Surface charge and growth of sulphate and carbonate green rust in aqueous media

We report the first determination of the point of zero charge of sulphated and carbonated green rust particles. Green rust has been recognised as a prevalent mineral in environments such as hydromorphic soils, groundwaters and anoxic Fe(II)-rich water bodies, and the evolution of its net surface charge with pH has direct implications for the uptake of contaminants, metals and nutrients in such settings. We find that the surface of both sulphated and carbonated green rust is positively charged at pH < 8.3, whereas it is negatively charged at pH > 8.3. Thus, alkaline settings will promote enhanced adsorption of metallic cations. However, the behaviour of ionic species surrounding green rust is more complicated than that predicted by simple pH-dependent adsorption, as our experiments suggest that green rust likely grows via dissolution-reprecipitation during Ostwald-ripening. This implies that adsorbed species are potentially subject to repetitive steps of release into solution, re-adsorption and co-precipitation during particle growth. The growth rate of green rust particles is highest within the first 50 min of aging, and appears to decrease towards an asymptote after 200 min, suggesting that particle growth controls on the uptake of dissolved species will be most important during the early steps of green rust growth. Our findings thus contribute to a better understanding of the controls that green rust may exert on dissolved ions in a variety of anoxic environments.