Radioiodide Sorption to Sediment Minerals

Abstract

Laboratory studies were conducted to quantify and understand the processes by which iodide (I) sorbs to minerals found in subsurface arid sediments. Little or no I 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 I sorbed to illite (Kd = 15.14 ± 2.84 mL/g).). Upon treating the iodide-laden illite with dissolved F, Cl, Br, or 127I, desorption (or isotopic exchange in the case of 127I) removed, respectively, 43 ± 3%, 45 ± 0%, 52 ± 3, and 83 ± 1 % of the I originally adsorbed to the illite. The fact that such large amounts of I could be desorbed suggests that the I 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, 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. Importantly, I sorbed to illite even under alkaline conditions. Together, these experiments suggest that illite removed I from the aqueous phase predominantly by reversible physical adsorption to the pH-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 I movement in these sediments.

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Correspondence to D. I. Kaplan.

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Kaplan, D.I., Serne, R.J., Parker, K.E. et al. Radioiodide Sorption to Sediment Minerals. MRS Online Proceedings Library 556, 1059 (1998). https://doi.org/10.1557/PROC-556-1059

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