Sorption of iodine on minerals investigated by X-ray absorption near edge structure (XANES) and 125I tracer sorption experiments

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The sorption of I on a set of selected minerals was examined by: X-ray absorption near edge structure (XANES) and high concentration chemical sorption experiments, as well as very low concentration tracer (using 125I) sorption experiments. A XANES method was used to determine I 2, I(V) and I (-I) in supernatant solutions and slurries of minerals. This is, we believe, the first reported use of XANES to investigate the redox state of I that is sorbed on minerals. With this technique, it was determined that pyrite, biotite and magnetite sorb I from solutions containing 1000 μg ml −1 I. In the pyrite/water system, IO 3 − was reduced to what appeared to be I 2 in the XANES scans and sorbed on the minerals surface. Magnetite sorbed I − from solution but not IO 3 −. In contrast, biotite sorbed IO 3 − from solution but did not take up any I −. No changes in I redox state were observed in the magnetite and biotite experiments. Based on this association the presence of Fe(II) in the sorbing phase appears to be related to uptake, but not necessarily to a change in I redox. Results of the XANES experiments, in which high concentrations of 1 were used, were corroborated for IO 3 − with pyrite, magnetite and biotite at very low concentrations of 125I. Most of the other minerals tested sorbed small fractions of the 125I tracer, but this level of uptake could not be observed in XANES scans. To examine the relationship between pyrite and IO 3 −, experiments were conducted in which concentrations of Fe, I, S 2− and SO 4 2− were determined in the aqueous phase of slurries containing pyrite. In the presence of I (starting as IO 3 −), concentrations of Fe in solution were significantly lower than without I, even at pH values sufficiently low that Fe should stay in solution. The XANES spectra indicate that I 2 is present on this material, eliminating the possibility of FeI 2 formation where I is present as I −. The implication is that I 2, sorbed on the surface, inhibits release of Fe to solution (or promotes its precipitation) while S is oxidized to SO 4 2− and released.