This thesis work is aimed at examining the oxygen atom mixing between common iron(III) oxides and Fe(II)aq solution during the sorption process. First, O labeled iron(III) minerals were used in batch sorption reactors to study the exchange of oxygen atom within the Fe(II)aq – iron(III) oxide system. Results showed a much lesser extent of oxygen mixing than reported Fe mixing from previous studies, even when dissolved Fe(II) is present. The oxygen mixing extents are 3.95 % more for goethite and 9.66 % more for ferrihydrite than their respective controls within 14 days. A consecutive atom exchange kinetic study indicated that O exchange is not inhibited by surface Fe sorptive site saturation, and that after 40d sorption experiment, the maximum O exchange is 54.96 % across reactors with different initial solid loadings and Fe(II)aq levels. The second part of this work examined the influence of aqueous Fe(II) on the sorption of selenium oxyanions (selenite and selenate) in Fe(II)aq – iron(III) systems. It was found that Fe(II)aq promotes selenium oxyanion sorption, and potential electron transfer occurred at the interface of goethite and selenite. Iron exists in nature in various forms including minerals in underground formations, freshwater and seawater sediments and in almost any living organisms. The diverse electrochemical properties of elemental Fe and its compounds have found way in many environmental applications; for example, Fe powder is used in permeable reactive barriers for groundwater remediation; Ferrate (+6 valent iron, FeO4 ) is used as an environmentally-friendly disinfectant for water and wastewater treatment. In recent decades, sorption of iron(III) oxides in reducing, Fe(II)-rich environment was extensively studied, with the advancement of spectroscopic techniques, researchers have found that the presence of aqueous Fe(II) changes the surface sorption activity of Fe minerals. Evidence of heavy metal incorporation and release, electron transfer between phases and through bulk solids, and the observation of recrystallization on the surface of the substrate mineral structures have positively confirmed the catalytic effects of aqueous Fe(II) on mineral-water interactions. More recent studies with Fe isotope tracers found near-complete mixing of structural Fe atoms with bulk solution dissolved Fe(II).