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 (K-d'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 (K-d = 0.04 +/- 0.01 mL/g), chlorite (K-d = -0.22 +/- 0.06 mL/g), goethite (K-d = 0.10 +/- 0.03 mL/g), montmorillonite (K-d = -0.42 +/- 0.08 mL/g), quartz (K-d = 0.04 +/- 0.02 mL/g), or vermiculite (K-d = 0.56 +/- 0.21 mL/g). Conversely, a significant amount of I- sorbed to illite (K-d = 15.14 +/- 2.84 mL/g). Treating the I-125(-)- laden illite mixtures with dissolved F-, Cl-, Br-, or I-127(-), 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 K-d 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 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. References: 29
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