Book Description

Due to the abundance and redox potential of iron (Fe), the solubility and oxidation state of trace metals may be strongly influenced by redox reactions with Fe minerals. The ability of a Fe mineral to participate in electron transfer with trace metals, such as chromium (Cr), is dependent on a variety of properties and processes. These include, but are not limited to, surface area, mineral stoichiometry, and geochemical conditions, such as redox potential. This dissertation work examines Fe mineral transformations and electron transfer reactions with Cr(VI) under varying redox conditions. In the second chapter, the mineralogical transformations of ferrihydrite ((Fe10O14(OH)2)) are investigated during rapid, abiotic redox oscillations, at different concentrations of dissolved Fe(II). The stoichiometry of the secondary mineral magnetite (Fe3O4) increases overall throughout redox oscillations, despite repeated exposure to oxygen. This work demonstrates structural Fe(II) in Fe minerals may not be oxidized in the presence of oxygen and could be a source of electrons in redox dynamic environments. In the third chapter, goethite ([alpha]-FeOOH) is exposed to varying numbers of abiotic redox cycles (e.g., 1, 2, 3, or 4) and subsequently reacted with Cr(VI). The reduction of Cr(VI) to Cr(III) is observed, demonstrating that Fe(II) substituted within the goethite lattice during anoxic periods is not only preserved during oxic periods but also available for electron transfer. Regardless of the number of redox cycles to which goethite is exposed Cr consistently is associated with the (100) crystallographic face, a predicted site of electron conduction. Finally, the fourth chapter examines the electron transfer between Cr(VI) and the reduced quinone species, AH2DS in the presence and absence of goethite at three different ratios of AH2DS:Cr(VI). Goethite inhibits of the extent of Cr(VI) reduction to Cr(III) most significantly at the highest ratio of AH2DS:Cr(VI) investigated. Possible production of semi-quinone radical species may limit electron transfer and decrease the percent yields of Fe(II) and Cr(III). Additionally, solid phase characterization of Cr confirms the formation of Cr(OH)3 phases. Understanding abiotic electron transfer reactions that occur in systems with multiple redox active species is important to elucidate the contribution of abiotic redox reactions to biogeochemical cycling in natural soils.