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The ability of mackinawite, an iron(II) monosulphide, to reduce Cr(VI) was studied at pH values of 5 and 7, Cr(VI) solution concentrations ranging from 10-4 to 6 × 10-3M and fixed ionic strength. Mackinawite suspensions (0.6gL-1 and 1.2gL-1) and Cr(VI) solutions were reacted for 4.5h in a nitrogen atmosphere. The oxidation of mackinawite was determined by analysing both solutions and solid phases after Cr(VI) reaction. The solid phases were characterised by using complementary electrokinetic measurements and X-ray Photoelectron Spectroscopy and by Transmission Electron Microscopy. Solutions were analysed using colorimetric methods for Fe(II) and Cr(VI) and capillary electrophoresis for sulphur speciation. Mackinawite was found to be a very effective reductant of Cr(VI) under the conditions of the experiments. The removal capacity was determined to be around 130mg Cr(VI)/g FeS at pH 7 and 240mg Cr(VI)/g FeS at pH 5. The presence of an iron(III) and chromium(III) (oxyhydr)oxide layer covering the mackinawite crystals was evidenced with a constant Cr to Fe ratio of 1.4 ± 0.2. The extent of the metal (oxyhydr)oxide layer increases with the initial Cr(VI) concentration, thus probably limiting the Cr(VI) reduction at high initial Cr(VI) concentrations. [Copyright &y& Elsevier]

In the goal of finding efficient scavengers for radioiodide in conditions (pH, pE) close to those encountered in deep geological sites, sorption of iodide ions on cuprous sulfide minerals (especially roxbyite, Cu1.75S) has been studied. Surface analysis by X-ray photoelectron spectroscopy has shown that commercial cuprous sulfides are covered by an oxidized overlayer (mainly in the form of CuSO4). Therefore, a synthetic procedure to get roxbyite (typically by mixing Na2S with an aqueous suspension of commercial Cu2O) was applied to produce pure samples with clean surfaces. Batch equilibration of cuprous sulfide particles suspended in aqueous solutions containing iodide species has revealed significant consumption of iodide. The sorption mechanism involves the formation of a surface complex via the exchange of surface hydroxyl groups by iodide anions, as highlighted by a transient pH increase during the immobilization process. Other copper and mixed copper–iron sulfides (e.g. CuS, CuFeS2), which are stable over wide pH and potential ranges are also likely to accumulate iodide species. Because of the specific interaction between iodide and copper(I) centers on the minerals, high distribution coefficients (>1000 ml/g) were observed. [Copyright &y& Elsevier]