SORPTION OF IODINE ON MINERALS INVESTIGATED BY X-RAY-ABSORPTION NEAR-EDGE STRUCTURE (XANES) AND I-125 TRACER SORPTION EXPERIMENTS

The sorption of I on a set of selected minerals was examined by: X-ray absorption near edge structure (XANES) and high concentration chemica l sorption experiments, as well as very low concentration tracer (usin g I-123) 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 th e redox state of I that is sorbed on minerals. With this technique, it was determined that pyrite, biotite and magnetite sorb I from solutio ns containing 1000 mu g ml(-1) I. In the pyrite/water system, IO3- was reduced to what appeared to be I-2 in the XANES scans and sorbed on t he minerals surface. Magnetite sorbed I- from solution but not IO3-. I n contrast, biotite sorbed IO3- from solution but did not take up any I-. No changes in I redox state were observed in the magnetite and bio tite 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 hi gh concentrations of I were used, were corroborated for IO3- with pyri te, magnetite and biotite at very low concentrations of I-125. Most of the other minerals tested sorbed small fractions of the I-125 tracer, but this level of uptake could not be observed in XANES scans. To exa mine the relationship between pyrite and IO3-, experiments were conduc ted in which concentrations of Fe, I, S2- and SO42- were determined in the aqueous phase of slurries containing pyrite. In the presence of I (starting as IO3-), concentrations of Fe in solution were significant ly lower than without I, even at pH values sufficiently low that Fe sh ould stay in solution. The XANES spectra indicate that I-2 is present on this material, eliminating the possibility of FeI2 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)wh ile S is oxidized to SO42- and released. (C) 1998 Elsevier Science Ltd . All rights reserved.