Dissolution kinetics for the Fe(II)-Fe(III) layered double hydroxide, green rust

Abstract

Green rust is a Fe(II)-Fe(III) layered double hydroxide. It occurs in nature and as a corrosion product and can participate in a range of environmental redox reactions, making it an attractive agent for remediation. However, green rust is highly sensitive to the presence of O₂, thus it is challenging to observe in the field. This means that prediction of its behaviour is important. Here, the dissolution rate of green rust at circumneutral pH and about 27 °C was determined from flow-through batch experiments and interpretation of earlier reported dissolution experiments. The results yielded a surface area normalised, far-from-equilibrium dissolution rate, R (logarithmic, mol m⁻² s⁻¹):  log R = log k_GR - n pH = −6.64 (± 1.30) - 0.23 (± 0.15) pH, based on a measured surface area of 13 m² g⁻¹. Here, k_GR represents the rate constant at about 27 °C and n, a factor describing pH dependence. This expression provides slightly slower dissolution rates than observed for brucite and despite uncertainties it allows reasonable simulation of the dissolution behaviour reported for the layered double hydroxides hydrotalcite and pyroaurite. In addition, the determined dissolution rates can be used by experimentalists to evaluate if redox reactions in the presence of green rust might be preceded by its dissolution or not.