Environ. Sci. Technol. 2000, 34, 399-405
Iodide Sorption to Subsurface Sediments and Illitic Minerals D A N I E L I . K A P L A N , * ,† R . J E F F S E R N E , ‡ KENT E. PARKER,‡ AND IGOR V. KUTNYAKOV‡ Westinghouse Savannah River Company, Aiken, South Carolina 29808, and Pacific Northwest National Laboratory, Richland, Washington
Laboratory studies were conducted to quantify and identify the key processes by which iodide (I-) sorbs to subsurface arid sediments. A surprisingly large amount of I- sorbed to three alkaline subsurface sediments that were low in organic matter content; distribution coefficients (Kd’s) ranged from 1 to 10 mL/g and averaged 3.3 mL/g. Experiments with pure mineral isolates, similar to the minerals identified in the clay fraction of the sediments, showed that there was little or no I- sorption to calcite (Kd ) 0.04 ( 0.01 mL/g), chlorite (Kd ) -0.22 ( 0.06 mL/g), goethite (Kd ) 0.10 ( 0.03 mL/g), montmorillonite (Kd ) -0.42 ( 0.08 mL/g), quartz (Kd ) 0.04 ( 0.02 mL/g), or vermiculite (Kd ) 0.56 ( 0.21 mL/g). Conversely, a significant amount of Isorbed to illite (Kd ) 15.14 ( 2.84 mL/g). Treating the 125I-laden illite mixtures with dissolved F-, Cl-, Br-, or 127I-, caused 43 ( 3%, 45 ( 0%, 52 ( 3%, and 83 ( 1%, respectively, of the adsorbed I- to desorb. Finally, I- 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. An appreciable amount of I- sorbed to illite even under alkaline conditions. These experiments suggest that illite removed I- from the aqueous phase predominantly by reversible physical adsorption to the pHdependent 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 Imovement in these sediments.
Introduction Iodine-129 is commonly among the largest contributors to the calculated health risk associated with long-term nuclearwaste disposal in the subsurface (1). The causes for the large risk include large inventories of 129I in many types of waste, its long half-life (1.6 × 107 yr), and a perceived high mobility (i.e., low adsorption tendency) through oxidized, low organic matter, alkaline environments. The high mobility of iodine is primarily due to its anionic nature in groundwater. It exists primarily as iodide (I-) or iodate (IO3-), both of which are repulsed from the negative surface charges on most sediments. Of these two species, the more reduced form, I-, is more commonly found in naturally oxygenated groundwaters (2). Iodate tends to exist only in highly oxygenated and alkaline systems and may be formed by interaction with radiolysis products (3). * Corresponding author phone: (803)725-2363; fax: (803)725-4704; e-mail:
[email protected]. † Westinghouse Savannah River Company. ‡ Pacific Northwest National Laboratory. 10.1021/es990220g CCC: $19.00 Published on Web 12/17/1999
2000 American Chemical Society
Iodide sorption to natural sediments has been closely correlated to organic matter concentrations, Fe-oxide concentrations, and pH levels of the sediments (2, 4-7). Iodide sorption to organic matter and Fe-oxides has been attributed to interaction with positively charged surface sites on these pH-dependent charge materials (2, 5). As the pH decreases, the number of positively charged surface sites available for I- or IO3- sorption increases (5, 7). The iodine oxidation state also has a significant effect on the degree of sorption. Iodate sorbs appreciably more than I- to several minerals (4, 8). Couture and Seitz (4) reported that >99.99% of the IO3- and only 30% of the I- sorbed to hematite in a pH 7 system. They also reported that >26% of the IO3- and none of the I- sorbed to kaolinite in a pH ∼5 system. They postulated that IO3- specifically sorbed on hematite by formation of Fe-OIO2 bonds, thereby displacing OH-. This mechanism is analogous to selenite sorption by goethite (9), phosphate adsorption to hematite (10), and adsorption of other oxyanions by ferric hydroxide (11). Ticknor and Cho (8) reported that more IO3- than I- sorbed to granitic fracture-filling minerals. The cause for the 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 I-, 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 (8) reported no I- sorption to calcite or muscovite using a pH 7.7 synthetic groundwater dominated by Ca, Na, and Cl ions. Muramatsu et al. (6) reported essentially no sorption of I- from distilled water onto bentonite or Fe2O3. Sazarashi et al. (12) reported no Isorption to montmorillonite (10-6 M KI and a 5-day contact period). Ticknor et al. (13) reported low I- distribution coefficients (Kd’s) 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 SO4 ions. De et al. (14) reported either no adsorption or negative adsorption (anion exclusion) for I- onto montmorillonite and kaolinite suspensions in contact with 3.175 and 2.595 mg/L KI solutions, respectively. Anion exclusion can occur when anions are repelled from negatively charged mineral surfaces (15). Anion exclusion has been reported for pertechnetate, chloride, and nitrate, primarily in alkaline sediments with little or no organic matter (16, 17). Subsurface arid sediments are the most common type of sediments currently contaminated with radioiodine or are at threat to be contaminated by radioiodine through the future storage and disposal of nuclear waste (e.g., the Hanford Site near Richland, WA, and the Idaho National Engineering and Environmental Laboratory in Idaho Falls, ID). These sediments tend to contain very low concentrations of organic carbon,
