Analysis of (bio)geochemical kinetics of Fe(III) oxides

The rates of abiotic mineral dissolution are commonly measured in batch or flow-through chemical reactors using established techniques and models. Interpretation of biogeochemical mineral dissolution rates in the presence of intact cells is more difficult and techniques for such approaches are not as well established. However, measurement and interpretation of the kinetics of enzyme-catalyzed reactions using both Michaelis-Menten and Monod models for purified enzymes and cell-containing systems respectively is well established for soluble substrates and may provide paradigms for analysis of such reactions with insoluble substrates. Two problems hinder the use of such approaches for biogeochemical mineral reaction systems. First, enzymes acting upon insoluble substrates for environmentally important reactions have generally not been isolated and purified. Second, little is known concerning how to model and interpret such characteristics as the catalytic efficiency of an enzyme that is transferring electrons to or from an insoluble substrate. These and other questions await new techniques and paradigms of study for mineral reactivity. Despite the complexities, a few generalizations can be made by comparing abiotic and biotic rates, and by comparing rates measured in vitro (i.e. fractions of cells interacting with mineral surfaces) and in vivo (i.e. intact cells interacting with mineral surfaces). This chapter summarizes approaches for analysis of both abiotic and biotic mineral reaction systems with examples drawn from reactivity of Fe (III) oxides. The rates of abiotic ligand-promoted, abiotic reductive, and biotic dissolution of goethite are shown to be slower than the rate predicted for abiotic proton-promoted dissolution of Fe(II) oxide, suggesting that this latter dissolution rate may be an upper limit for all reductive and nonreductive dissolution of Fe(III) oxides, including dissimilatory Fe(III) reduction. While comparison of in vivo to in vitro studies will become more useful in the future as environmentally important enzymes are isolated and purified, comparisons of systems using membrane fractions to systems using whole cells demonstrate how it might be possible to scale up rates of biotic reactions from enzymes to cultures. In this endeavor, computer-based (in silico) investigation of complex reactions must be utilized in order to identify rate mechanisms that are consistent with experimental observations.