OBSERVATOIRE DE GRENOBLE et LABORATOIRE DE GEOPHYSIQUE INTERNE ET TECTONOPHYSIQUE THESE Physics of natural nanoparticles – water interfaces: chemical reactivity and environmental implications

Precise structural studies of nano-crystalline minerals using diffraction techniques have been hindered by the broad diffraction peaks found in their diffraction patterns. In this thesis, I have applied molecular scale techniques such as High-Energy X-ray Diffraction and Molecular Dynamics simulations to study the structure and reactivity of two nano-minerals of important environmental significance: imogolite and schwertmannite. These minerals have nanotube or channel-like structures, and both are strong anion adsorbers. Imogolite is a nanotubular aluminosilicate present in the clay fraction of volcanic soils. It has high specific surface areas and it is one of the few minerals reactive towards anions and cations, which makes it a very important mineral in soils where it is found. However, the prediction of imogolite chemical reactivity is hindered by its nano-crystalline character. Structural studies up to now have been restricted to X-Ray Diffraction and Electron Diffraction analyses, where the diffraction peaks were used mainly as fingerprints for the identification of the mineral. In this thesis I have performed a detailed structural characterisation of the structure of synthetic imogolite by Transmission Electron Microscopy (TEM), High-Energy X-ray and Molecular Dynamics methods. In addition, the structure of water at the imogolite / water interface has been investigated by theoretical and experimental methods. Using these structural inputs, I have developed a geochemical MUSIC model of imogolite, and compared it to the existing models for Gibbsite (planar equivalent of the external surface of imogolite), evaluating the effect of the curvature on geochemical reactivity. Parameters investigated include, the number of hydrogen bonds per surface adsorption site or the protonation constants of surface hydroxyl groups. Finally, these molecular-scale investigations have been linked to the biogeochemistry of Selenium in volcanic soils (andisols). Andisols are frequently rich in Selenium but controversially often low in bioavailable selenium. At the same time, imogolite is often found in the clay fraction of these soils. The adsorption of Selenium oxyanions at the Imogolite/water interface has been studied using X-ray Absorption Spectroscopy and DFT methods. In contrast to imogolite, schwertmannite structure is thought to be akin to that of akaganeite, with sulphate molecules substituting chlorine atoms in channels. The structure of the octahedral iron frame and the positions of the sulphate molecules within the structure have been object of an intense debate during the last 15 years. I present here a combined Pair-Distribution Function and X-ray Diffraction Study of the structure of schwertmannite. A structural model is proposed and discussed in terms of the retention of oxyanions. te l-0 07 71 93 7, v er si on 1 9 Ja n 20 13