Role of phosphate species during the formation and transformation of the Fe(II-III) hydroxycarbonate green rust

The Fe(II-III) hydroxycarbonate green rust, GR(CO32-), can be prepared by oxidation of aerated aqueous suspensions of Fe(II) hydroxide. The effects of phosphate species on the formation and transformation of GR(CO32-) were then studied by adding Na2HPO4 before the precipitation of the initial Fe(II) hydroxide or once GR(CO32-) was obtained. The final products of the oxidation processes were characterized by X-ray diffraction (XRD) and Raman spectroscopy. When Na2HPO4 was added to a suspension of well crystallized GR(CO32-), various effects were observed depending on the Na2HPO4 concentration. (i) The formation of alpha-FeOOH was inhibited. (ii) Nanophase lepidocrocite was the main final product for the lowest phosphate concentrations (0.001 M, that is Fe/P = 120). (iii) Ferric GR(CO32-), a Fe(III) oxyhydroxycarbonate keeping a layered structure similar to that of GR(CO32-), was the only final product when [Na2HPO4] was increased. When phosphate species were present together with Fe2+, OH- and HCO3- ions before precipitation, the crystallization and growth of GR(CO32-) particles were inhibited. The intermediate Fe(II-III) product was characterized for the first time. It is amorphous and may consist of nanoparticles showing a local ordering of atoms similar to that of GR(CO32-), isolated from each other by adsorbed phosphate species. Its oxidation led to ferrihydrite. The effects of phosphate species may also depend on the type of GR considered. (c) 2006 Elsevier B.V. All rights reserved.