The use of stable metal isotopes in environmetal geochemistry

Robert Zachary Porter

Master's thesis

Abstract:

Zinc and cadmium are naturally occurring elements but are also present at increased levels in the environment as a result anthropogenic activity. Both elements, specifically cadmium are potentially toxic from an environmental standpoint and is it vital to identify the pathways these contaminants may take and in what form they are present. Advances in instrumentation and technologies such as the development of TIMS (Thermal Ionization Mass Spectrometry) and MC-ICPMS (Multi-Collector Inductively Coupled Plasma Mass Spectrometry) have made it possible to analyze non-traditional isotopes. Traditionally, zinc and cadmium isotopes were unable to be readily analyzed in terms of isotopic fractionation. To help understand the processes in which these two elements become either isotopically enriched or depleted, it is essential to know how they react with soil phases in question.The primary objective of this thesis was to evaluate the pH and ionic strength’s influence on the adsorption of zinc and cadmium on four representative soil phases, i.e., varying crystalline Fe oxides, Mn oxides and clay minerals. As the representatives, synthetic 2-line ferrihydrite, goethite, synthetic birnessite, and illite clay were chosen for the purpose of our experiment. The main aim of the work was to provide a more in-depth view into the interaction between each soil phases’ surface chemistry with zinc and cadmium, respectively. Experiments were performed to construct adsorption edges of 1x10-4M solutions of both Zn and Cd at varying ionic strengths of 0.1M, 0.01M, and 0.001M NaNO3. The adsorption was evaluated by adjusting the pH of each solution through a progressive range of pH values under an inert atmosphere. The adsorption of both Zn and Cd was highly dependant on the specific mineral phase; it is worth noting that the influence of pH on each minerals adsorption also varied on the respective element, inferring that although both metals share similar properties, they behave differently. Birnessite exhibited the smallest range of pH required to transition from minimal to maximum adsorption of Zn, and for Cd, ferrihydrite was found to be the most dependant. Experimental research provided by this thesis offers access to parameters essential to imminent isotopic fractionation studies regarding the preferential adsorption of Zn and Cd on each soil phase. Results from this study will help provide additional insight into the surface chemistry of these representative phases. To conclude, the comparison of these experimental results can aid in the fitting of experimental parameters to equilibrium models. As visible from the literature review dealing with fractionation of Cd/Zn during the sorption onto the various soil mineral phases, there are still significant gaps in knowledge. Because of that, this study will serve as a basis for large study aiming to elucidate the isotopic fractionation of Zn/Cd during the sorption on various soil phases and its connection with the types of surface complexes formed.

Keywords

Zinc, Cadmium, Adsorption, Isotopic Fractionation, Soil, Geochemistry

Language used: English

Date on which the thesis was submitted / produced: 17. 4. 2019

Thesis defence

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

Czech University of Life Sciences Prague

Faculty of Environmental Sciences

Master programme / field:
Geology / Environmental Geosciences