A Novel Approach for theSimultaneous Determination ofIodide, Iodate and Organo-Iodide for127I and 129I in EnvironmentalSamples Using GasChromatography-MassSpectrometryS . Z H A N G , * , † K . A . S C H W E H R , †

Y . - F . H O , † C . X U , † K . A . R O B E R T S , ‡

D . I . K A P L A N , ‡ R . B R I N K M E Y E R , †

C . M . Y E A G E R , ‡ A N D P . H . S A N T S C H I †

Department of Oceanography and Marine Science, Texas A&MUniversity, Galveston, Texas 77551, United States, andSavannah River National Laboratory, Aiken,South Carolina, United States

Received June 16, 2010. Revised manuscript receivedAugust 28, 2010. Accepted October 8, 2010.

In aquatic environments, iodine mainly exists as iodide,iodate, and organic iodine. The high mobility of iodine in aquaticsystems has led to 129I contamination problems at siteswhere nuclear fuel has been reprocessed, such as the F-areaof Savannah River Site. In order to assess the distribution of129I and stable 127I in environmental systems, a sensitive and rapidmethod was developed which enables determination ofisotopic ratios of speciated iodine. Iodide concentrations werequantified using gas chromatography-mass spectrometry(GC-MS) after derivatization to 4-iodo-N,N-dimethylaniline.Iodateconcentrationswerequantifiedbymeasuringthedifferenceof iodide concentrations in the solution before and afterreduction by Na2S2O5. Total iodine, including inorganic andorganic iodine, was determined after conversion to iodate bycombustion at 900 °C. Organo-iodine was calculated as thedifference between the total iodine and total inorganic iodine(iodide and iodate). The detection limits of iodide-127 and iodate-127 were 0.34 nM and 1.11 nM, respectively, whereas thedetection limits for both iodide-129 and iodate-129 was 0.08nM (i.e., 2pCi 129I/L). This method was successfully applied towater samples from the contaminated Savannah River Site, SouthCarolina, and more pristine Galveston Bay, Texas.

IntroductionIodine is a biophilic and essential trace element that existsas one stable isotope, 127I and 25 radioactive isotopes. 129I isof particular concern due to its extremely long half-life(16 000 000 years) and because it is perceived to be highlymobile in the environment. The primary source of 129I in theaquatic environment is from accidental and purposefulreleases associated with nuclear fuel reprocessing worldwide

(1, 2). For example, groundwater from F-area at the Depart-ment of Energy’s Savannah River Site (SRS) in South Carolinais highly contaminated with 129I and other radionuclides (3).Approximately 7 billion liters of predominantly acidic aque-ous waste from nuclear processing facilities were disposedin three unlined basins from 1955 until 1988. The groundwaterstill remains acidic, with pH as low as 3.2 in the middle ofthe plume, increasing to background pH levels of 5-6 at theplume fringe. It was found that plutonium and other actinidesare mostly bound to sediments beneath the basins and onlyvery low concentrations occur in groundwater. Other moremobile radionuclides, such as 129I, have been detected in thisgroundwater, at concentrations that are exceeding theprimary drinking water limit for this nuclide. The SavannahRiver flows along a portion of southwestern border ofSavannah River Site (Figure S1 of the Supporting Information(SI)).

Different iodine species exhibit dramatically differentmobility in aquatic and sedimentary environments, asinorganic and organic species may exhibit different hydro-philic and biophilic properties (4). The importance of organo-iodine species has recently been investigated in freshwater(5-9) and marine surface waters (10, 11), but little is knownabout the prevalence and role of organo-iodine in ground-water. Complexation with organic matter could significantlymodify iodine transport and bioavailability, even thoughinorganic iodine has long been assumed to be the dominantand also most mobile species in groundwater (7, 12).Moreover, field data have shown that the speciation ofanthropogenic 129I in the environment can be different fromthat of stable iodine 127I with iodide/iodate ratios of 129I twotimes higher than that of 127I along the European coastalarea (13).

Several methods have been utilized to determine stableiodine species (iodide and iodate) in the literature. Forexample, ion chromatography (IC) and high performanceliquid chromatography (HPLC) have been used for the directdetermination of iodide (14, 15). However, high levels ofchloride in seawater media affect the efficiency of ionseparation, and can thus compromise analytical accuracy.Although the salt effect could be avoided by adding chlorideto the mobile phase (7), the limitation of the routine use ofthe technique is the inadequate sensitivity of detection anddifficulty in maintaining the exchange capacity of the column.The voltammetric method, which was used for the deter-mination of inorganic iodine in open ocean waters (16), isalso limited by its low sensitivity and fouling of the electrodes.Most recently, a chemical vapor generation technique, whichcommonly is used in analyses of trace metals, has beenproposed to analyze iodide by alkylation with trialkyloxoniumtetrafluoroborates (17).

The contemporary methods used to measure 129I wererecently reviewed by Hou et al. (13). Only neutron activationanalysis (NAA) and accelerator mass spectrometry (AMS)provide the sensitivity required for low level environmentalsamples (129I/127I ratio of 10-6!10-10 with NAA and 129I/127Iratios down to 10-14 with AMS). Before AMS measurementscan be conducted, iodine needs to be separated and purifiedfrom a sample and prepared as an AgI target. As AMS is arelative analytical method, the absolute concentration of 129Iis calculated by measuring separately, apart from the 129I/127Iratio, the content of 127I in the samples. For samples with129I/127I ratios higher than !10-10, 1000 fold higher amountsof 127I must be artificially added to the sample prior tochemical separation. Alternatively, liquid scintillation and !spectrometry can be applied to determine 129I after iodine

* Corresponding author. E-mail: saijinzhang03@hotmail.com;phone: 409-740-4772; fax: 409-740-4787.

† Texas A&M University.‡ Savannah River National Laboratory.

Environ. Sci. Technol. 2010, 44, 9042–9048

9042 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 23, 2010 10.1021/es102047y ! 2010 American Chemical SocietyPublished on Web 11/11/2010

is separated from other radionuclides. Both methods havevery low detection limits, 20 mBq (equivalent to 0.03 nM) for! spectrometry and 0.3 Bq (equivalent to 0.4 nM) for liquidscintillation counting. As a powerful technique, inductivelycoupled plasma mass spectrometry (ICP-MS) has beensuccessfully applied to analyze 127I and 129I in different kindsof samples (18-20). However, many difficulties inherent tothis technique were raised, especially the presence of isobaricinterferences (i.e., xenon 129), molecular ions, that is, 127IH2

and tailing of 127I. Special systems allowing for the directintroduction of the sample into the plasma, or by reactionwith a gas mixture (collision or reaction cell) have beendeveloped to overcome the problems (21, 22). In addition,a variety of sample preparation techniques have beenproposed to facilitate the use of ICP-MS in a wide series ofenvironmental samples by eliminating the matrix effects,such as alkaline fusion and back extraction (23), and UVphotochemical generation of volatile iodine (24, 25).

In this study, we propose a new approach for thedetermination of 127I and 129I speciation by using a morecommon and relatively maintenance-free instrument, gaschromatography-mass spectrometry, and derivatization ofiodine species to 4-iodo-N,N-dimethylaniline. Even thoughthe same derivatization technique has been proposed fordetermination of iodine species (26, 27) and elemental iodineof 127I and 129I in the atmosphere (28), it is for the first timeapplied here to analyze different iodine species (iodide,iodate, iodine, organo-iodine), for 127I and 129I in groundwater.This is particularly useful, as normally, highly 129I-contami-nated samples require special sample preparation. In ad-dition, the determination of total (and speciated) iodine ingroundwater and soils (which are more difficult) is alsodemonstrated in this work and thus give a vast overview overchallenges in iodine speciation determinations. The compre-hensive analyses in iodine species including 129I and 127I in SRSgroundwater enables one to understand the equilibrationmechanism and kinetics of 127I with respect to anthropogenic129I to provide a rationale for improved remediation strategies.

Materials and MethodsEquipments. GC-MS instrumentation consisted of an au-tosampler AS3000, Finnigan Trace GC and Polaris Q EI-MSfrom Thermo. A TR-5MS capillary column (30 m " 0.25 mmi.d., 0.25 µm) was used for separation. The injector temper-ature was set at 220 °C and injections (2 µL) were made inthe splitless mode. For each sample run, the oven temperaturewas held at 90 °C for 3 min and then increased to 220 °C ata rate of 30 °C/min. The GC transfer line was set at 280 °C.The MS ion source temperature was set to 250 °C. All themass spectra were collected in full scan mode. ThermoXcalibur software was used for data acquisition andprocessing.

A LS 6500 multipurpose scintillation counter from Beck-man Coulter was used to measure 125I radioactivity in samples.All samples were counted for 10 min.

Reagents and Standard Solutions. Sodium 2-iodosoben-zoate reagent was prepared by mixing 200 mg of free benzoicacid (Alfa Aesar, U.S.) with 3.8 mL of 0.2 M sodium hydroxideon a Touch Mixer (model 231, Fisher Scientific) and dilutedto 50 mL with ultrapure water. The solution was filteredthrough a 0.45 µm polycarbonate membrane. This solutionis stable for at least 4 months when stored at ambienttemperature.

N,N-dimethylaniline solution was prepared by diluting20 µL of N,N-dimethylaniline to 10 mL with methanol.Phosphate buffer (pH 6.5) was prepared by dissolving 10 geach of NaH2PO4. H2O and Na2HPO4 ·7H2O in 250 mL ofultrapure water. Fresh solutions of 0.01 M of Na2S2O5 weredaily prepared by dissolving 0.019 g of sodium metabisulfite(Fisher Scientific) in 10 mL of ultrapure water.

An internal standard stock solution was prepared bydissolving 25 mg of 2,4,6-tribromoaniline in 50 mL ofmethanol. To generate the working internal standard solution,50 µL of the stock solution was added to 10 mL methanol.

To prepare an iodide stock solution (1000 mg/L), 65.4 mgof potassium iodide was dissolved in 50 mL of ultrapure water.Working solutions of 100 µg/L and 1 µg/L iodide were thenprepared and used to generate 1-16 µg/L and 0.1-1 µg/Lcalibration curves, respectively. An iodate stock solution (1000mg I- eq/L) was prepared by dissolving 84 mg of potassiumiodate in 50 mL of ultrapure water. Both the iodide and iodatestock solutions were stored in glass vials at 4 °C in light-proof containers and are good for one week. Workingsolutions were freshly prepared daily from the stock solutions.

Sampling. Groundwater samples were collected from theSRS F-area plume in February 2010 at well FPZ6A (SI FigureS2), where 129I contamination had previously been measuredat >100 pCi/L (SRS report, contract number: DE-AC09-08SR22470). Groundwater was filtered through 0.45 and 0.2µm in parallel. Then 1.2 L of 0.45 µm-filtered permeate wasdivided into three aliquots of 400 mL for fractionation byultrafiltration using cartridges with pore sizes of 100 k Da,10 k Da, and 1 kDa. Each ultrafiltered fraction consisted of40 mL retentate and 360 mL permeate. Surface seawatercollected from Galveston Bay was filtered through a 0.22 µmmembrane immediately after sampling.

Determination of Iodide in Aqueous Samples. A 5 mLaliquot of sample or iodide standard solution was mixedwith 0.5 mL of 1% acetic acid and 1 mL of phosphate bufferin a culture tube (16 " 150 mm). The acetic acid was addedto maintain the solution at background conditions, pH 3.5,the wetland groundwater pH at the Savannah River Site.Internal standard (50 µL), N,N-dimethylaniline solution (50µL), and 2-iodosobenzoate solution (0.4 mL) were then addedto each tube and shaken on a Touch Mixer for 1 min. Next,cyclohexane (0.5 mL) was added to the tubes and shaken ona Touch Mixer for 20 s. The top cyclohexane layer wasremoved and placed into an auto sampler vial for GC-MSanalysis.

Determination of Iodate in Aqueous Samples. A 5 mLaliquot of sample or standard was mixed with 50 µL of 1 MHCl and 100 µL of 0.01 M of sodium meta-bisulfite in a culturetube (16 " 150 mm). This solution was reacted at roomtemperature for 30 min. Next, 1 mL of phosphate buffer wasadded and mixed. Internal standard (50 µL), N,N-dimethyl-aniline solution (50 µL), and 2-iodosobenzoate solution (1.0mL) were added, and the solution was shaken on a TouchMixer for 1 min. This solution was then extracted withcyclohexane for GC-MS analysis, as described in the previoussection. Iodate concentration was then calculated by dif-ference using the iodide concentrations before and afterNa2S2O5 treatment.

Determination of Iodine. A 5 mL sample aliquot wasmixed with 0.5 mL of 1% acetic acid and 1 mL of phosphatebuffer. Then 50 µL of internal standard and 50 µL of N,N-dimethylaniline solution were added and mixed on a TouchMixer for 1 min. Cyclohexane extraction for GC-MS analysiswas performed as described above.

Determination of Iodide and Iodate in Aqueous Sampleswith High Concentrations of 129I. A Strata SAX SPE column(anion exchange column, Phenomenex) was used to purifysamples before measurement by eliminating interferencesfrom inorganic ions and charged organic compounds insamples. The Strata SAX SPE column was conditioned with3 mL of acetone, followed by 3 mL of methanol and 3 mLof ultrapure water. Next, the Strata SAX SPE was equilibratedwith 3 mL of 1 M NaOH and twice with 1.5 mL of ultrapurewater. After column conditioning and equilibration, 5 mL ofsample that had been filtered through a 0.45 µm polycar-bonate membrane was loaded and effluent was collected.

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The column was then eluted with 5 mL of 1 M NaCl. TheNaCl eluent and the previous effluent were combined andbrought to a final volume of 10 mL using ultrapure water.This solution was then split into two 4.8 mL subsamples toseparately measure iodide and iodate as described above.

Determination of Organo-Iodine in Groundwater. Theorgano-iodine concentration was determined by calculatingthe difference between total iodine and total inorganic iodine.Total iodine concentration was determined by combustionof aqueous samples. The procedure was based on Schnetgerand Muramatsu (29), but was modified for aqueous samplesmeasurements (SI Figure S3). Aqueous sample (2.5 mL) wasmixed with 20 mg of vanadium pentoxide (a catalyst for therapid combustion of environmental samples) in a porcelaincombustion boat. Higher amounts of vanadium pentoxideare required for samples with high organic carbon content(>5 mg/L). The boat with the sample was placed into a quartzcombustion tube and subjected to combustion in a tubefurnace (Lindberg/Blue M Mini-Mite, Thermo Scientific). Thetemperature of the furnace was programmed as follows: Thetemperature was increased to 200 °C in 1 min and held for8 min. The temperature of the furnace was then increasedto 900 °C over 10 min and held steady for an additional 10min. Oxygen was used as a carrier gas at a flow rate of 200-250 mL/minute. A glass tube containing 1 mL of ultrapurewater was used as a receiver. The carrier was directed intothe receiver by connecting glass tubing to the tapered endof the quartz combustion. After combustion, the furnace wascooled down to 700 °C and then the carrier gas was switchedto nitrogen. The nitrogen stream kept blowing for additional30 min at the same rate of oxygen. Then the glass tubing andthe tapered end of the combustion tube were rinsed twicewith 0.75 and 0.5 mL of ultrapure water, respectively. Therinses were combined with the receiving solution (!6 mLfinal volume) and subjected to the iodate quantification asdescribed above.

Determination of Total Iodine in Soils. The procedurefor the determination of total iodine in aqueous samples canbe applied to the measurement of total iodine in soils byappropriate modifications. The temperature was increasedto 300 °C within 1 min and then held constant for 3 min.After that, the temperature of the furnace was increased to850 °C over 10 min and held steady for an additional 5 min.The rate of oxygen was reduced to 100 mL/minute. Ad-ditionally, a glass tube containing 2 mL of ultrapure waterwas used as a receiver. Due to the wide range of soilcharacteristics, the amount of vanadium pentoxide for soilsis a critical factor to affect the precision and accuracy of themeasurement. In our experiments, for soils with low organiccarbon (<2%), a vanadium pentoxide to soil ratio of 1, asrecommended by Schnetger and Muramatsu (29), gavesatisfying results, that is, 20 mg of vanadium pentoxide wasadded to 20 mg of soil. However, for soils with high organiccarbon (10% or more), for example, soils from the SavannahRiver Site, reduced amounts of soil samples (5 mg) andincreased amounts of vanadium pentoxide (100 mg) wereneeded for complete combustion and thus, to obtain reliableresults.

The three separate methods necessary for the determi-nation of iodide, iodate, and iodine after their derivatizationto 4-iodo-N,N-dimethylaniline and a fourth method todetermine organo-iodine after combustion are illustrated inSI Figure S3. The pH optimum for the two reactions, theoxidation of iodide with 2-iodosobenzoate to iodine andsubsequent iodination of N,N-dimethylaniline, has beenverified by Mishra et al. (26) as 6.4. A phosphate buffer wasused to control the pH of the reactions.

At 25 °C, the redox potential of 2-iodosobenzoate wasreported as: 1.21 V at pH 1, 0.53 V at pH 4 and 0.48 V at pH7 (30). Therefore, in neutral and weakly acidic solutions,

2-iodosobenzoate oxidizes iodide to iodine without furtheroxidization to iodate (the redox potential of iodate is >0.8 V)(13 and references therein).

Aromatic amines and phenols are exceptional iodinationreagents (26, 30). Mishra et al. (26) proposed that N,N-dimethylaniline could act as an iodine scavenger that formsjust a single isomer of the derivative (at the para position),because substitution at the two ortho positions are impededby the large dimethylamino group.

ResultsReduction of Iodate to Iodide. Iodate was prepared forquantification by a two-step chemical process: (1) reductionto iodide followed by (2) oxidation and derivitization of iodide.Researchers have previously used ascorbic acid as a reducingagent to convert iodate to iodide (26, 27), however, in ourhands the redox reaction was not successful, possibly becauseof the high sensitivity of ascorbic acid to light and air. Instead,we used sodium metabisulfite (Na2S2O5) as the reductant atpH 2 (7, 31).

An experiment was conducted to determine the optimumconcentration of Na2S2O5 to use as reductant for iodate at aworking concentration of 78.7 nM. As shown in Figure 1,addition of 0.2 mM Na2S2O5 provided sufficient reducingpower to recover nearly all the iodate present.

Evaluation of Iodosobenzoate Dose for Iodate Measure-ment. During the two-step chemical process that we usedto enable iodate quantification, iodosobenzoate is used asan oxidizing reagent to convert iodide to iodine, however, itcan also react with excessive Na2S2O5 from the iodatereduction step. Insufficient iodobenzoate will result in lowrecovery due to incomplete converstion of iodide to iodine.Therefore, to determine the optimum level of iodobenzoate,an assay of iodosobenzoate, 0.4, 0.6, 0.8, and 1 mL, was testedin a dose experiment in which concentrations of Na2S2O5

and iodate were 0.2 mM and 78.7 nM, respectively. Eventhough three batches of experiments showed that 0.4 mL ofiodosobenzoate resulted in an iodate recovery of 91.4(1.0%,1 mL of iodosobenzoate (i.e., 4.3 mM) was eventually chosenfor the assay because it consistently worked for all ranges ofstandards and water samples (data not shown).

Distinguishing 129I from 127I. The chromatographic peakof iodinated N,N-dimethylaniline was identified by theretention time and verified by its mass spectrum at full scan(50-400, Figure 2). 129I was distinguished from 127I by thedifferent mass of their iodinated product, 4-iodo-N,N-dimethylaniline, which are 249 and 247 g/mol, respectively.Therefore, to quantify 127I the mass range was set to 247(Figure 3b) and to quantify 129I the mass range was set to 249(Figure 3c). The setting of single mass range works analo-gously to the single ion mode (SIM) of mass spectrometry.However, the postrun setting is more informative and flexibleby providing a full scan chromatogram.

FIGURE 1. Effect of Na2S2O5 concentration on iodate reduction.

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Calibration Curves. For quantitative GC-MS analysis ofiodinated N,N-dimethylaniline, 2,4,6-tribromoanline wasused as the internal standard. The peak area of the internalstandard was gained from the full scan chromatogram (Figure3a). Quantification of iodinated N,N-dimethylaniline (for 127I)was performed by integrating the appropriate peak in thechromatogram using a mass range 247 filter. The ratios ofthe respective areas for peaks representing 4-iodo-N,N-dimethylaniline and the internal standard, 2,4,6-tribro-moanline, were plotted against the concentrations of iodide.Two calibration curves were obtained. One was for a highconcentration of iodide, with a range of 1-16 µg/L (7.9-126nM). The regression equation was Y) 0.0231 "-0.0093. Theother one was for low concentrations of iodide, with a rangeof 100-1000 ng/L (0.8-7.9 nM). The regression equation isY)0.0137 "+0.0009. Their correlation coefficients are 0.9998and 0.9992, respectively. 127I and 129I are proportionallycollected, therefore the calibration curve for 127I can be usedfor 129I.

Detection Limits. Due to the application of mass spec-trometry, the sensitivity of detection was significantlyincreased. As a result, reagent blanks for the measurementof 127I species could not be overlooked. The reagent blank foriodide was 2.51 ( 0.11 nM, and the detection limit of 127I was0.34 nM. During the quantification of iodate, the inclusionof iodosobenzoate increased the reagent blank of 127I to 8.74( 0.32 nM and the detection limit to 1.11 nM. For thedetermination of 129I species, no detectable 129I was found inthe reagents, which leads to a more sensitive and the detectionlimit of 0.08 nM (2 pCi 129I/L).

Quality Control by Standard Addition to Samples. Forgeneral groundwater and seawater samples with a neutralpH of 7, iodide and iodate could be directly measured withthe procedures for aqueous samples described above. Qualitycontrol was carried out by monitoring the recoveries of iodideor iodate standards added to the samples. The averagerecoveries of iodide and iodate were 99.8 ( 0.9% and 102.6( 1.6%, respectively, for near neutral natural samples.However, direct measurement of iodide or iodate in SRSgroundwater samples was difficult due to the acidic natureof the F- area plume (pH 3.5-4) and the presence of unknowninterfering compounds. These low pH values altered thechemistry of the groundwater and interfered with thereduction of iodate and subsequent oxidation of iodide toiodine. Therefore, we applied a Strata SAX SPE column forcleanup of interfering compounds in these samples, withspecific conditioning and equilibration steps. The averagerecoveries for the two batches of contaminated samples were90.5(1.3% for iodide and 95.1(0.2% for iodate, respectively.

Validation of Total Iodine Quantification with KnownAmounts of Iodide, Iodate,125I, and Thyroxine. Iodide andiodate (2.5 mL of 78.7 nM) were added to samples andcombusted to verify the recovery efficiency of our method.The recoveries were 81.0 ( 2.1% for iodide and 85.4 ( 1.3%for iodate. In addition, 100 µL of an 125I standard (0.18 mCi/mL iodide) was diluted to 2.5 mL with ultrapure water andcombusted. The recovery calculated by the 125I activity was90.0%. Finally, thyroxine, as a proxy of organic iodine, wasmixed with a NIST 2709 reference material (1:200). A 20 mgsubsample of the thyroxine mixture was combusted with 20mg of V2O5. The recovery of thyroxin was 92.7 ( 3.0%.

Iodine Speciation of Samples. Iodine speciation in surfacewater from Galveston Bay was determined and comparedwith earlier determinations in the same estuary as Schwehret al. (11), which used HPLC methods. The concentration ofiodide was 67.6 ( 0.5 nM, 116.7 ( 1.0 nM for iodate and 28.5( 0.2 nM for organo-iodine in this study. This result iscomparable to that of Schwehr et al. (11), who determined66-116 nM for iodide, 50-111 nM for iodate, and 12-158nM for organo-iodine for this estuary. Higher concentrationsof IO3

- than I- are reasonable as there is ample oxygen inthe surface waters. Both 129I and 129IO3 were measured aslower than 0.08 nM (i.e., 73 mBq 129I/L).

Size-fractionated samples from SRS F-Area well FPZ 6Awere analyzed and their iodine species are shown in Figure4. In all fractions, iodine mainly existed as iodide > iodate> organo-iodine. No detectable elemental iodine was foundin all the groundwater samples analyzed, which might bedue to the high reactivity of I2 toward organic matter in aquaticenvironments. For 127I, iodide accounted for 45-57% of totaliodine, with a concentration of 95.7 nM in the <0.45 µmfraction. Iodate and organo-iodine contributed nearly evenlyto the remaining total iodine, except the <100 kDa fractionwhere significantly elevated concentrations of organo-iodinewas detected due to an experimental artifact. 129I speciesshowed similar distributions in their respective fractions.The concentration of total 129I was 5.3 nM (133 pCi/L) in the<0.45 µm fraction. The ratios of 129I/127I for iodide, iodate andorgano-iodine were similar (0.02-0.03, in all fractions) which

FIGURE 2. (A) Chromatogram of 8 µg/L iodide (5 mL sample)after derivatization to 4-iodo-N,N-dimethylaniline (retention time7.87 min), and 25 µg/L 2,4,6-tribromoanline (retention time 9.05min) used as internal standard. (B) mass spectrum of4-iodo-N,N-dimethylaniline.

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is dramatically elevated when compared to values (129I/127Iratios !10-12) typically observed in pristine waters (32).

DiscussionHere we present a novel method for the quantification ofiodine species, which distinguishes itself by providing 10-50fold higher sensitivity than HPLC or IC without compromisingaccuracy. In addition, this method provides a much simplerand more convenient way to analyze 129I species whencompared to currently available techniques such as AMS orneutron activation analysis methods. As such, it is a valuablecomplementary method to AMS for the analysis of envi-ronmental samples that are highly contaminated with 129I,such as groundwater near nuclear reprocessing facilities.However, the detection limit of our method (0.08 nM or 2

pCi 129I/L) does not permit the determination of 129I in morepristine, natural samples. Finally, the primary innovation ofthis method is that it permits full speciation at ambientconcentrations of 127I and 129I, thereby permitting greatermechanistic understanding of the terrestrial biogeochemicalfate and transport of radioiodine.

Currently there are no sensitive methods available for thedirect determination of iodate, instead iodate is typicallyreduced to iodide and then measured by HPLC, IC or AMS.This approach can be problematic though, in that samplechemistry can greatly influence the percentage of iodaterecovered as iodide. Indeed, the common iodate reductant,Na2S2O5, completely failed to yield detectable iodide whenapplied to groundwater samples collected from a contami-nated plume in this study. Waste that was deposited in the

FIGURE 3. Strategy for distinguishing 129I from 127I. Arrows point to the peak with RT 7.86 that represents 4-iodo-dimethylaniline. Fullscan of GC chromatogram representing a sample containing 4-iodo-dimethylaniline and the internal standard, 2,4,6-tribromoanline (RT9.03), (B) GC chromatogram filtered by a mass range of 247 for identification of 4-127Iodo-dimethylaniline, (C) GC chromatogramfiltered by a mass range of 249 for identification of 4-129Iodo-dimethylaniline.

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F-Area seepage basin at SRS was strongly acidic andsignificantly changed the chemistry of the downgradientgroundwater by leaching cations (e.g., Mn2+) and lowmolecular weight organic compounds (e.g., organic acids)from aquifer sediments into the aqueous phase. Therefore,pretreatment of these acidic groundwater samples wasrequired to enable effective iodate reduction by Na2S2O5. Astrong anion exchange column with a sorbent functionalgroup of +R3N-, Strata SAX (Phenomenex), was used topretreat the samples after being equilibrated by 1 M NaOH.After equilibration with NaOH, the functional groups of thesorbent were activated and would remove charged organicor inorganic compounds via anionic exchange. These resultshighlight that the inherent chemical diversity of environ-mental samples necessitates the validation of iodate reduc-tion prior to iodide oxidation, derivatization, and detection.

The distribution of iodine species in natural watersdepends on many environmental factors, including chemicalcomposition, pH, Eh, primary and secondary productivity.Under normal conditions (pH 3-10, Eh <0.8 V), iodine shouldtheoretically exist in freshwaters as iodide (13). However, atthe F-area of SRS, iodide only accounted for 48.8% of totaliodine. The relatively high concentration of iodate (27.3%)and organo-iodine (23.9%) implies that chemical and bio-logical factors, other than pH and Eh, are involved inregulating iodine speciation in the system. In addition, theeven distribution of iodine species within each of the fractionsexamined suggests that organo-iodine is associated primarilywith low molecular weight organic moieties. Tremendouslyelevated but relatively constant ratios of 129I/127I for iodide,iodate and organo-iodine (129I/127I ratios !0.03) can beattributed to the spread of 129I from nuclear waste into thegroundwater where it equilibrated with stable 127I. The similarmagnitude of ratios of 129I/127I for the three iodine speciesimplies that the conversion of iodine species of 129I musthave been of recent origin, that is, the conversion occurredwithin the decadal time frame after it was released to thesurrounding groundwater. These consistent ratios of 129I/127I

also support the application of using 129I/127I ratios as anenvironmental tracer on time frames of decades (8, 32).

AcknowledgmentsThis work was funded by the Department of Energy’sSubsurface Biogeochemical Research Program within theOffice of Science (DE-PS02-07ER07-18) S.Z. was partiallysupported by Welch Grant BD0046.

Supporting Information AvailableThe maps of Savannah River Site and its correspondingsampling site as noted in the text (Figure S1, S2). The flowchartfor determination of iodine species was provided in FigureS3. The setting up for combustion of aqueous samples orsoils was provided in Figure S4. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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FIGURE 4. Iodide speciation in samples from well FPZ6A at theSanvannah River Site. (A) 127I species, (B) 129I species.

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