (pH 1.54) to an unlined, 27 200 m2, seepage basin. Basin closure activities included adding limestone and blast furnace slag to the basin bottom and then covering it with a low permeability engineered barrier system to reduce groundwater infiltration. Since 2000, a funnel-and-gate groundwater remediation effort has been underway that involves r 2011 American Chemical Society
the installation of a low permeability barrier wall and the annual injection of base solution at the “gate”.2 Below the base injection point, the concentrations of several radionuclides and metals have decreased, including 90Sr and 238U. A key factor that determines the mobility of iodine in the environment is its chemical form. In terrestrial system, iodine mainly exists as iodide (I), iodate (IO3) and organo-iodine (OI), of which the interconversion is primarily dependent on a number of factors, for example, redox potential, pH, the presence of organic matter and/or iron and manganese oxide, as well as microbial enzymatic activity.1 For example, along a groundwater flow path between the basins and a wetland area ∼800 m away, a general increase of pH from 3.2 to 6.8 and a steady decrease in Eh was measured within the contaminant plume, which was coupled Received: April 19, 2011 Accepted: October 28, 2011 Revised: October 21, 2011 Published: October 28, 2011 9975
dx.doi.org/10.1021/es201343d | Environ. Sci. Technol. 2011, 45, 9975–9983
Environmental Science & Technology with a dramatic change in iodine speciation, switching from an iodide-dominant pattern near the seepage basin to a pattern that both iodate (20%) and organo-iodine (66%) were present in substantial amount relatively compared to iodide in the downgradient region close to the wetland area (data recalculated from ref 3). The results from this field speciation study clearly did not support the conventional wisdom that dictates iodide should be the dominant species under these thermodynamic conditions. In addition, while both laboratory batch experiment2 and field work2,3 indicated that pH is the primary control in iodine speciation of the groundwater at SRS F-area, one would expect that radioiodine concentration in the groundwater should increase as a result of increasing pH due to recent site baseinjection remediation work, from a thermodynamic viewpoint not considering the presence of organic matter. This is the case in a well closest to the seepage basin where 129I concentration increased by about 4-fold (from 200 to 800 pCi/L), concurrent to an increase of pH by 0.7 in the recent 17 years; however, 129I concentrations did not change too much in the groundwater ∼220 and 375 m downgradient from the recharge area, where pH values generally increased by ∼1 and ∼2 from 1989 to 2010, respectively.2 Thus immobilization and remobilization of iodine species did not seemingly follow a consistent pattern in this region, which adds to the complexity of site remediation action that is attempting to minimize the groundwater flux of multiple contaminants. Elevated 129I/127I ratios (0.0590.118) were recently reported in soil organic matter extracted from a contaminated soil of F-Area,4 which were significantly higher than those found in the nearby groundwater (∼0.03).3,5 129I, incorporated into mobile soil organic matter, can enter the groundwater by infiltration or the wetland area downgradient from it through surface runoff or stormflow,6 and thus becomes a possible carrier and source of radioactive organo-iodine detected in the groundwater and wetland area. Through batch7 and soil column8 experiments, organoiodine formation was observed at ambient iodine input concentrations using soils or sediments collected from the contaminated sites. However, little is known about the chemical properties of this organic iodine carrier, the nature of iodine binding to it, and its relevance to iodine mobility. Up to now, except for the studies mentioned above,7,8 laboratory investigations into the interactions between iodine and soil organic matter (SOM) were mostly carried out at elevated iodine concentrations for experimental convenience or different purposes.915 Moreover, the association of iodine with colloidal organic matter (COM) in aquatic systems was reported in the Mississippi River,16 as well as in the groundwater of the SRS,5 but only a few cases have been described in the soil literature so far, yet without further exploring the nature of this COM.17 Lastly, though recent field and laboratory experiments 15,18 showed iodine speciation and transport at SRS were also partially controlled by some redox pairs (Fe(II)/Fe(III) and Mn(II)/Mn(IV)), which were also present in substantial amounts in the soil mineral phase (goethtite, hematite, etc.) at SRS, the significance of the presence of natural organic matter (NOM) was mostly ignored. However, NOM can act as the substratum and mediator in the process.4,5,7,8 Therefore, the objectives of our study were to (1) investigate the role of SOM in the F-Area soils and sediments in sequestering radioiodine released from the seepage basins, by looking at the partitioning of newly introduced inorganic iodine among the particulate (soil, > 0.45 μm), colloidal (3 kDa0.45 μm) and truly dissolved phases (
