Impact of green rust structure on anion exchange processes
Impact de la structure des rouilles vertes sur les processus d'échanges anioniques
Résumé
Layered materials such as clay minerals or oxides contribute significantly to soil reactivity because of their large specific surface area that promotes adsorption/desorption reactions.
Among them, Foug`erite is a Fe(II)/Fe(III) layered double hydroxide that was identified twenty
years ago in hydromorphic soils. Foug`erite has a layer structure resembling that of the so-called
Green Rust (GR) whose color is due to the mixed Fe valence in their structure. GR layers are
positively charged because of the presence isomorphic substitutions (e.g. Fe3+ in place of Fe2+)
and possibly vacancies. Layer charge compensation at the mineral scale is ensured by anions
located in the interlayer space. These interlayer anions can be exchanged with anions present in
surrounding pore water. Consequently, GR participate to the biogeochemical cycling of anions
in the environment, including nutrients or pollutants. In this study, we carried out a detailed
analysis of green rust structure in order to better quantify these biogeochemical cyclings. In
particular, we performed laboratory and synchrotron experiments that allowed deciphering the
mechanisms of chloride for sulphate exchange. The effect of particle size was investigated using
two types of samples, one being nanometric and the other one micrometric, that we synthesized
and characterized. It could be demonstrated that sulphate replaces chloride in the interlayer
space. These experiments shed light on the relationship between structure and reactivity of
these phases.