Iron oxides are ubiquitous in Earth’s crust. Its redox reactions in the environment not only control the cycling and availability of important elements such as C, N, S, but also influence the speciation and mobility of contaminants. During the last few decades, significant advances have been made in understanding the reaction of Fe(II) with Fe(III) oxides. However, there are knowledge gaps in the mechanistic understanding, specifically on what drives Fe(II)-Fe(III) electron transfer. Despite substantial experimental evidence for Fe(II)-Fe(III) electron transfer, recent studies in computational chemistry calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited, unless surface defects are present. In this thesis, we explored the connections between surface defects and Fe redox chemistry. More specifically, we investigated how the presence of surface defects influence (i) Fe(II)-goethite electron transfer, (ii) microbial respiration of goethite, and (iii) the stability and reversibility of Fe(II)-goethite electron transfer. Isotope labeled 56Fe goethite was synthesized by hydrolysis and further hydrothermally treated to remove surface defects such as Fe vacancies. Goethites with more and fewer defects were reacted with 57Fe(II) and 57Fe Mössbauer spectroscopy and revealed that Fe(II)-goethite electron transfer is inhibited in goethite with fewer defects, demonstrating that surface defects play an important role in enabling electron transfer. We further used isotope-labeled (natural abundant or 56Fe) goethite to track the origin of the Fe(II) reduced by Geobacter sulfurreducens when goethite with more and fewer defects were both present. The isotopic composition of the microbially reduced atoms revealed that most of the reduced atoms arise from goethite containing more defects and demonstrate that surface defects facilitate the respiration of Fe(III) reducing bacteria. Finally, we used selective isotope labeling (56goethite + 56Fe(II) + 57Fe(II)) to investigate the fate of new Fe(II) that sorbed onto goethite that has been previously exposed to Fe(II). The longer goethite is pre-exposed to Fe(II), the more Fe(II)-goethite electron transfer was exposed, and we demonstrated that a passivation layer of sorbed Fe(II) contributed to the observed inhibition. Importantly, however, electron transfer was partially restored upon removal of the layer of Fe(II) by extraction or oxidation. Our results help resolve the disagreement between the computational chemical calculations and experimental observations. We further demonstrated that even small changes at the surface of iron minerals might change their bioavailability and determine which minerals will be reduced. Finally, in this work, we showed that electron transfer is a process that can be self-inhibitory unless the minerals are exposed to geochemical fluctuations, such as changes in pH and water table fluctuations.