Evolution of vernadite crystal structure as a function of pH: Implications for reactivity
Résumé
Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environment. Similar to birnessite, its layers are built of (MnO6)8− octahedra connected through their edges and frequently contain vacancies and (or) isomorphic substitutions. Both create a layer charge deficit that can exceed 1 valence unit per layer octahedron and thus induces a strong chemical reactivity. In addition, vernadite has a high affinity for many trace elements and possesses a redox potential that allows for the oxidation of redox-sensitive elements or molecules. As a result, vernadite acts as a sink for many trace metal elements.
The present talk will report on the evolution of vernadite crystal chemistry as a function of pH using a variety of analytical techniques including modelling of powder X-ray diffraction patterns. In particular, the number of vacant layer sites, that of layer and interlayer Mn3+, the resulting average Mn oxidation degree, the Na/Mn ratio, unit-cell parameters, and crystallite size will be determined as a function of equilibration pH.
Reactivity of structurally contrasted samples with respect to metal sorption or to their conversion to tunnel structures will be reported next.