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  • 35 The Fate and Removal of Radioactive Iodine in the Aquatic Environment

    R. Scott Summers1, Friedrich Fuchs, and Heinrich Sontheimer

    Engler-Bunte-Institut, Universitt Karlsruhe, 7500 Karlsruhe, Federal Republic of Germany

    The reaction of iodine with aquatic humic substances (HS) and the subsequent removal of the products by typical drinking-water-treat-ment processes was investigated. Both iodine and iodide react com-pletely with isolated HS in the concentration range below 0.03 mg of I per mg of HS and behave similarly with Rhine River water. The reaction is independent of pH, initial HS concentration, and HS molecular size. However, at higher I-HS ratios iodine reacts slightly more than iodide. Kinetic studies indicate that the reaction is complete within 10 min. No interaction was found between methyl iodide and HS. Flocculation and activated-carbon (AC) adsorption were effective for the removal of the I-HS complex, and the dissolved organic carbon measurement served as a good surrogate parameter. Volatil-ization and AC adsorption were effective for methyl iodide removal.

    /JLFTER A NUCLEAR REACTOR ACCIDENT the release of radionuclides poses a problem for drinking-water-treatment facilities using surface waters as their raw water source. A recent nuclear power plant accident resulted in high levels of radioactivity in the environment throughout Europe, as shown in Table I for the Federal Republic of Germany. The highest activity levels occurred in the southern part of the country where, fortunately, 95% of the potable water originates from ground-water aquifers that were not directly contaminated. However, for communities that use surface waters, an un-

    1Current address: Civil and Environmental Engineering Department, University of Cincinnati, Cincinnati, O H 45221-0071

    0065-2393/89/0219-0623$06.00/0 1989 American Chemical Society

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 624 AQUATIC HUMIC SUBSTANCES

    Table I. Radioactivity Levels in the Federal Republic of Germany after the Chernobyl Accident

    Maximum Reported Main Ref. Source Activity Component Ref.

    Air (Bq/m3) 150 1-131 1 Ground (Bq/m2) 280,000 1-131 1 Aqueous (Bq/L)

    Rain 35,000 1-132 or 1-131 2 River 370 1-131 2 Reservoir 570 1-131 2 Ground

  • 35. SUMMERS ET AL. Radioactive Iodine in the Aquatic Environment 625

    were also measured in order to find an indicator parameter that could be measured more easily than radioactivity.

    One problem encountered was the determination of the appropriate species of iodine to use in such an investigation. Metal iodide is the chemical form that escapes from the reactor core (7); however, the form of iodine predominating after exposure to the atmosphere is not completely understood. The form is thought to be dependent on the conditions in the containment building; the volatile elemental iodine and methyl iodide are important with respect to atmospheric release.

    Experimental Details The humic substances used in this study were isolated by a strong basic anionic resin (Lewatit MP 500 A, Bayer Chemical Co.) used in the treatment of ground water with a high humic content (7 g of DOC/m3) in Fuhrberg, Federal Republic of Germany. The resins were regenerated with a solution containing 10% NaCl and 2% NaOH (8). The regenerate has a molecular size (MS) range of 200-4000, with an average of 1500 as estimated by gel-permeation chromatography (GPC). The experimental solution properties for the Fuhrberg humic-substances (FHS) are shown in Table II.

    The GPC of the Rhine River sample indicates a MS range of 160-5000, with an average of about 1500. Samples of the Rhine River were taken at Karlsruhe, Federal Republic of Germany, approximately 360 km downstream of Lake Constance. Their properties are also shown in Table II. The radionuclide 1-131 was supplied in a carrier solution of Nal at a ratio of 4.72 104 Bq^g of I, and the solution activity was analyzed by 7-ray spectrometry. The concentrations of I" and I2 were analyzed by the leuco crystal violet method (6) with a detection limit of 5 mg/m3 in the Rhine River and FHS solutions. The DPD (N,N-diethyl-p-phenylenediamine) photometric method was used to measure chlorine (6). The concentration of CH3I was measured with an electron-capture detector-gas chromatograph with a detection limit of less than 0.1 mg/m3.

    The reaction experiments between I~, I2, or CH3I and the FHS or Rhine River water were conducted in 0.1- or 0.25-L closed volumetric flasks. Flocculation was conducted in 1-L glass beakers with the addition of iron sulfate at high mixing intensities (250 rpm) for 5 min, followed by 10 min of flocculation at 50 rpm and 30 min of settling. The cationic polyelectrolyte, poly aery lam ide (PAA), when used, was added 2.5 min after the addition of iron sulfate. Filtration utilized glass-fiber filters or 0.45- membrane filters. Adsorption experiments were conducted with closed 0.25-L bottles on a shaker-table (250 rpm) at a contact time of 2 days with pulverized activated carbon (F300, Chemviron Corp.). In the combined floccula-

    Table II. Properties of Fuhrberg Humic Substances and Rhine River Sample DOC UV-254 Redox Temperature Turbidity

    Sample (g/m3) (m-*) pH (mV) (C) (FTUa) Rhine River 2.62 6.52 7.65 240 19.6 2.9 Fuhrberg humic substances 4.20 16.3 6.5 515 20 "Formazine turbidity units.

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 626 AQUATIC HUMIC SUBSTANCES

    tion-activated-carbon adsorption experiments, the pulverized carbon was added to the beaker at high mixing intensities 10 min prior to the addition of iron sulfate; it was filtered out with the sludge; the result was a 1-h contact time.

    Results and Discussion Reaction with Humic Substances. The reaction kinetics of ele

    mental iodine, I 2 , and iodide, I", with F H S over a 2-week period are shown in Table III. A l l initial solutions of I 2 used throughout this study contained 27% iodide. For both iodine and iodide the reaction with F H S is very fast, with no additional reduction in solution concentration occurring after 10 min. The reaction may even be faster, but this possibility could not be assessed, as 10 min was required for analytical sample preparation. The reaction kinetics of iodine and chlorine with the F H S can be seen in Figure 1. The iodine reaction is much faster than that of chlorine, which displays continued formation of organic-bound chlorine over a 17-h period.

    A l l nonorganic-bound iodine is reduced to iodide in the reaction between 1 2 and F H S , as shown in Table HI . This result can also be seen in Figure 2, where the initial concentrations of iodine were five times higher than in Table HI . In the system of I 2 and F H S , the total amount of iodine in solution decreases from 4.38 to 1.02 g / m 3 after 0.5 h; the I 2 component decreases from 3.20 to 0.23 g /m 3 . After 3.5 h the total amount of iodine does not change, but the I 2 component is completely reduced to iodide. In the system with both I" and C l 2 , all iodide is initially oxidized to I 2 . After reaction with F H S (0.5 h), most of the iodine left in solution has been reduced to iodide. After 3.5 h all solution iodine is in the form of iodide, but no additional organic-bound iodine was formed.

    The relationship between organic-bound iodine and the added or total system iodine is shown in Figure 3 for both elemental iodine and iodide. Below a total iodine concentration, normalized for the D O C concentration of F H S , of 0.05 g of I per g of D O C , both forms of iodine react completely with the F H S . This reaction represents iodine concentrations as high as

    Table . Reaction of Iodine and Iodide with Fuhrberg Humic Substances

    Time Iodine (l2) Iodide (/)

    Time h I 2 / h / - / 0 650 240 890 ndf l 650 650 10 min nd 370 370 nd 280 280 8 h nd 400 400 nd 270 270 1 day nd 380 380 nd 285 285 4 day nd 400 400 nd 290 290 14 day nd 400 400 nd 290 290 NOTE: All values are concentration in milligrams per cubic meter. Solution conditions: 3.2 g of DOC/m3; pH 6.5. end, not detected.

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 35. SUMMERS ET AL. Radioactive Iodine in the Aquatic Environment 627

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    S 80 X I

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    Fuhrberg humic substances (FHS) /

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