MAGNETITE (111) SURFACE STRUCTURE AND REACTIVITY UNDER ENVIRONMENTALLY RELEVANT CONDITIONS

Mineral surfaces, particularly Fe-(hydr)oxides, play a predominant role in controlling the composition of natural waters and regulating transport and bioavailability of aqueous contaminants as the mineral surface reactivity is directly related to the surface structure and composition. X-ray crystal truncation rod (CTR) diffraction was used to determine surface structure, relaxations, and chemical identity of surface moieties of a magnetite (111) surface. Exposing the magnetite (111) to hydrated conditions resulted in a surface with two co-existing structural domains that continues undergo structural modification as a function of a hydration time (hours to days) where the mixed-iron termination lattice sites are becoming less occupied with time indicating that the magnetite (111) surface is undergoing a weathering-type process. To control and further understand how the surface structure/composition changes with varying environmentally relevant redox conditions, electrochemical impedance spectroscopy (EIS) was used where the resultant surface was then characterized using in-situ CTR, X-ray surface diffraction phase and texture analyses, and flow injection analysis of Fe(II) along with ex-situ atomic force microscopy (AFM). The EIS results show the oxidized and then re-reduced magnetite surface has a slightly higher resistance compared to the initially reduced surface, and the surface became extremely rough as no CTR’s were obtained. However, X-ray surface diffraction phase and texture analyses found the co-existence of textured iron-oxide phases of hematite and goethite in the near surface region. Complementary AFM images show the formation of surface precipitates suggesting the ordered surface is buried under these globules and the presence of other iron-oxide phases. The terminating surface structure of oxidized and reduced magnetite (111) surfaces determined as a function of time and variable redox conditions is used to model a pathway for how a magnetite surface might evolve under environmental conditions in nature and its implication on the surface reactivity will be discussed.