Crystal-chemistry of iron-bearing minerals and implications in the geochemical cycling of metal pollutants – CrIMin

In soils and sediments, chemical reactions at the interfaces between iron oxyhydroxides and water play an important role in numerous natural processes, from controlling the fate and transport of environmental contaminants to the biological availability and geochemical cycling of iron. In this context, the present project aims at taking advantage of the most recent theoretical developments in ab initio calculations to improve our understanding of the crystal-chemistry of iron-bearing minerals and of their implication in the geochemical cycling of minor and trace metal pollutants. To do so, the group of young mineralogists and physicists will focus on the three following tasks:
- The modeling of bulk iron-bearing minerals that will progress from pure bulk minerals to minerals containing cationic substitutions (chemical impurities) and atomic defects. The minerals considered will be the iron oxyhydroxides commonly found at the redox interfaces of soils, whose surface reactivity favours the oxidation or reduction of adsorbed molecules. The vibrational properties will be investigated in details in order to improve the interpretation of infrared and Raman spectra, and will lead to the determination of the equilibrium isotopic fractionation factors.
- The development of an innovating method to determine the isotopic properties of liquid solutions, which is currently the limiting step in the theoretical investigation of isotopic fractionation between solids and solutions. This will be applied to pure water first, before looking at solvated ions (i.e. Fe and metal pollutants like Cr, Zn, Ni).
- The modeling of solid-solution interfaces. First we will study the hydrated mineral surfaces. This will allow us to address the question of the isotopic signature of mineral surfaces. Then we will investigate the adsorption mechanisms of the metal elements. Structural and vibrational properties of sorption complexes will be compared with the experimental data that have been and will be obtained by our colleagues of the IMPMC laboratory in companion projects. These structural data will then be connected to the thermodynamical properties (reaction energies, isotopic fractionations).
The present project will provide a general theoretical framework for interpreting the experimental data derived from the different analytical techniques (e.g. diffraction techniques, vibrational spectroscopies, X-ray absorption spectroscopies, isotopic analysis). Beyond the results relevant to environmental mineralogy, the substantial methodological work that will be conducted in this project (improvement of the modeling of mineral phases containing transition elements and theoretical determination of the isotopic properties of liquid solutions) will find a wide range of applications in Earth Sciences, Material Sciences and Physics.