| Quinones, a term that encompasses dihydroxybenzenes, semiquinone radicals, and benzoquinones, are abundant redox-active moieties within natural organic matter. The roles of quinone redox chemistry in biogeochemical processes such as microbial iron respiration have received considerable interest in recent years. In order to understand the involvement of quinones in such processes, both thermodynamic and kinetic aspects of quinone redox chemistry must be understood for a wide range of environmental conditions.; This study investigates the extent, rate, and mechanism of redox reactions between benzoquinones and FeII, and between different quinone structures in a process called quinone cross reactions. Thermodynamic calculations allow us to predict the equilibrium acid-base speciation of quinones and iron, and the energetics of reactions between quinones and iron. Experiments were conducted to determine the influence of pH, benzoquinone structure, and iron speciation on the rate and mechanism of reactions.; Our studies showed that the rate of cross reactions can be successfully related to the energetics of reaction through a Marcus correlation, if the reactants and products are stable toward side reactions. Depending upon the cross reaction pair and pH, benzoquinone oxidants and oxidation products having high standard reduction potentials are prone to side reactions. Side reactions alter mechanism, stoichiometry, equilibrium concentrations, and rate of cross reactions.; In the absence of iron-coordinating ligands, the time courses of benzoquinone reduction by FeII show an autocatalytic behavior with a lag phase followed by an exponential decay phase. An increase in the driving force with pH and the standard reduction potential of the benzoquinone oxidant results in an increased rate. In the presence of excess malonate, oxalate, citrate, and EDTA, reactions became much faster and time courses followed an exponential decay without an initial lag period. Reaction rate was controlled by the FeII speciation; lowered FeIII/II reduction potential upon ligand addition resulted in an increased rate. The rate of the "reverse" reaction, the oxidation of hydroquinones by FeIII, was controlled by the FeIII speciation. Decreased Fe III reduction potential upon ligand addition lowered the rate of Fe III reduction. |
