Abstract
The sorption of iodide, iodate, and periodate ions by kaolinite, hematite and pelagic red clay in buffer solutions and sea water was studied. Iodate ions (IO 3 −) are strongly absorbed by hematite (Fe 2O 3) at pH values up to 9. The reaction is rapid and reversible and apparently occurs by replacement of OH- ions on the surface. Periodate ions (IO 4 −) are more strongly sorbed by Fe 20 3 than IO 3 − ions; iodide ions (I −) appear to be less strongly sorbed. At pH 4, hematite-free, goethite-free kaolinite sorbs IO 3 − only slightly and I − not at all. IO 3 − is stable relative to I − in the presence of oxygen, and occurs in surface waters and in seawater. Small amounts of finely divided iron oxides, which are present in many types of sediments, may play a major role in the geochemistry of iodine by absorbing IO 3 −. For example, in one experiment pelagic red clay was found to be about as active as reagent Fe 2O 3 in removing dissolved IO 3 − from seawater. This result suggests that pelagic red clay may be an appropriate host for disposal of radioactive iodine-129. Experiments on sorption of radioactive I − by sedimentary minerals are reported in the literature. In many of these experiments, some of the iodine is likely to have been in the IO 3 − or IO 4 − form, which would have been sorbed by iron oxide impurities. The results of sorption experiments with radioactive iodine are considered dubious unless the oxidation state and the mineralogical purity are established.
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