Role of nitrite 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 nitrite ions on the formation and oxidation of GR(CO32−) were then studied by adding NaNO2 just after the precipitation of Fe(OH)2 or once GR(CO32−) was obtained. The oxidation products were characterized by X-ray diffraction and Raman spectroscopy. In aerated conditions the oxidation of GR(CO32−) in the presence of nitrite species (0.004 to 0.1 mol L−1) is mainly due to dissolved O2. Nitrite however influences the transformation of GR(CO32−), promoting lepidocrocite at the detriment of goethite at 0.02 mol L−1 and hindering the crystallization of lepidocrocite at 0.1 mol L−1. In anoxic conditions GR(CO32−) transforms spontaneously into a mixture of magnetite, siderite and chukanovite whether nitrite species are present or not. The proportion of magnetite is larger with nitrite which can be attributed to a partial reduction of NO2− by GR(CO32−). Finally, nitrite ions are able to oxidize rapidly Fe(OH)2 into GR(CO32−) even in the presence of O2. With a sufficient nitrite concentration, the formation of GR(CO32−) is entirely due to nitrite reduction.