HR: 14:10h
AN: B12D-03    [PDF]
TI: Molecular Simulation of Electron Exchange Between Ferrous and Ferric Iron in Hydrolyzing Aqueous Solutions
AU: * Rustad, J R
EM: rustad@geology.ucdavis.edu
AF: Department of Geology University of California-Davis, One Shields Avenue, Davis, CA 95616-8605 United States
AB: We have constructed a molecular model for ferrous-ferric electron transfer in hydrolyzing aqueous solutions. The model is explored at two pH values, where the pH is defined by the influence of added protons on the ratios of the hydrolysis species of ferric iron. We find two important effects on electron transfer rates as a funciton of pH. First, it is shown that the barrier to electron transfer at any given ion separation increases with increasing pH due to the stabilization of ferric iron. A similar increase in the transfer barrier would result, for example, from increasing the dielectric constant, effectively pinning the electron on the ferrous ion. The second effect, which acts opposite to the first, is the decrease in the potential of mean force between the ferrous and ferric ions as the pH increases. These two contributions, of opposite sign, are of approximately equal magnitudes. Electron transfer reorganization energies for the hydrolysis species show a surprising degree of overlap, indicating that fluctuations in hydrolysis state can be viewed on a continuum with other solvent contributions to the reorganization energy. This continuum picture is odds with current interpretations of the pH dependence of the transfer rate, which ascribe the observed increase in rate with increasing pH to hydrogen atom transfer involving Fe$^{2+}$-FeOH$^{2+}$, where the FeOH$^{2+}$ species present at vanishingly small concentrations.
DE: 0330 Geochemical cycles
DE: 1045 Low-temperature geochemistry
SC: Biogeosciences [B]
MN: 2003 Fall Meeting