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 (K_d = −0.42 ± 0.08 mL/g), quartz (K_d = 0.04 ± 0.02 mL/g), vermiculite (K_d = 0.56 ± 0.21 mL/g), calcite (K_d = 0.04 ± 0.01 mL/g), goethite (K_d = 0.10 ± 0.03 mL/g), or chlorite (K_d = −0.22 ± 0.06 mL/g). A significant amount of I⁻ sorbed to illite (K_d = 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 K_d 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.