127I in Seawater and Implications for Sources and Transport

concentrations (2r3 × 1011 atoms/L) were observed in the northern part of the English Channel and in the southeastern North Sea. The atomic ratio of ...
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Environ. Sci. Technol. 2007, 41, 5993-5999

Speciation of 129I and 127I in Seawater and Implications for Sources and Transport Pathways in the North Sea X I A O L I N H O U , * ,† A L A A L D A H A N , ‡ SVEN P. NIELSEN,† GO ¨ RAN POSSNERT,§ HARTMUT NIES,| AND JIM HEDFORS‡ Risø National Laboratory, NUK-202, Technical University of Denmark, DK-4000 Roskilde, Denmark, Department of Earth Sciences, Uppsala University, SE-752 36 Uppsala, Sweden, Tandem Laboratory, Uppsala University, SE-751 21 Uppsala, Sweden, and Bundesamt fuer Seeschifffahrt und Hydrographie, D-22589 Hamburg, Germany

Surface seawater samples collected from the North Sea and English Channel were analyzed for total 129I and 127I, as well as for iodide and iodate. Relatively high 129I concentrations (2-3 × 1011 atoms/L) were observed in the northern part of the English Channel and in the southeastern North Sea. The atomic ratio of 129I/127I decreases from the eastern (1.0-1.9 × 10-6) to the western (4-6 × 10-8) parts of the North Sea and from the northeastern (1.5 × 10-6) to southwestern (1-5 × 10-8) parts of the English Channel. The ratios of iodide to iodate are 0.1-0.5 and 0.5-1.6 for 127I and 129I, respectively, in open seawaters, whereas these ratios range from 0.6 to 1.3 and 0.8 to 2.2, respectively, in coastal waters. The results suggest that (1) imprints of the La Hague facility dominates the 129I distribution in the surface water of the North Sea, (2) reduction of iodate to iodide is relatively fast during the transport to the European continental coast, (3) oxidation of newly produced 129I- to 129IO3- is insignificant during water exchange between the coastal area and open sea, (4) reduction of iodate and oxidation of iodide in the open sea seems to be a slow process.

certain areas (1-4). However, the mechanism of conversion among iodine species is still not clear because of the difficulties associated with distinguishing the origin of newly produced and converted iodine species. Isotopic tracers are an excellent tool for the distinction and detection of the source of chemical species. Laboratory research on the conversion of different chemical species of iodine using short-lived isotopes of iodine has been carried out (5-6), but the results are only qualitative because of inadequate simulation of the real seawater environment and consideration of complex interactions among a variety of minor and trace components in seawater. 129I (T1/2 ≈ 15.7 Ma) is a naturally produced long-lived radioisotope of iodine, which has a natural atomic ratio (129I/127I) of about 10-12 in the ocean (7). Releases from human nuclear activities dominate the present 129I level in the environment (8-11), where the nuclear reprocessing facilities at Sellafield (U.K.) and La Hague (France) are responsible for about 90% of the anthropogenic releases (9, 10, 12). These sources and their rapid increase since 1990 (Figure S-1) provide a unique temporal and spatial field tracer for the investigation of the iodine marine geochemical cycle by chemical speciation of 129I combined with that of stable iodine. The occurrence of a relatively huge anthropogenic 129I input in marine waters has been used to trace ocean currents and water transport in the North Atlantic and Arctic Oceans and related seas (9-10, 13-17). However, none of these studies have used the chemical speciation of iodine to further quantify effects on the mixing of water masses and fingerprinting of transport mechanisms. Schwehr et al. (18) showed a potential application of 129I and 127I speciation in the estuarine surface waters of Galveston Bay for tracing terrestrial organic carbon. A main objective of this study was to investigate the source of iodide in the coastal water and interconversion process of iodide and iodate by chemical speciation of 129I and 127I in surface seawater collected from the English Channel and the North Sea. A second objective of this study was to investigate the distribution and transport pathways of different species of 129I and 127I in the North Sea surface waters. Such knowledge will provide significant information about the use of 129I as an environmental tracer.

Experimental Section Introduction Iodine exists in the ocean surface waters predominantly as dissolved iodate, iodide, and a minute amount of organic iodine (1). Iodide is a thermodynamically unfavorable species in oxygenated water, so its formation through the reduction of iodate cannot occur spontaneously by chemical means alone. Although iodate is a thermodynamically favorable species of iodine in seawater, the kinetic barrier prevents the direct oxidation of iodide to iodate (1). Numerous studies have been carried out to investigate the origin of iodide, the conversion of iodine between different species, and the marine geochemical cycle of iodine by determination of the concentrations of various species of iodine in seawater in * Corresponding author fax: +45 4677 5347; e-mail: xiaolin.hou@ risoe.dk. † Risø National Laboratory, NUK-202, Technical University of Denmark. ‡ Department of Earth Sciences, Uppsala University. § Tandem Laboratory, Uppsala University. | Bundesamt fuer Seeschifffahrt und Hydrographie. 10.1021/es070575x CCC: $37.00 Published on Web 08/04/2007

 2007 American Chemical Society

Surface water was collected from 42 sites in the English Channel and the North Sea in August-September 2005 (Table S-1 and Figure 1). The samples were filtered through a Φ 0.45 µm membrane (Sartorius AG, Go¨ttingen, Germany) on site and then tightened and stored in clean polyethylene containers under normal laboratory conditions until analysis. A modified method of Hou et al. (19-20) was used for the separation of iodine species. Wet AG1-×4 resin (Bio-Rad laboratories) in NO3- form was packed in column of φ 10 × 200 mm. A 50-100 mL of seawater spiked with 50 Bq of 125Iand 125IO3- tracers was loaded onto the column, and the column was washed with 30 mL of deionized water and then 50 mL of 0.2 M KNO3. The effluent and the washes were combined for the determination of iodate. Iodide on the column was eluted by 150 mL of 2.0 M KNO3. One milliliter of separated iodide, iodate solution, or original seawater was diluted to 20 mL with 0.15 M NH4OH, and Cs+ as (CsCl) was added as an internal standard. The concentration of iodine (127I) was determined using an X SeriesII ICP-MS (Thermal Electron Corporation). The detection limit, calculated as 3SD of blanks, was 0.27 nM. VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Sampling locations of surface water from the North Sea (0) and German Bight (O) in August 2005 and Danish coast in November 1999 (b). About 50 Bq of a 125I- solution was added to 20 mL of original seawater as chemical yield tracer. About 2.0 mg of stable iodine (as NaI prepared from Woodward iodine) was added as a carrier. To convert all iodine to iodide, 0.2 mL of 2.0 M NaHSO3 and 3 mL of 3.0M HNO3 were added to the solution. Iodine was first extracted with CCl4 after oxidization using NaNO2, and then back-extracted with NaHSO3 solution. 125I in the separated solution was measured using an NaI γ-detector, and the chemical yield of iodide, iodate, and total iodine was 80-85, 90-95, and 94-98%, respectively. Iodine in the separated solution (as iodide) was precipitated as AgI. The dried AgI precipitate was mixed with niobium powder, pressed into a copper holder, and analyzed for 129I using a Pelletron (NEC machine) and the AMS system at the Tandem Laboratory, Uppsala University. A diluted 129I standard NISTSRM 4949C with an 129I/127I ratio of 1.1 × 10-11 was used. Blank samples were prepared using the same procedure as for samples for total iodine, iodide, and iodate. The measured 129I/127I ratio in blanks (1.5 ( 0.5 × 10-13), which was subtracted from the measured value in the samples, was 30-1000 times lower than that in the samples (0.5-15 × 10-11). The statistical error of the measurements at 1 standard deviation was 35S) and iodate-rich Atlantic Ocean water by fresh water input from European continent has contributed to the relatively low iodate and total iodine, as well as to the salinity in the North Sea, particularly along coastal areas. An insignificant correlation between iodide concentration and salinity was observed (Figure 3), which confirms that the iodide in the coastal waters was produced locally by a likely reduction of iodate. Relatively high 129I-/127I- values compared to the 129IO3-/ 127IO - values were found in all the investigated samples 3 (Table 1, Figure S-4). The differences between the 129I-/127Iand 129IO3-/127IO3- values are significantly larger in the English Channel than those in the North Sea, especially in the coastal waters. This observation also suggests that part of the 129IO3in the source water from the English Channel was reduced in the North Sea, particularly in the coastal areas. Conversion of Iodine Species in the North Sea Water. Some processes, such as biological activity involving bacteria and enzyme and photochemical and chemical interactions, have been suggested to control the conversion of iodine species. However, it is still unclear which process or which type of organism controls the reduction of iodate in surface seawater and how fast the conversion process proceeds (14). Insignificant changes of 127I and 129I speciation in water from the English Channel to the southern North Sea were found here. The transit times of the water from La Hague to VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Borkumriff on the Dutch coast and to the German Bight were estimated to be 11 and 14 months, respectively (24). It can be assumed that the water transited to the southern North Sea during 6-8 months, thus implying a rather sluggish change of 129I speciation from La Hague in the English Channel to the southern North Sea. The observation suggests that reduction of iodate and oxidation of iodide in open sea surface water is a relatively slow process. However, 129IO3is rapidly reduced to 129I- when it reaches the Dutch coast. This significant and rapid increase of 129I-/129IO3- values in the Dutch coastal seawater indicates that the reduction of iodate to iodide is a fast process in the coastal area. A relatively low oxidation-reduction potential (ORP ) 60-90 meV) observed in the Dutch coastal water (locations 7, 9, and 10) imply that chemical process is an important factor, in addition to biological activity, that participates in the reduction of 129I. The significantly high iodide to iodate ratios for both 129I and 127I found in the water from German Bight (Figure 2 and Table S-2) might be attributed to local reduction of iodate in the area or that a transport of reduced 129I occurred in the Dutch coast. Absence of published reports on the chemical speciation of iodine in water from the European continental coast of the North Sea or the German Bight hinders temporal comparison. Therefore, it is difficult to establish whether anoxic conditions were recently developed or have existed for long time in these regions. However, Truesdale and Upstill-Goddard (4) have foreseen an exceptionally low iodate concentration in the Dutch and German coastal waters and in the Jutland coast current based on a modeling approach. The existence of anoxic conditions in the sediment of the German Bight (27-28) associated with shallow depths (1429 m) may cause reduction of iodate in the area. In addition, the effect of exceptionally high anoxic conditions is illustrated by a iodide to iodate ratio of 68-70 in sample 20 collected in the Elbe River, which also shows a low oxidation-reduction potential (ORP ) 22 mV; Table S-1 and Figure S-3), indicating reductive water. Such reductive waters can be also the cause of the low ORP (52-56 mV) observed in the estuary of the Elbe River (locations 19 and 25) and contribute to the anoxic condition in the German Bight. A much higher 129I-/129IO3and 127I-/127IO3- values observed in the water from the estuary of Elbe River suggests a rapid reduction of iodate in the Elbe River estuary, which could be mainly a locally controlled chemical process. Accordingly, some of the iodide was produced locally in the German Bight. However, local chemical process cannot fully explain the high iodide to iodate ratios in most surface water along the European continental coast of the North Sea. These waters have relatively high ORP (Table S-1 and Figure S-3) and oxygen concentration (29) and do not show significant correlation between ORP and the 127I-/127IO3- values (Figure S-6). Biological activity and the concentration of bacteria, which may affect the reduction of iodate, were not monitored, which make establishing a dominant mechanism for the reduction of iodate in the coast water difficult. As one moves away from the German Bight to the west coast of Jutland, the iodide to iodate ratios become higher than those in the English Channel and similar to those along the Dutch coast, without significant change northwards along the European continental coast. Such a trend can be explained either by negligible reduction of iodate with time spent in the coastal area or by establishment of a dynamic equilibrium in the iodate-iodide redox pair in the coastal water of the northern North Sea. The 129I concentrations in the central North Sea (1.1-3.5 × 108 atoms/L) are one order of magnitude lower than that in the coastal and offshore waters. As discussed above, 129I in the west and central North Sea may partly originate from the Sellafield marine discharges. However, the gradually east 5998

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to west decrease in 129I concentration and 129I/127I value in the central North Sea surface water analyzed here suggests strong imprints of contribution from the La Hague facility. Regardless of the source, the much higher 129I-/129IO3- (0.92.7) compared to the 127I-/127IO3- (0.17-0.45) values in the central North Sea surface water indicate that iodide oxidation is a slow process in open waters of the North Sea. The similar high 129I-/129IO3- and low 127I-/127IO3- (0.17-0.45) behavior in the open water of the northern North Sea suggest insignificant oxidation of newly produced 129I- during transport after production in the European continental coast. This pattern again confirms the slow oxidation of iodide in the open waters of the North Sea. These observations also support insignificant reduction of iodate during transit along the coastal areas and further oxidation of iodide to iodate in the coastal water, which might explain the insignificant correlation between ORP and the 127I-/127IO3- and 129I-/129IO3- observed in the North Sea water (Figure S-6). The distribution of chemical species of 129I and 127I in the surface water investigated in this work shows that (1) a rapid reduction of iodate to iodide occurs in the coastal area, (2) the oxidation of the new produced iodide to iodate does not occur during its transit along the coast, and to the open sea, and (3) reduction of iodate or oxidation of iodide in the open sea seems to be a slow process. The difference between 129I and 127I species in seawater represents a potential tracer of iodide sources and conversion of iodine species in marine system, as well as in studying details of spatial and temporal evolution of ocean water. A future study of depth-related chemical speciation of 129I and 127I in the North Sea will shed more light on the conversion rates of iodine species, geochemical cycle of iodine in various reservoirs of marine system, and the magnitude and transport pathways of the Sellafield plume.

Acknowledgments The authors wish to thank the Villum Kann Rasmussen Foundation for financial support.

Supporting Information Available Discussions of analytical accuracy and stability of iodine species during storage, two tables, and seven figures as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review March 7, 2007. Revised manuscript received June 26, 2007. Accepted June 27, 2007. ES070575X

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