Environ. Sci. Technol. 2009, 43, 7258–7264
Organo-Iodine Formation in Soils and Aquifer Sediments at Ambient Concentrations K . A . S C H W E H R , † P . H . S A N T S C H I , * ,† D.I. KAPLAN,‡ C.M. YEAGER,‡ AND R. BRINKMEYER† Laboratory for Environmental and Oceanographic Research, Department of Marine Sciences, Texas A&M University, 5007 Avenue U, Galveston, Texas 77551, and Savannah River National Laboratory, Aiken, SC 29803
Received March 16, 2009. Revised manuscript received May 19, 2009. Accepted May 26, 2009.
One of the key risk drivers at radioactive waste disposal facilities is radioiodine, especially 129I. As iodine mobility varies greatlywithiodinespeciation,experimentswith 129I-contaminated aquifer sediments from the Savannah River Site located in Aiken, SC, were carried out to test iodine interactions with soils and aquifer sediments. Using tracer 125I- and stable 127I- additions, it was shown that such interactions were highly dependent on I- concentrations added to sediment suspensions, contact time with the sediment, and organic carbon (OC) content, resulting in an empirical particle-water partition coefficient (Kd) that was an inverse power function of the added Iconcentration. However, Kd values of organically bound 127I were 3 orders of magnitude higher than those determined after 1-2 weeks of tracer equilibration, approaching those of OC. Under ambient conditions, organo-iodine (OI) was a major fraction (67%) of the total iodine in the dissolved phase and by implication of the particulate phase. As the total concentration of amended I- increased, the fraction of detectable dissolved OI decreased. This trend, attributed to OC becoming the limiting factor in the aquifer sediment, explains why at elevated Iconcentrations OI is often not detected.
Introduction Iodine is a biophilic element, with several short-lived isotopes (e.g., 131I, t1/2 ) 8 d), one long-lived isotope, 129I (t1/2 ) 15.6 million years), and one stable isotope, 127I. The inventory of 129I in surface environments has been overwhelmed by anthropogenic releases over the past 50 years. Radioiodine isotopes account for the largest fraction of short-term and long-term doses from accidental releases and fallout from atomic bomb tests. 129I and 99Tc are the two long-lived nuclides with highest mobility in stored radioactive waste. 129 I is also among the top three risk drivers at the proposed waste disposal site at Yucca Mountain (1) and is the lead risk driver at the low activity waste disposal facilities at the Savannah River Site (2) and the Hanford Site (3). Its large risk is due in part to its perceived high mobility in the environment, toxicity (via accumulation in the thyroid gland), high inventory, and long half-life. Releases from nuclear waste disposal sites occur through groundwater aquifers, where * Corresponding author e-mail:
[email protected]. † Texas A&M University. ‡ Savannah River National Laboratory. 7258
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migration depends on the species tendency to be retarded due to sorption and other reactions to aquifer materials. Iodine sorption to sediments varies greatly with iodine speciation. While iodate, IO3-, and certain organo-iodine (OI) species sorb relatively strongly, I- does not. As determined in laboratory experiments, I- distribution coefficients (Kd values, the ratio of the concentrations of I- sorbed onto sediments to I- in the aqueous phase) are relatively low, e1 cm3 g-1, whereas Kd values of IO3- and OI are on the order of 103 cm3 g-1, depending on sediment type and microbial biomass richness in the sediment (4, 5). Thus, microorganisms can strongly influence iodine sorption by regulating iodine speciation, including OI formation. With the exception of a few studies, most groundwater 129 I field studies deal with atmospheric fallout after atomic bomb tests, the Chernobyl accident, and releases from nuclear reprocessing facilities in Europe (6-8). The behavior of 129I was reported for surface waters (9, 10), groundwater aquifers (11-14), and in recharge zones by infiltrating river water (15, 16). In previous studies on iodine mobility research, 127 I and 129I have been described as being mobile in groundwater (9, 15, 16). It was found that iodine isotopes appear to be retarded less in the few aquifers that had been studied in more arid parts of the United States (16) than those that had been studied in wetter parts of Europe (15, 17). Very little is known about how the chemical speciation of iodine in the environment or how biologically mediated processes that form OI affect the overall sorption of iodine and migration in groundwater. A number of laboratory studies have clearly demonstrated the incorporation of iodine into naturally occurring high molecular weight (HMW) organic matter (e.g., humic acids). For example, humic substances in soils and surface waters can react with I2 (18) or with I(17). The possibility that iodine binds covalently to aromatic moieties in natural humic substances was also demonstrated by using elevated iodine concentrations and XANES/EXAFS analysis (9, 18, 19) and electrospray mass spectrometry (20). In field studies, I binding to aromatic substances has so far not been demonstrated, even though it had been proposed that binding of iodine by HMW natural organic matter (NOM) may be promoted by microbial enzymatic activity (21). In a few cases, it was shown that groundwater 127I existed in part as OI (14, 22). It is also known that OI species can form in surface waters (17, 23, 24), soils and sediments (25, 26), and biological systems (26). Most of the analytical methods employed were, however, semiquantitative or operational; thus, the results from field data do not present a consistent picture of iodine behavior, likely due to the different experimental conditions that were employed. The objective of our study was, therefore, to quantify iodine speciation and sorption to sediments in order to investigate the formation of OI as a function of experimental conditions, e.g., concentrations of I- and OC.
Materials and Methods Soil and Aquifer Sediment Samples. A surface soil (referred to as the “surface sediment”) and a subsurface sediment were collected with a 7.6 cm diameter auger from a riparian zone located in F-Area on the Savannah River Site, Aiken, SC. F-Area was formally used as a nuclear fuel reprocessing facility for the production of radionuclides, including plutonium and uranium. It was selected for study because portions of the groundwater aquifer are contaminated with 129 I, a neutron-induced fission byproduct of fuel reprocessing. The sediment samples were collected from an uncontaminated site that borders Four Mile Branch, upstream of the 10.1021/es900795k CCC: $40.75
2009 American Chemical Society
Published on Web 06/11/2009
contaminant plume. The surface sample was collected from an area that is occasionally submerged under water. Leaf litter was removed prior to collecting the upper 60 cm core, which was dark brownish-black, characteristic of high organic matter sediments. The subsurface sediment was collected from a zone that is continuously saturated with water. It came from a depth interval of 90-120 cm. It was white and contrasted sharply with the color of the surface sediment. The samples were quickly separated in the field and stored in zip lock bags on ice until they were transferred to a 4 °C refrigerator. All characterization and subsequent resuspension tests were conducted with the sediment in the moist state. Moist samples were used in an effort to minimize experimental artifacts introduced by drying sediments such as changing hydrophobicity or hydrophilicity of organic matter (27) that can severely impact availability and nature of surface reactive sites on organic matter and possibly clays. Such potential changes in the nature and reactivity may also have a profound effect on the interaction of substrate with microbes that are likely to have an important role in the speciation and mobility of iodine. The sediments were characterized for pH in a 1:1 water-sediment slurry, particle size distribution by the sieve and hydrometer method, and total free Fe concentration by the dithionite-citrate buffer method (28). Mineralogy was determined by X-ray diffraction analyses of the
