Hudson, Jeffrey2023-01-062023-01-062022https://udspace.udel.edu/handle/19716/31988Electroanalytical methods are widely applicable in a variety of scientific fields and have proven to be a useful tool for fundamental and applied scientific investigations involving redox reactions and species. Over the past decades, electrochemical methods have been increasingly applied to investigate environmental processes. Environmental applications of electrochemistry range from large-scale environmental monitoring to bench-top investigations of fundamental processes at a micro-scale. This dissertation primarily applies two analytical techniques, voltammetry and mediated electrochemical oxidation (MEO), to directly investigate in situ redox cycling in estuarine and lacustrine environments, and to probe iron complexation and redox dynamics with organic ligands in a controlled laboratory setting. ☐ Voltammetric microelectrodes were used in conjunction with a pump-profiling system to measure concentrations of redox-active species (O2, MnII, FeII, H2S) associated with a stratified water column in Chesapeake Bay. Electrochemical measurements of high vertical resolution were achieved in the water column with the pumping system, which was also used to measure other constituents. Microelectrodes displayed high precision and accuracy when compared with other sensors onboard a research vessel, and measurements from two separate years showed distinct redox zonation between bottom and surface waters that were influenced by freshwater inputs from tributaries. ☐ Voltammetric microelectrodes were then applied to Arctic lacustrine systems near Toolik Lake, AK to investigate the distribution of redox-active species (O2, MnII, FeII, FeIII-complexes) associated with terminal electron acceptors (TEA). High vertical resolution profiles of TEAs in Toolik Lake sediment pore waters and iron-rich seeps were paired with microbiological analysis of iron and carbon cycling bacteria. Results enable the interpretation of relationships between the location the distribution of TEAs, while gaining insight into the influence of TEAs and other geochemical parameters on carbon cycling in the Arctic, which is particularly sensitive to climate change. ☐ MEO was used to investigate the influence of model organic ligands on FeII oxidation across fixed thermodynamic conditions (i.e. EH and pH). Iron oxidation reactions play an important role in biogeochemical processes, such carbon cycling and sequestration, as well as environmental transformation of nitroaromatic explosives. FeIII-stabilizing ligands containing oxygen or nitrogen donor atoms induced high-spin FeII and increased oxidation extent at lower pH by decreasing the thermodynamic stability of FeII. Additionally, a linear relationship exists between measured apparent reduction potentials (EHФ) of the Fe-ligand complexes and the ratio of their known stability constants (log KFe(III) /KFe(II)), thus enabling the determination of stability constant ratios of unknown ligands. This method improves on traditional probe compound methods that indirectly determine redox properties of complexes based on probe compound reduction kinetics. ☐ Finally, MEO was used in conjunction with the previously generated to broadly assess iron-dissolved organic matter (DOM) stability constant ratios. Preliminary results with 3 DOM isolates imply that weak Fe-DOM complexes form at low pH, while higher pH conditions enable the formation of strong FeIII-stabilizing complexes with low EHФ. Although a disadvantage of the method is that it does not allow for precise determination of individual FeII or FeIII-DOM stability constants, a weaker correlation between measured EHФ and known FeIII stability constants of model ligands allows for the estimation of strong FeIII-DOM stability constants, which are similar to other values reported in the literature.ElectrochemistryIronLigandsMediated electrochemical OxidationVoltammetryThesis1356965676https://doi.org/10.58088/ta2p-k3232022-08-10en