Cadmium adsorption onto Fe and Mn oxides – Implications for Cd isotopic fractionation

B.Sc. Nasrin Moradikoochi

Master's thesis

Abstract:

Abstract Cadmium is a naturally occurring element, but it is also found in higher concentrations in the environment as a result of anthropogenic activity. Cadmium is potentially toxic to the environment, so it is critical to identify the pathways of these contaminants that may be present in the environment. Instrument and technology advancements, such as TIMS development (Thermal Ionization Mass Spectrometry) and MC-ICP-MS (Multi-Collector Inductively Coupled Plasma Mass Spectrometry), have enabled the analysis of non-traditional isotopes. Cadmium isotopes have traditionally been challenging to analyze in terms of isotopic fractionation. To determine the processes by which this element becomes isotopically enriched or depleted in the different environments, it is critical to understand how it reacts with soil phases and in an aqueous solution. The primary goal of this thesis was to determine the effect of pH and ionic strength on Cd adsorption as well as the adsorption and isotopic kinetics of Cd onto two different Fe oxides the present in soil, ferrihydrite, and goethite. For our experiment, we chose synthetic ferrihydrite and goethite as representatives. The study's main goal was to provide a more in-depth understanding of the interaction between the surface chemistry of each soil phase and Cd. Experiments were conducted to create adsorption edges of 1x10-4 M Cd solutions with various ionic strengths of 0.1M, 0.01M, and 0.001M NaNO3. Adsorption was assessed by varying the pH of each solution over a progressive pH values in an inert atmosphere. The adsorption of Cd was highly dependent on the specific mineral phase; it is worth noting that the effect of pH on each mineral adsorption varied depending on the element, implying that while Cd shares similar properties, it behaves differently. Furthermore, the adsorption and isotopic kinetics of Cd onto ferrihydrite and goethite has reached equilibrium in the 24 hours of times intervals, and according to acquired results, the isotopic kinetic of adsorption onto goethite was faster than ferrihydrite. According to the results of the experiments, Cd fractionation during adsorption was more significant in the adsorption onto ferrihydrite, the δ114/112Cd values for the 24 hour time interval have shown values at the corresponding standard deviation: i.e., 0.372 ± 0.025‰ for ferrihydrite and 0.106 ± 0.014‰ for goethite. Meanwhile, there was no discernible effect of solution ionic strength on ferrihydrite adsorbed Cd. Adsorption of Cd onto goethite reached equilibrium (89%) in 30 minutes, whereas ferrihydrite adsorption reached its peak (98%) after 24 hours. The results implied that a 24-hour equilibration time is sufficient to achieve both adsorption and kinetic equilibrium.

Keywords

cadmium, adsorption, isotopic fractionation, goethite, ferrihydrite

Language used: English

Date on which the thesis was submitted / produced: 30. 3. 2022

Thesis defence

• Supervisor: prof. RNDr. Michael Komárek, Ph.D.

Czech University of Life Sciences Prague

Faculty of Environmental Sciences

Master programme:
Environmental Geosciences