The Formation of Green Rust and Its Transformation to Lepidocrocite

The formation of Fe n,m hydmxy salts (green rusts) from Fe n sulphate and chloride solutions and their transformation to lepidocrocite, 7-FeOOH, was studied at ambient temperature, constant pH of ~7.0 and under controlled air flow. The formation of green rust and its subsequent transformation to lepidocrocite could be distinguished by the reaction rate and solution chemistry. The SO4 2-and CI-concentrations in solution attain a minimum at the break between the two reaction steps and are completely restored to their initial values after complete transformation to lepidocrocite. While the green rust is being formed, its FeU/FO n ratio shows a minor decrease and its Fern/anion ratio a minor increase. These ratios, however, deviate from the ideal pyroaurite-type structure (M2+/M 3+ = 3; M3+/A = 1; M: metal; A: anion) and possibilities to accommodate this deviation are discussed. A period during which no protons were produced (i.e. no base consumed) was found to separate the two reactions in the chloride, but not in the sulphate system. During this period a partial solid state oxidation of the green rust took place, leading to its destabilization and eventual decomposition. Green-blue iron hydroxide compounds occur under reducing and weakly acid to weakly alkaline conditions as intermediate phases in the formation of Fe oxides (goethite, lepidocrocite, magnetite). Because these green compounds were first discovered in conjunction with corrosion of steel in earth surface environments, they were called green rusts. Their well-known structure is of a pyroaurite type. It consists of sheets of Fen(OH)6 octahedra (called hydroxide layer in this article) in which some of the Fe n is replaced by Fe m, creating a positive layer charge which is balanced by anions located between the layers and sulphate are the most common interlayer anions, but a carbonate form of green rust also exists (McGill et al. have identified green rust in an ochre sludge in Denmark. The oxidation of Fe(OH)2 to FeOOH via green rust in unbuffered chloride and sulphate systems was extensively studied by Derie & Ghodsi (1972) and Detournay et al. (1974, 1976). They monitored the overall degree of oxidation, pH, Eh, C1-and SO42-concentration in solution and the 02 consumption. During the reaction, the pH dropped from 8 to below 4 and the Eh rose from-0.5 V to +0.4 V with strong intermediate buffering at pH-6 and an Eh of-0 V. A break in the rate of oxidation was combined with a minimum …