Iodide sorption to other sediment minerals is generally quite limited. Ticknor and Cho. (6) reported no F sorption to calcite or muscovite using a pH 7.7 synthetic ...
RADIOIODIDE SORPTION TO SEDIMENT MINERALS D.I. KAPLAN*, R.J. SERNE**, K.E. PARKER**, I.V. KUTNYAKOV** *Westinghouse Savannah River Company, Aiken, SC 29808, daniel.kaplan(Thsrs.gov **Pacific Northwest National Laboratory, Richland, WA 99352 ABSTRACT Laboratory studies were conducted to quantify and understand the processes by which iodide (F) sorbs to minerals found in subsurface arid sediments. Little or no F sorbed to montmorillonite (Kd = -0.42 ± 0.08 mL/g), quartz (Kd = 0.04 ± 0.02 mL/g), vermiculite (Kd 0.56 ± 0.21 mL/g), calcite (Kd = 0.04 ± 0.01 mL/g), goethite (Kd = 0.10 ± 0.03 mL/g), or chlorite (Kd = -0.22 ± 0.06 mL/g). A significant amount of F sorbed to illite (Kd 15.14 ± 2.84 mL/g).). desorption (or isotopic Upon treating the iodide-laden illite with dissolved F-, Cl1, Br-, or 1271-, exchange in the case of 1271) removed, respectively, 43 ± 3%, 45 ± 0%, 52 ± 3, and 83 ± 1% of the F originally adsorbed to the illite. The fact that such large amounts of F could be desorbed suggests that the 1-was weakly adsorbed, and not chemically bonded to a soft metal, such as mercury or silver, that may have existed in the illite structure as trace impurities. Finally, l sorption to illite was strongly pH-dependent; the Kd values decreased from 46 to 22 mL/g as the pH values increased from 3.6 to 9.4. Importantly, F sorbed to illite even under alkaline conditions. Together, these experiments suggest that illite removed F from the aqueous phase predominantly by reversible physical adsorption to the p1l--dependent edge sites. Illites may constitute a substantial proportion of the clay-size fraction of many arid sediments and therefore may play an important role in retarding F movement in these sediments. INTRODUCTION Iodine-129 is commonly among the largest contributors to the calculated health risk associated with long-term nuclear-waste disposal in the subsurface. The cause for this is because 1291 has a large inventory in many types of waste, a long half-life (1.6 x IO' yr), a high toxicity, and a perceived high mobility through oxidized, low organic matter environments. The high mobility of iodine is primarily due to its anionic nature in groundwater. It exists primarily as iodide (F) or iodate (103), which can be repulsed from the negative charge of most sediments. Of these two species, the more reduced form, F, is more commonly found in natural oxygenated groundwaters (1). Iodate tends to exist only in highly oxygenated and alkaline systems and may be formed by interaction with radiolysis products (2). Iodide sorption to natural sediments has been closely correlated to organic matter concentrations, Fe-oxide concentrations, and pH levels of the sediments (2-5). Iodide sorption to organic matter and Fe-oxides has been attributed to interaction with positively charged sites on these pl1-dependent charge materials (3,4). As the pH decreases, the number of positively charged surface sites available for I- or 10; sorption increases (4,5). The iodine oxidation state also has a significant effect on the degree of sorption. Iodate sorbs appreciably more than P to several minerals (2,6). Couture and Seitz (2) reported that >99.99% of the 103- and only 30% of the F sorbed to hematite in a pH 7 system. They also reported that >26% of the 103- and none of the F sorbed to kaolinite in a pl- 5 system. They postulated that 10; specifically sorbed on hematite by formation of Fe-010 2 bonds, thereby displacing OH-. This mechanism is analogous to selenite sorption by goethite, phosphate adsorption to hematite, and adsorption of other oxyanions by ferric hydroxide. Ticknor and Cho (6) reported that more 103- than F sorbed to granitic fracture-filling minerals. The cause for the
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difference in I and IO3; sorptive behavior is not known but is presumably the result of the "harder" base nature of IO3-, as compared to F, which would favor "hard-hard" interactions with the "hard" acid sites on the mineral surfaces. Iodide sorption to other sediment minerals is generally quite limited. Ticknor and Cho (6) reported no F sorption to calcite or muscovite using a pH 7.7 synthetic groundwater dominated by Ca, Na and Cl. Muramatsu et al. (7) reported essentially no sorption of F from distilled water onto bentonite and Fe2O3 using distilled water. Sazarashi et al. (8) reported no P sorption (F Kd = 0 mL/g) to montmorillonite (10-6 M KI and 5-day contact period). Ticknor et al. (9) reported low F-Kd values for biotite, 0.7 mL/g, and montmorillonite, 1.9 mL/g, when measured in a pH 7.7 synthetic groundwater that was dominated by the Ca, Na, Cl, and S04 ions. De et al. (10) reported either no adsorption or negative adsorption (anion exclusion) for F onto montmorillonite and kaolinite suspension in contact with 3.175 and 2.595 mg/L KI solutions, respectively. Subsurface arid sediments are the most common type of sediments contaminated by radioiodine or at risk to be contaminated by radioiodine by proposed nuclear waste storage (e.g., Hanford Site in Richland, Washington, Idaho National Engineering and Environmental Laboratory in Idaho Falls, Idaho). These sediments tend to contain very low concentrations of organic carbon ( vermiculite > goethite >> montmorillonite. The fact that the surface-area normalized Kd value of the illite remained appreciably greater than the other minerals, suggests that the surface area alone was not responsible for the high P sorption capacity of the illite. The unique mineralogical properties of the illite that permitted it to sorb large amounts of I are not known. A computer-assisted review of the literature revealed that high I-sorption to illite has previously been reported, albeit these reports are few and separated by many years. De et al. (10) reported that illite sorbed -99% of the F initial added as 31.75 mg/L I-(KI) in deionized water. They also reported that I tended to sorb appreciably more to natural sediments with clay fractions dominated by illite rather than sediments dominated by kaolinite or montmorillonite. Unfortunately, the conditions of the experiments conducted by De et al. (10) were not fully described and those that were described were quite different from the conditions of this study. They contacted the illite for only 2 hr with
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Ticknor et al. (9) reported a low F Kd value of 0.4 ± 2.9 mL/g for a goethite sample acquired from the same location (Biwabik, Minnesota) as the goethite used in this study. The goethite Kd values measured in this study were 0.10 ± 0.03 and 0.07 ± 0.05 mL/g for the nontreated and treated samples, respectively. The standard deviation values associated with these Kd values permit us to conclude that a small amount of I- sorption occurred. A similar conclusion can not be made at a 95% level of confidence based on the data from Ticknor et al. (9). Finally, Ticknor et al. (9) reported an IFKd value for biotite, a type of vermiculite, of 0.7 ± 1.2 mL/g, which is quite similar to the vermiculite Kd values reported in Table 1, 0.56 and 0.80 mL/g. This detailed description of previously reported F sorption results is presented to show that F sorption to these minerals, with the exception of illite, either does not occur or does so to a very limited extent. Illite Desorption Experiment It is possible that some chemical impurities in the illite mineral composition may have promoted enhanced I complexation onto or into the mineral surface. Soft metals, such as mercury or silver, form very strong bonds with I- (Allard et al. 1980). In an attempt to evaluate the tenacity that ' 5I sorbed to illite, dissolved F-, Cl-, Br-, and '27I were added to systems containing illite and sorbed I, and the amount of I released was measured. These anions desorbed a large percentage of the sorbed F. Fluoride desorbed 43 + 0.3%, Cl desorbed 45 ± 0%, Br- desorbed 52 ± 3%, and 1271- isotopically exchanged 84 ± 1% of the adsorbed 1251-. The percentage of F desorbed from the illite decreased as the size of the competing anion decreased. These findings suggests that the I-was weakly sorbed to the illite, perhaps by anion exchange or ligand exchange, and was not strongly sorbed, as would be the case if the I bonded to soft metal in the illite structure. Consistent with these findings, Muramatsu et al. (7) reported that I adsorbed to mineral isolates and natural sediments was isotopically exchangeable. Similarly, Sheppard et al. (17) reported that Cl- could compete with I-for sediment sorption sites. Illite/pH Sorption Experiment
Among the possible mechanisms by which I-may have sorbed to the illite in the Mineral Sorption Experiment is via interaction with the edge sites, which possess a pH-dependent charge. This experiment was conducted to evaluate the extent that I sorption to the illite was pH dependent. As the pH of the system decreased below 8, the amount of I sorbed increased appreciably (Table 2). Importantly, the I Kd values at the very high pH levels, pH 7.9 and 9.4, were significant, 24 ± 1.2 and 22 ± 0.2 mL/g, respectively. The concentration of positively charged edge sites at these pH levels is generally extremely low, but apparently there were a sufficient number of sites to adsorb the low mass of I- in the test system (approximately 10-12 M Iin the initial spike solution). The ionic strength was not identical in each of the pH-adjusted suspensions. The electrical conductivity values of the suspension, which provides an index of the ionic strength, shows that the salt concentrations did not vary greatly and, perhaps more importantly, not in a covariant manner with the Kd values (Table 2).
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Table 2. Iodide
Kd
on illite as a function of pH(a)
Kd Electrical Conductivity (mL/g) (mS/cm) 46 ± 3.9 2.52 ± 0.03 3.6 59 ± 2.2 2.31 ± 0.03 5.0 24 ± 1.2 2.35 ± 0.04 7.9 22 ± 0.2 2.41 ± 0.04 9.4 (a) Background electrolyte was 0.01 M CaCl 2 ; pH adjustment with NaOH and HCI, three replicates. pH
CONCLUSIONS
Experiments with pure mineral phases showed that there was little or no I sorption to montmorillonite, quartz, vermiculite, calcite, goethite, or chlorite. A significant amount of Isorbed to illite (Kd = 15.14 ± 2.84 mL/g). A number of subsequent tests were conducted in an attempt to identify the mechanism by which the illite sorbed the I-. Normalizing the Kd values by surface area, rather than by mass, did not alter the ranking of the minerals by their I capacity. This suggests that the mineralogical properties of illite were responsible for its exceptionally high I sorption capacity. The sorbed I could be easily desorbed with halide salts. As the ionic radius of the additions halides decreased, the percent of I desorbed increased. Dissolved F-, Cl-, Br-, and 127IT desorbed (or isotopically exchanged) 43 ± 3%, 45 + 0%, 52 + 3 and 84 ± 1% of the sorbed iodide on the illite, respectively. The fact that such large amounts of I could be desorbed suggested that the I was weakly adsorbed, and not adsorbed via surface complexation with soft metals included in the illite structure as trace impurities. Soft metals can form extremely strong complexes (and sparingly soluble phases) with I-. Finally, P Kd values on illite showed a strong pH dependency. Importantly, I- sorption to illite was substantial even under alkaline conditions. It is not clear whether soil scientists have underestimated the amount of positively-charged surface sites on illite at alkaline pH values or whether our use of extremely low F masses (10-12 M) lead to the high Kd values. Future tests should vary F concentrations to see if the positively charged surface sites rapidly saturate, exhibiting a non-linear adsorption isotherm. Together, these experiments suggest that illites may be largely responsible for F sorbed to the subsurface arid soils used in this study. The illite appears to have adsorbed I predominantly by reversible physical adsorption to the pH-dependent edge sites, and not by irreversible fixation into the illite structure. Illites may constitute a substantial proportion of the clay-size fraction of many arid sediments and therefore may play an important role in retarding P movement in these sediments. However, for risk- or performance-assessment considerations it may be important to keep in mind that P could easily desorb from illites, and presumably natural sediments, should the concentration of competing anions or pH levels increase.
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ACKNOWLEDGMENTS The authors thank Dr. James Amonette (PNNL) for supplying us with mineral isolates and help in interpreting the data. This work was supported by the U.S. Department of Energy under contracts with Westinghouse Savannah River Company (DE-AC09-89SR-18035) and the Pacific Northwest National Laboratory (operated by Battelle Memorial Institute, DE-AC0676RLO 1830). This work was funded by the Hanford Immobilized Low-Activity Waste Performance Assessment Project. REFERENCES 1. R.J. Lemire, J. Paquette, D.F. Torgerson, D.J. Wren, and J.W. Fletcher, Atomic Energy of Canada Limited Report, AECL-6812 (1981). 2. D.C. Whitehead, J. Soil Sci. 25, 461 (1974). 3. B.B. Allard, B. Torstenfelt, K. Andersson, and J. Rydberg, in Scientific Basis for Nuclear Waste Management, Vol. 2, edited by C.J.M. Northrup, (Mater. Res. Soc. Proc. 2, 1980) pp. 673-679. 4. R.A. Couture and M.A. Seitz, Nucl. Chem. Waste Management. 4, 301 (1983). 5. N. Hakem, B. Fourest, R. Guillaumont, and N. Marmier, Radiochimica Acta 74, 225 (1996). 6. K.V. Ticknor and Y.H. Cho, J. Radioanal. Nucl. Chem. 140, 75 (1990). 7. Y. Muramatsu, S. Uchida, P. Sriyotha, and K. Sriyotha, Water Air Soil Pollution. 49, 125 (1990). 8. M. Sazarashi, Y. Ikeda, R. Seki, and H. Yoshikawa, J. Nucl. Sci. Technol. 31, 620 (1994). 9. K. Ticknor, V.P. Vilks, and T.T. Vandergraaf, Applied Geochem. 11, 555 (1996). 10. S.K. De, N.S.S. Rao, C.M. Tripathi, and C. Rai, Indian J. Agric. Chem. 4, 43 (1971). 11. D.I. Kaplan and R.J. Semne, PNL-10379, SUP. 1, (Pacific Northwest National Laboratory, Richland, WA, 1995), p. 57. 12. S. Assemi and H.N. Erten, J. Radioanal. Nucl. Chem. 178, 193 (1994). 13. M.I Sheppard, D.H. Thibault, J. McMurry, and P.A. Smith, Water, Air Soil Pollution, 83, 51 (1994). 14. J. Bors, H. Erten, and R. Martens, Radiochimica Acta. 52/53, 317 (1991).
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