ARTICLE pubs.acs.org/est
Concentration-Dependent Mobility, Retardation, and Speciation of Iodine in Surface Sediment from the Savannah River Site S. Zhang,*,† J. Du,‡ C. Xu,† K. A. Schwehr,† Y.-F. Ho,† H.-P. Li,† K. A. Roberts,§ D. I. Kaplan,§ R. Brinkmeyer,† C. M. Yeager,§ Hyun-shik Chang,|| and P. H. Santschi† †
Department of Marine Science, Texas A&M University, Galveston, Texas 77553, United States State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China § Savannah River National Laboratory, Aiken, South Carolina, United States Savannah River Ecology Laboratory, University of Georgia, Aiken, South Carolina, United States
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‡
bS Supporting Information ABSTRACT: Iodine occurs in multiple oxidation states in aquatic systems in the form of organic and inorganic species. This feature leads to complex biogeochemical cycling of stable iodine and its long-lived isotope, 129I. In this study, we investigated the sorption, transport, and interconversion of iodine species by comparing their mobility in groundwaters at ambient concentrations of iodine species (108 to 107 M) to those at artificially elevated concentrations (78.7 μM), which often are used in laboratory analyses. Results demonstrate that the mobility of iodine species greatly depends on, in addition to the type of species, the iodine concentration used, presumably limited by the number of surface organic carbon binding sites to form covalent bonds. At ambient concentrations, iodide and iodate were significantly retarded (Kd values as high as 49 mL g1), whereas at concentrations of 78.7 μM, iodide traveled along with the water without retardation. Appreciable amounts of iodide during transport were retained in soils due to iodination of organic carbon, specifically retained by aromatic carbon. At high input concentration of iodate (78.7 μM), iodate was found to be reduced to iodide and subsequently followed the transport behavior of iodide. These experiments underscore the importance of studying iodine geochemistry at ambient concentrations and demonstrate the dynamic nature of their speciation during transport conditions.
’ INTRODUCTION Anthropogenic 129I is found in the environment mainly due to releases from fuel reprocessing facilities, with smaller amounts from atmospheric bomb testing (19451970s) and natural production (1 and references therein). Due to its long halflife (1.6 107 yrs), high inventories, and high mobility, 129I accidentally released from fuel reprocessing facilities has migrated into groundwaters. For example, groundwater within several areas of the Savannah River Site (SRS) in South Carolina is contaminated with 129I and other radionuclides.2 The F-Area Seepage Basins plume has the highest concentrations of 129I in the United States, with numerous wells having concentrations 10100 times the drinking water standard (1 pCi/L). The highest concentration reported so far in 2009 is 1060 pCi/L in a well adjacent to the down gradient side of the largest basin.3 For both iodine isotopes, 127I and 129I, iodide (I), iodate (IO3), and organo-iodine are the dominant forms in aquatic environments. In seawater, iodate is the dominant species due to the relatively high concentration of oxygen, whereas iodide is often the main species in freshwater, as well as coastal and estuarine environments (1, 4 and references therein). As a biophilic element, iodine in mammals is almost entirely concentrated r 2011 American Chemical Society
in the thyroid, in the form of triiodothyronine (T3) and thyroxine, for example, 90% of iodine in the human body exists in the thyroid (T4).5 Other forms of iodine in body tissues are associated with proteins, polyphenols, and pigments.6 Seaweeds are known to contain water-soluble and -insoluble iodine, composed of iodide, organic iodine, and minor amounts of iodate.6,7 The distribution of iodine species in soils and sediments depends on environmental chemistry, such as pH, redox, and organic carbon content. Generally, iodine in soils is found to be associated with organic matter, mainly with humic substances. There is a significant body of laboratory and field studies that indicate that iodine can react with natural organic matter (NOM) and become covalently bound, i.e., to the aromatic carbon on phenolic moieties of NOM.812 Iodate and iodide mobility in the subsurface environment has been studied by a number of researchers. Iodate was found to be retarded in the soils to a significantly greater degree than iodide. Received: December 2, 2010 Accepted: April 8, 2011 Revised: April 2, 2011 Published: June 10, 2011 5543
dx.doi.org/10.1021/es1040442 | Environ. Sci. Technol. 2011, 45, 5543–5549
Environmental Science & Technology
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Table 1. Characterization of the Savannah River Site Wetland Sediment, FSI-18a surface area
FSI-18 wetland
m2 g1
pHb
1.89
5.63 ( 0.01
sand
silt
clay
LOI
OC
ON
wt-%
wt-%
wt-%
wt-%
wt-%
wt-%
total I μg/g
127
total 129
I μg/g
92.1 ( 0.1 7.0 ( 0.1 0.9 ( 0.1 10.8 ( 0.7 10.8 ( 0.1 0.5 ( 0.02 6.2 ( 0.3 0.4 ( 0.05
extractable inorganic iodine ng/g 4.9 ( 0.5
sediment a
LOI: loss on ignition, a quick way to estimate organic matter when carbonate is not present. Weigh sample before and after heating to some temperature to burn off organic matter but not minerals. OC: Organic carbon. ON: Organic nitrogen. b pH was measured at a ratio of 2 to artificial freshwater.
In batch experiments, iodide distribution coefficients (Kd, the concentration ratio of I sorbed to the solid phase versus that in the aqueous phase) are relatively low, 10% of total iodine. Additionally, iodine speciation of soil, including the soil before and after introducing the iodine tracer, was determined, especially the extractable inorganic iodine and total iodine. This enables the estimation of a mass balance of total iodine and thus improves our understanding of the interconversion and transport of iodine species in the surface soil, especially that of organo-iodine. The difference between total iodine and extractable inorganic iodine is considered to be the representative of organo-iodine in soil. Soil was subjected to extraction by dispersing 200 mg of air-dried soil in 2 mL of 0.1 M KCl according to the Yamaguchi et al.27 procedure. After that, extractable inorganic iodine was measured by following the iodate procedure as above. The analysis of total iodine in soil is described in Zhang et al.25
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Figure 1. Breakthrough curves of tritium and iodide in Savannah River Site wetland sediment, FSI-18 (relative concentration = concentration in effluent/concentration in influent solution). Iodide was applied to the columns at two concentration levels, 7.87 nM and 78.8 μM. The two solutions were spiked with 125I or 127I. For the loading of high iodide concentration (78.7 μM), iodide concentrations (b) and total 125I activities (2) in the effluents were measured. For the loading of low iodide concentration (7.87 nM), only total 125I activities ( ) in the effluents were measured. Spike solutions (1520 mL) were added and then flushed with artificial groundwater for either 30 or 45 days. The relative standard deviation for determination of stable iodide in effluents using GC/MS was
