Reversible redox chemistry of quinones: impact on biogeochemical cycles.

The role of quinone biomolecules and quinone moieties of natural organic matter (NOM) as the electron transfer mediator in essential biogeochemical processes such as iron bioreduction and contaminant degradation has received considerable interests in the past decade. Hypothesized electron shuttling mechanism must be evaluated based on the availability and stability of quinones under a given environmental setting. The goal of this review is to examine the source, reactivity, and fate of potential quinone catalysts with respect to chemical interactions (e.g., with other quinones and nucleophiles) that will inevitably occur in complex environmental media. We will first discuss natural and anthropogenic sources of quinones in aqueous environments, and fundamental transformation pathways including cross reaction, autoxidation, and addition reactions. We will then assess how the described sources (molecular structure) and transformation pathways (stability) will impact the ability of a quinone molecule to catalyze a biogeochemical process. Thermodynamics and kinetics of electron transfer reactions with both the electron donor (e.g., hydrogen sulfide as a bulk reductant) and the terminal electron acceptor (e.g., nitroaromatic explosives in contaminant degradation), and stability towards irreversible side reactions are the key factors determining the geochemical conditions under which the catalysis by a quinone molecule will be operative.