Radioiodide Sorption to Sediment Minerals

AbstractLaboratory 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.