The isotopic fractionation of cadmium during adsorption on goethite and ferrihydrite

Bc. Bakhtiyar Mammadov

Master's thesis

Abstract:

The isotopic fractionation can be defined as the distribution of relative amount of heavier and lighter isotopes between the two coexisting phases. This distribution of isotopes can be driven by kinetic or equilibrium effect. The equilibrium isotope fractionation of metals can occur during formation of different complexes due to thermodynamic differences in the bonding environments. It is important to investigate the impact of sorption reactions on the isotopic fractionation of Cd before it can be applied as isotopic signature for the tracing the different environmental processes and sources. The study of Cd isotopic fractionation during sorption on goethite and ferrihydrite was conducted to investigate the difference in isotopic fractionation of Cd at the Fe-oxyhydroxide/water interface. Our result demonstrates that the isotopes of Cd are fractionated during adsorption on goethite and ferrihydrite with the heavier isotope enrichment in aqueous solution. According to our experimental data, we observed predominantly the equilibrium isotopic effect since the fractionation of Cd proceeded through the trend line of isotopic equilibrium. The calculated average equilibrium isotopic fractionation values of Cd during adsorption on goethite and ferrihydrite in aqueous solution were ∆114/110Cdsorbed-aqueous = +0.48‰ and ∆114/110Cdsorbed-aqueous = +0.18‰, respectively. We hypothesize that the different magnitudes of Cd fractionation in aqueous solution can be associated with the difference in bond valence of Сd-O and different complexation mechanisms that could be driven by coordination chemistry of originating metal complexes. The presented results need further investigation since we did not provide the directly measured data on isotopic fractionation of Cd on studied minerals. The data from EXAFS analyses (currently under preparation) could also further elucidate the relations between the measured adsorption and isotopic data and explain more the complexation processes involved.

Keywords

cadmium, adsorption, isotopic fractionation, ferrihydrite, goethite

Language used: English

Date on which the thesis was submitted / produced: 11. 12. 2020

Thesis defence

• Supervisor: Ing. Zuzana Vaňková, Ph.D.

Czech University of Life Sciences Prague

Faculty of Environmental Sciences

Master programme / field:
Geology / Environmental Geosciences