The Formation of Green Rust and Its Transformation to Lepidocrocite

Abstract

The formation of Fe^II,III hydoxy salts (green rusts) from Fe^II sulphate and chloride solutions and their transformation to lepidocrocite, γ-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 SO₄²⁻ and Cl⁻ 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 Fe^II/Fe^III ratio shows a minor decrease and its Fe^III/anion ratio a minor increase. These ratios, however, deviate from the ideal pyroaurite-type structure (M²⁺/M³⁺ = 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.