Removal of hexavalent chromium from solutions by mackinawite, tetragonal FeS

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 × 10−3 M and fixed ionic strength. Mackinawite suspensions (0.6 g L−1 and 1.2 g L−1) and Cr(VI) solutions were reacted for 4.5 h 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 130 mg Cr(VI)/g FeS at pH 7 and 240 mg 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.