Transformation of Iodide in Natural and Wastewater Systems by

Separation of different fractions of humic substances (HS) by their molecular weight was carried out by size exclusion chromatography (SEC). The fixat...
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Environ. Sci. Technol. 2000, 34, 3932-3936

Transformation of Iodide in Natural and Wastewater Systems by Fixation on Humic Substances G U N T H E R R A¨ D L I N G E R A N D KLAUS G. HEUMANN* Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg-University Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany

Separation of different fractions of humic substances (HS) by their molecular weight was carried out by size exclusion chromatography (SEC). The fixation of inorganic iodide by HS of these fractions was determined by online coupling of SEC with inductively coupled plasma mass spectrometry (ICP-MS). Possible changes of HS during this transformation process of HS/iodine species could be followed by UV absorption. A 129I-labeled sodium iodide solution was used to determine the transformation of iodide into iodine fixed on humic substances. In the case of natural samples, iodide was exclusively fixed by HS fractions where natural HS/iodine species have also been observed in the original sample. In wastewater samples from sewage disposal plants organoiodine compounds were also identified which were not affected by the transfer of iodide. A strong influence of the transformation process was seen on the microbiological activity of the sample and a complicated transfer mechanism by the formation of different intermediate HS/iodine species was observed. In general, the results contribute to a better understanding of the biogeochemical cycle of the important trace element iodine, but they are also important for the possible fixation of radioactive iodine, e.g. emitted from nuclear plants, in natural aquatic systems. On the other hand, it must be assumed that substantial amounts of iodine are removed as HS/iodine species during water processing, where humic substances are usually separated. This reduces the iodine content in drinking and mineral waters with respect to the natural level of this essential trace element.

Introduction Iodine is an essential trace element for both man and other biological species. To prevent goiter, a typical disease of iodine deficiency, at least 100 µg of this element should be the daily uptake by human consumption. Drinking and mineral water are one of the most important alimentary substances to contribute to this daily iodine uptake. Information of possible iodine compounds present in natural aquatic systems, from which drinking and mineral waters are produced, is, therefore, of special interest. On the other hand, nuclear plants produce radioactive iodine isotopes, e.g. 129I and 131I, which can strongly affect the health of the individual after incorporation (1). During the nuclear reactor accident in Windscale (1957) and in Chernobyl (1986) considerable amounts of radioactive * Corresponding author phone: +49/6131/39-25882; fax: +49/ 6131/39-23369; e-mail: [email protected]. 3932

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iodine were emitted into the atmosphere (2, 3). Continuous emission of radioactive iodine takes place at reprocessing plants for nuclear fuel (4). A global distribution of radioactive iodine easily occurs because of the volatile character of many iodine compounds involved (4, 5) and also due to the fact that particulate atmospheric iodine is preferably associated with the smallest aerosol particles (6, 7). Volatile organoiodine compounds, e.g. methyl iodide, are biogenically produced in the marine environment (8-10), and it is assumed that the global input of methyl iodide from the oceans into the atmosphere is in the range of (3-13) × 105 tons per year (11). This results in a global average concentration in the atmosphere of about 2 pptv (12). Although methyl iodide is one of the most stable iodine compounds in the atmosphere, its atmospheric lifetime is not more than a few days (13). Methyl iodide and also other biogenic organoiodine compounds, for example CH2I2 or CH2ClI, are decomposed in the atmosphere by radiation. This results in iodine radicals and other reactive iodine species such as HIO and HI (14), which are quickly transformed into more stable inorganic species, such as iodide and iodate. Dry and wet deposition from the atmosphere transport these iodine species on the earth’s surface where they can easily be absorbed by aquatic systems. This deposition process enhances the iodide and iodate concentrations in natural aquatic systems which originate from iodine containing minerals. It is, therefore, of great importance to elucidate whether the inorganic iodine in the aquatic system remains in its chemical form or if it can be converted into other iodine compounds. Both inorganic iodine species, iodide and iodate, have been determined in fresh and ocean water samples. Iodate is usually found to be the most abundant species in seawater and iodide is often the most abundant species in freshwater samples (15-17). However, in freshwater samples substantial amounts of organoiodine species can also be found (16, 17). It must be assumed that in the case of organoiodine compounds iodine is preferably associated with humic substances (HS). Experiments with radioactive iodine have shown that iodide can undergo reactions with humic substances (18-20). In addition, it was also assumed that the formation of HS/iodine species is enhanced by microorganisms (19, 20). Thus, the aim of this work was to get more detailed information on the possible conversion of inorganic iodide into HS/iodine species in aquatic systems and address the question whether microorganisms are able to enhance such a transformation. These investigations became possible due to the development of an analytical method by Rottmann and Heumann for the characterization of HS complexes with heavy metals, using high performance liquid chromatography (HPLC) coupled with inductively coupled plasma mass spectrometry (ICP-MS) (21, 22). Applying size exclusion chromatography (SEC) as a separation method it was recently demonstrated that such a HPLC/ ICP-MS system allowed characterization of HS/iodine species in different fractions and that the additional use of the isotope dilution (ID) technique also enabled the determination of “real time” concentrations of separated species (23).

Experimental Section Samples. A groundwater sample with a low HS concentration level (dissolved organic carbon DOC ) 11.6 mg/L), a wastewater sample from a brown coal pyrolysis process with high HS concentrations (DOC ) 236.1 mg/L), and samples from receiving watercourses of two different sewage disposal plants (city of Mainz and community of Neureuth near 10.1021/es000868p CCC: $19.00

 2000 American Chemical Society Published on Web 08/11/2000

Karlsruhe, Germany) were investigated. In the sewage disposal plant of Mainz half of the wastewater is of industrial, the other half of communal origin, whereas the wastewater in Neureuth is mainly of communal origin. The DOC contents of these two wastewater samples were 16 and 8.6 mg/L, respectively. The groundwater (Fuhrberger Feld near Hannover, Germany), the brown coal wastewater (Schwellvollert near Leipzig, Germany), and the sewage sample from Neureuth were also used as reference samples (code numbers FG1, SV1, and ABV2) within the research project “Refractory Organic Substances in Waters (ROSIG)” of the German Research Society DFG. All water samples were stored in precleaned polyethylene bottles in a refrigerator at 5 °C until they were used for the experiments. To minimize modifications of HS by aging, the investigations were started a few days after sampling. All samples were filtered (0.45 µm Teflon filter) prior to their chromatographic separation to avoid plugging of the column by particles. Analytical Procedures. Size exclusion chromatography (SEC) was applied to separate the humic substances and their iodinated species by molecular weight. Two different SEC columns were used, TSKgel 3000 PW from TosoHaas and HEMA SEC BIO 300 from Alltech, because the former column showed decreasing separation efficiencies during the period of investigations. This was tested by the resolution between peaks of standard reference substances. Elution of the HS fractions was carried out by pure water, produced by a MilliQ system from Millipore. Calibration of the molecular weight of the HS compounds in the different fractions by their retention time was not exactly possible because no HS reference material exists. However, dextrane reference materials with a molecular weight of 1 kDa, 10 kDa, and 44 kDa correlated with retention times of 15, 13.3, and 11.6 min, respectively, for the HEMA column. The TSK column showed slightly higher retention times for identical HS fractions but a similar fractionation pattern as the HEMA column. A HPLC system, type S 1000 from Sykam, was coupled with a quadrupole ICP-MS (Elan 5000 from Perkin-Elmer) equipped with a cross-flow nebulizer and a Scott spray chamber. Between the SEC column and the ICP-MS a UV absorption cell was installed to receive additional information on the structure of the separated HS compounds. The chosen wavelength of 254 nm indicates aromatic character or structural elements of HS with conjugated double bonds, whereas the ICP-MS results in the iodine-specific detection. The aqueous sample was introduced into the SEC/ICP-MS system by a sample injection valve fitted with a 500 µL sample loop. More detailed information on this SEC/ICP-MS system is given elsewhere (21-24). To follow the formation of HS/iodine species from inorganic iodine, an 129I-enriched sodium iodide spike solution (isotopic composition 129I ) 84.3%, 127I ) 15.7%) was used. Because natural iodine is monoisotopic (127I), the long-lived radioactive 129I isotope with a half-life of 1.6 × 107 years was applied for these labeling experiments. The spike solution was prepared from 129I-enriched sodium iodide (NEN Chemicals). Approximately the same amount of 129I-enriched iodide, as was analyzed directly by ICP-IDMS for the total original iodine content in the corresponding sample, was added, and the samples were stored in airtight sealed vessels for several weeks at room temperature in the dark. To investigate the assumed enhancement of HS/iodine species formation by microorganisms, bacteria cultures, cultivated by well-known standard microbiological procedures (25) from the same original sample, were added to one of the original samples (wastewater from the sewage disposal plant of Mainz). Another original sample from this source was filtered using a 0.2 µm pore sized Teflon filter. By determining the colony forming units (CFU) of bacteria, the

FIGURE 1. Distribution of natural iodine (127I isotope) in different fractions of humic substances determined by SEC/ICP-MS (separation by TSK column; note that the x-axis starts with a retention time of 5 min and that fraction numbers F2-F4 correspond with those of the same sample in Figure 2). filtered sample had a reduced bacteria population by a factor of about 200, whereas the microbiologically enriched one showed an increase by a factor of about 400 when compared with the untreated sample (1.1 × 105 CFU/mL). More detailed information on the biological parameters of such microbiologically enriched samples are described elsewhere (26). In the case of a microbiological influence, the filtered sample should show a reduced formation and the microbiologically enriched sample an enhanced formation of HS/iodine species when compared with the untreated sample. Spectroscopic interferences, especially when using a quadrupole mass spectrometer with its low mass resolution, can be a problem in ICP-MS. For 127I and 129I measurements only 129Xe has to be taken into account for possible interferences because xenon is sometimes an impurity in the argon plasma gas. However, it is easy to correct the 129I chromatogram for this interference because 129Xe results in a constant background signal during the whole measuring time, whereas 129I only appears in the corresponding HS fractions labeled with this isotope. Typical parameters used for running the ICP-MS are described elsewhere (21-23).

Results and Discussion Iodine Distribution in HS Fractions of Different Aquatic Samples. Figure 1 represents the distribution of natural iodine VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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(127I is the only stable iodine isotope) in three different samples (wastewater from the sewage disposal plant in Neureuth ABV2, groundwater FG1, and wastewater from a brown coal pyrolysis process SV1) obtained with the SEC/ICP-MS system. The UV absorption curve at a wavelength of 254 nm is also shown and indicates aromatic structural elements of the HS fractions or those with conjugated double bonds. The broad UV absorption band at the low retention time scale (at about 15 min), measured for the wastewater sample from the brown coal pyrolysis process, is due to a high molecular HS fraction. A UV active HS fraction, similar in its molecular weight, was also observed in the groundwater sample. This similarity indicates an influence of brown coal on the groundwater, which agrees with knowledge on the hydrological origin of this water. On the other hand, only a sharp UV absorption band could be measured for the sewage water sample at 20 min retention time, which corresponds to compounds with lower molecular weight. As can be seen from Figure 1 the distribution of iodine in the different HS fractions is different with respect to its association with UV active substances but also with respect to the molecular weight of the HS/iodine species. HS/iodine species are preferably formed in the sewage water sample with the HS of fraction F3 at 20 min retention time, which show a high UV absorbance. Small iodine amounts are also associated with fraction F2 in this sample, which is a little higher in its molecular weight but smaller in its UV absorbance. Iodine is also bound in the low molecular weight fraction F4 where no UV absorption is observed. It is assumed that the substance of fraction F4 may be an iodine-containing contrast agent used in medicine for diagnostic purposes. This assumption is confirmed by the stability of this compound, which can be seen from Figure 2 where fraction F4 was found to be unchanged after 4 weeks. Ku ¨ mmerer et al. also suggested that contrast agents can contribute to the iodine content of wastewater in sewage disposal plants (27). HS/iodine species in ground and brown coal pyrolysis water are exclusively formed with a narrow fraction F2, which shows a more or less intensive UV absorption. However, it is interesting that in these samples iodine is not bound to the high molecular substances with their distinct aromatic behavior or structural elements with conjugated double bonds. Formation of HS/Iodine Species by Iodide Transformation. Fifty milliliters of the three different samples, for which the natural iodine distribution in the different HS fractions is shown in Figure 1, was mixed with the 129I-enriched sodium iodide solution and stored in the dark at room temperature for 4 weeks. The amount of 129I-enriched iodide added was always identical to the amount of natural iodine in the corresponding samples. After 4 weeks, SEC/ICP-MS chromatograms were produced, and the distribution of 127I and 129I in the different HS fractions was measured (Figure 2). Because the SEC separation, shown in Figures 1 and 2, was obtained by two different separation columns, the retention times for HS fractions of identical molecular weight are different (TSK retention times are higher than those obtained with the HEMA column). However, the numbering system of HS/iodine fractions to be compared is identical in both figures for samples of the same origin. A substantial change in the composition of humic substances is observed when the UV absorption curves of the original samples at the beginning of the experiment (Figure 1) are compared with those after 4 weeks storage (Figure 2). For example, UV active substances with high molecular weights were newly formed in the sewage water sample. In the case of the two other samples a significant transformation of the high molecular UV active HS compounds occurred. In addition to these changes in the UV absorption behavior of the different HS fractions transfor3934

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FIGURE 2. Distribution of natural 127I and of spiked 129I in different fractions of humic substances determined by SEC/ICP-MS after 4 weeks of equilibration with 129I-labeled iodide (separation by HEMA column; note that HS/iodine fractions F1-F4 correspond with those of the same sample in Figure 1; for a better presentation of the isotope chromatograms the 127I curves are shifted upward with respect to the corresponding scale on the y-axis). mations of originally HS bound iodine (represented by 127I; Figure 1) into HS fractions of higher molecular weight took place (Figure 2). Transformation of HS fractions is a known aging effect of refractory organic substances. Recently, conversion of HS/iodine fractions was also found as a significant aging process (26). However, no loss of iodine was observed during these investigations with respect to the total iodine fixed on humic substances. Inorganic iodide (represented by the 129I chromatogram) was transformed into HS/iodine species within 4 weeks of equilibration (Figure 2). An identical distribution of 127I and 129I in the different HS fractions of the ground and brown coal pyrolysis water sample was found. In addition, the ICPMS count rates of both iodine isotopes are identical. This means that the iodide was completely transformed into HS/ iodine species and equilibrated with the iodine of the original HS/iodine species (represented by the 127I chromatogram). A slightly different situation was observed for the sewage water sample where the 127I and 129I chromatograms do not exactly fit one another. In this case the 129I-labeled iodide was preferably fixed by fraction F3, which was the main HS/ iodine species in the original sample. In addition, a new HS/iodine species (F1) with a higher molecular weight was

FIGURE 4. Distribution of natural 127I and of spiked 129I in different HS fractions of the microbiologically enriched wastewater sample from the sewage disposal plant in Mainz after 2, 4, and 8 weeks of equilibration, respectively, with 129I-labeled iodide (separation by HEMA column; for a better presentation the 129I curves are shifted upward on the y-axis). FIGURE 3. Distribution of 129I in different HS fractions of a wastewater sample from the sewage disposal plant in Mainz after 8 weeks of equilibration with 129I-labeled iodide dependent on the microbiological activity (separation by HEMA column; chromatogram on top of this figure shows the situation in the untreated sample at the beginning of the experiment). also formed. On the other hand, no spike iodide was bound by fraction F4, which demonstrates that this compound is stable with respect to an iodine exchange. This indicates a stable synthetic organoiodine product, possibly a contrast agent used for medical purposes, as was discussed previously. This is the first time that transformations of inorganic iodide into HS/iodine species could directly be measured, and it confirms previous results where iodine fixation by humic substances has been followed by only measuring the reduction of iodide ions in the corresponding system (19, 20). The results of this work also show that radioactive inorganic iodine, possibly introduced into the aquatic system by emissions from nuclear plants, will be stabilized by transformation into HS/iodine species and, therefore, becomes quite mobile in the hydrological system. Another important aspect of these results is related to the processing of drinking and mineral water. This processing usually includes separation of humic substances. Because of the strong association of iodine with HS it must be assumed that substantial amounts of iodine are removed during water processing which may significantly reduce the natural content of the essential trace element iodine in drinking and mineral waters. Microbiological Influence on the HS/Iodine Formation. Three 50 mL wastewater samples from the sewage disposal plant of the city of Mainz, collected at the same time, were used to investigate the microbiological influence on the

formation of HS/iodine species from inorganic iodide. One of these samples remained untreated, whereas another one was filtered to reduce the biological activity. The third sample was inoculated with bacteria cultivated from the same sewage disposal plant (microbiological enriched sample). The same amount of 129I-labeled iodide, comparable with the iodine amount determined in the original sample, was added to each of the samples. The samples were then stored for 8 weeks at room temperature in the dark. The corresponding results, showing the 129I and UV absorption chromatograms, are summarized in Figure 3, where the 127I and UV chromatograms of the original sample at the beginning of the experiments are also plotted for comparison (diagram on top of Figure 3). A distinct microbiological influence was found for the formation of HS/iodine species by HS fixation of 129I-labeled iodide, which also significantly depends on the different fractions of humic substances involved. After 8 weeks of storage, 129I was only detected in the high molecular HS fractions F1 and F2. The formation of HS/iodine species F2 seems to be almost independent of the microbiological activity, whereas the formation of species F1 strongly depends on it. This becomes obvious from the results for the microbiological enriched sample (chromatogram at bottom of Figure 3), where a strong transformation of iodide into species F1 correlates with a distinct increase of the UV active character of this high molecular HS fraction. On the other hand, transformation of the 129I-labeled iodide into fraction F1 was a little lower for the filtered sample compared with the untreated (original) one. It is interesting to point out that in this sewage water sample iodide was no longer fixed in the low molecular fractions F3 and F4 after 8 weeks of storage where iodine species were also found in the original sample at the VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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beginning of the experiment. When following the transformation of iodide into HS/ iodine species with time for the microbiologically enriched sample, it was found that the biologically activated mechanism of this conversion process is obviously not a direct fixation of inorganic iodide on the corresponding HS fractions. A much more complicated mechanism must be assumed from the results represented in Figure 4, where the chromatograms of the natural (127I) and the spiked (129I) iodine are shown after 2, 4, and 8 weeks of storage, respectively. After 2 weeks, transformation of iodide was exclusively observed into HS fraction F2, which is represented by the single 129I peak at a retention time of 16 min. After 4 weeks, 129 I-labeled HS/iodine species were also formed by fractions F1 and F3 but not by the HS compounds of fraction F4. After 8 weeks of equilibration most of the iodide was transformed into the high molecular HS/iodine species F1. Previous quantitative investigations on the aging of HS/iodine species by HPLC/ICP-IDMS have shown that no iodine is removed from humic substances, but transformation can occur from one into other HS fractions (26). This result is confirmed by the 127I chromatograms in Figure 4, where iodine, originally bound in HS fractions F3 and F4, totally disappeared after 8 weeks (F3) or was significantly reduced (F4). The results of this work confirm earlier investigations where transformation of inorganic iodide into HS/iodine species was assumed to be enhanced by microorganisms (19, 20), which could not directly be measured by the analytical methods available at that time. In addition, more detailed knowledge on the complex mechanism of this transformation process is now available by applying SEC/ ICP-MS. Because iodine fixation on humic substances plays an important role in the biogeochemical cycle of this element, the investigations of this work contribute to a better understanding of the behavior of iodine in the environment. This is essential not only for natural iodine but also for anthropogenically produced iodine, such as radioactive emissions from nuclear plants.

Acknowledgments We wish to thank the Deutsche Forschungsgemeinschaft for financial support and all members of the research project on “Refractory Organic Substances in Waters (ROSIG)” for their excellent cooperation. We especially thank Dr. G. Abbt-Braun and Prof. F. H. Frimmel, Karlsruhe, for the availability of samples, used as reference substances in the “ROSIG” project, and the corresponding DOC data as well as Dr. H. Claus, Mainz, for the bacteria cultures applied in the microbiologically enriched samples.

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Received for review January 4, 2000. Revised manuscript received June 13, 2000. Accepted June 20, 2000. ES000868P