Molybdenum, vanadium and iron are important micro and macro nutrients, respectively, for all living organisms. Their cycles and budgets in the natural environment is affected by - and affects many different processes.\ud \ud Poorly ordered ferrihydrite nanoparticles characterised for their particle size (3-5nm), crystallinity, surface area (~200m2g-1) and surface charge (point of zero charge =\ud 7.96) helped to understand better their adsorption and co-precipitation interactions mechanisms with molybdenum and vanadium.\ud \ud Comparative studies of molybdenum and vanadium adsorbed onto or coprecipitated with ferrihydrite as well as the influence of different conditions such us pH, metal concentration, particles concentration and matrix composition were carried out. The obtained kinetic parameters were modelled with various geochemical software packages to evaluate their behavior.\ud \ud Metastability of ferrihydrite under hydrothermal conditions was the second big theme of this thesis. The crystallisation kinetic (transformation rates) and thermodynamics (activation energies) of the hematite formation were assessd with in situ synchrotronbased\ud difraction tehnique in the presence and the absence of molybdenum and vanadium under conditions mimicing the geochemistry of deep sea hydrothermal systems (ionic\ud strength = 0.7, pH = 8). The data showed that hematite crystallization (from ferrihydrite) followed a temperature dependent trends and that the transformation requires an apparent activation energy of 26 kJmol-1 . The presence of molybdenum and vanadium delayed the transformation reaction by 32% and 38% respectively.\ud \ud The transformation also leads to the sequestration of more than 90% of the initial ferrihydrite associated molybdenum and vanadium in the hematite structure, making it thus\ud non-bioavailable for further reaction.\ud \ud Finally, synchrotron-based X-ray Absorption Spectroscopy revealed that the initial molybdate speciation in the starting ferrihydrite changes bonding and coordination in the end-product hematite and molybdenum replaced iron in the hematite structure further supporting the fact that molybdenum is immobilized in the hematite structure.\ud \u
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