Keoni Young, Milwaukee School of Engineering

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Abstract:

The Ce anomaly has been used for years as a paleoredox probe when analyzing ocean sediments, but the specifics of Ce’s redox chemistry have not been well characterized. Since Ce’s redox pathways are not well defined, its use as a proxy is questionable. The goal of this research is to gain a clearer understanding of Ce(III)’s oxidation while in complex with the organic ligand, desferrioxamine B (DFOB). This was achieved using spectrophotometry to gain kinetic data of Ce(IV)DFOB’s formation. Spectrophotometry was also used to identify one or more reductants that can slow the oxidation enough to define the Ce(III)DFOB complex through titration. Ascorbic acid was identified as a reductant that could not only slow, but also reverse the oxidation, and when used for titrations of Ce and DFOB, stability constants (logβ values) of 5.23, 8.44 and 11.31 were measured for the Ce(III)DFOB complex. These measurements are critical for describing the kinetics and mechanisms for Ce(III)DFOB’s oxidation to Ce(IV)DFOB. Curve fitting to kinetic data of Ce(IV)DFOB’s growth also showed a pH dependency in the reaction. Understanding more about Ce’s redox chemistry in this complex should affect how its presence in sediment is interpreted since this research indicates that Ce’s oxidation is more complicated than what it is currently assumed to be. Experimentation on tin was also performed as a comparison to Ce’s redox chemistry since Sn has some similarities in that it can also be oxidized to a valence of IV.