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. F or this reason, Fe(II)-Fe(III) layered double hydroxysalt green rusts, rece ntly identified as a mineral in hydromorphic soils, were evaluated as poten tial Fe(II)-bearing phases for hexavalent chromium reduction. Both consider ed synthetic varieties, the hydroxysulfate GR(SO42-) and the hydroxychlorid e GR(Cl-), proved to be very reactive; their interaction with potassium chr omate 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, Mossb auer, X-ray photoelectron, and Raman spectroscopies, was determined to be a poorly ordered Cr(III)-Fe(III) oxyhydroxide, similar to the "2 the line fe rrihydrite". The comparison between the experimental redox potential and pH values for a theoretical equilibrium diagram bearing Cr and Fe phases indi cated that the solubility of this solid solution, which may govern the beha vior of chromium in the environment, is of the same order as that of pure C r(OH)(3).