Trapping of Cr by Formation of Ferrihydrite during the Reduction of Chromate Ions by Fe(II)−Fe(III) Hydroxysalt Green Rusts

Hexavalent chromium, a byproduct of many industrial processes, is toxic and produces mobile aqueous oxyanions, whereas Cr(III) is relatively immobile in the environment and, moreover, essential in human glucidic metabolism. For this reason, Fe(II)−Fe(III) layered double hydroxysalt green rusts, recently identified as a mineral in hydromorphic soils, were evaluated as potential Fe(II)-bearing phases for hexavalent chromium reduction. Both considered synthetic varieties, the hydroxysulfate GR(SO4 2-) and the hydroxychloride GR(Cl-), proved to be very reactive; their interaction with potassium chromate solutions leads to the rapid and complete reduction of Cr(VI) into Cr(III). The Cr(III)-bearing solid phase, studied by X-ray diffraction, Mössbauer, X-ray photoelectron, and Raman spectroscopies, was determined to be a poorly ordered Cr(III)−Fe(III) oxyhydroxide, similar to the “2 the line ferrihydrite”. The comparison between the experimental redox potential and pH values for a theoretical equilibrium diagram bearing Cr and Fe phases indicated that the solubility of this solid solution, which may govern the behavior of chromium in the environment, is of the same order as that of pure Cr(OH)3.