A stable isotopic exchange kinetic (SIEK) study of Chromium mobility in ultramafic systems

Chromium is one of the most potentially dangerous heavy metals which have been raising severe ecological and environmental problems. It occurs in the environment primarily in two valence states, trivalent chromium (Cr (III)) and hexavalent chromium (Cr (VI)), which may transform to each other by redox processes. Due to its increasing toxicity and associated consequences, it is crucial to explore the mobility and chemical behaviors of Chromium in contaminated soil and water, which could help understand and control its pollution. This makes the main objective of the thesis. In the first place, the mobility of Cr is investigated by quantifying the contribution of Cr-bearing mineral phases to the exchange pools of Cr(III) and Cr(VI), using the stable isotope exchange kinetics (SIEK) technique, as previously done for Nickel. Meanwhile, we also examine whether oxidation and reduction occur during the exchange and, if it is the case, which process appears to be faster. Furthermore, for each Cr-bearing phase, the adsorption is studied to figure out different mechanisms occuring for the Cr(III) and Cr(VI) surface interactions together with their relative contributions. The analyses are performed on pure Cr-bearing phases in laboratory and on natural samples collected in ultramafic systems.