Sorption of anions of iodine by iron oxides and kaolinite

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.