Intellectual Merit: Biogeochemical iron cycling involves primarily the alternation of iron between Fe(II) and Fe(III) oxidation states. This iron redox cycling is connected to the biogeochemical cycles of carbon, oxygen, phosphorus, and sulfur, and plays an important role controlling the fate of contaminants such as arsenic, uranium, and trichloroethylene. Aqueous Fe(II) and Fe(III) oxide minerals often coexist during biogeochemical iron cycling, and secondary abiotic reactions between these species may transform poorly crystalline iron oxides into more crystalline phases, fractionate iron isotopes, affect contaminant fate and speciation, and possibly acts as a negative feedback on microbial iron reduction. As these secondary processes have important geochemical and environmental implications, we need to obtain a mechanistic understanding of the fundamental reactions that occur between aqueous Fe(II) and Fe(III) oxides. Recent studies have observed electron transfer and atom exchange between aqueous Fe(II) and crystalline Fe(III) oxides. Preliminary measurements reveal that the reaction of Fe(II) with hematite (α-Fe2O3) varies with crystallographic orientation, with the (001) surface experiencing growth and other surfaces dissolution. Similar effects were seen at pH 3 and 7. As these reactions appear to be independent of Fe(II) adsorption and do not affect the bulk mineralogy or fluid composition, they are effectively ?hidden? redox processes. We hypothesize that Fe(II) serves a catalytic role, with iron atoms transferring to (001) surfaces through solution as Fe(II), and electrons transferring away from this surface through the hematite structure. The objectives of this proposal are to: (1) characterize the nature of the dynamic hematite surface redox processes operating in the presence of Fe(II), including how they vary with solution conditions and time and whether they are continuous of self-limiting; (2) determine how this coupled growth and dissolution is affected by the presence of the common inorganic species aluminum, phosphate, and silicate, all known hematite growth modifiers; (3) determine how these processes affect the speciation of structurally compatible [Ni(II)] and incompatible [As(V)] contaminants; and (4) investigate whether these processes can be activated during reductive dissolution by sulfide, an important process in marine sediments. These studies are expected to demonstrate a new complex abiotic interfacial redox process that occurs when biogeochemical iron cycling creates systems with coexisting Fe(II) and Fe(III) species. These studies may also provide new insight into iron biomineralization and nanoparticle synthesis. Finally, similar processes may occur for many elements that coexist in different oxidation states having different solubilities, such as S, Mn, or U; the expected results may thus serve as a guide for exploring complex redox processes in other geochemical systems. Broader Impacts: This project will facilitate the training of two new graduate researchers in the field of biogeochemistry. It will also allow a number of undergraduate researchers (2-3 per year) to be educated in the practice of science, training them in the formulation of research questions and the use of the tools and methods needed to answer scientific questions. Each graduate researcher will be given the opportunity to mentor an undergraduate student as part of their preparation as future educators. Results of this research will be incorporated into a graduate course taught by the PI. This research also may have societal impacts, as the studies of contaminant fate during these surface redox reactions may provide the basis for future development of new remediation strategies or methods to recharge water filtration systems.
