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Elucidating the Role of Electron Shuttles in Reductive Transformations in Anaerobic Sediments
Citation:
Zhang, H. AND E. J. WEBER. Elucidating the Role of Electron Shuttles in Reductive Transformations in Anaerobic Sediments. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 43(4):1042-1048, (2009).
Impact/Purpose:
The general goal of this project is to provide the process science necessary for the development of an environmental fate simulator for chemical transformations that includes reductive transformations. This requires a clear understanding of the molecular descriptors and system parameters necessary to describe the reaction kinetics and pathways for the reductive transformation of organic chemicals in anoxic environments.
Description:
Model studies have demonstrated that electron shuttles (ES) such as dissolved organic matter (DOM) can participate in the reduction of organic contaminants; however, much uncertainty exists concerning the significance of this solution phase pathway for contaminant reduction in natural systems. To compare the identity and reactivity of ES in anaerobic sediments with those in model systems, two chemical probes (4-cyano-4'-aminoazobenzene (CNAAzB) either free or covalently bound to glass beads) were synthesized that allowed for differentiation between surface-associated and solution-phase electron-transfer processes. The feasibility of these chemical probes were demonstrated in abiotic model systems (Fe(II)/Fe(III) oxide) and biotic model systems (Fe(II)/Fe(III) oxide or river sediment amended with S. putrefaciens strain cells). Experiments in the abiotic systems revealed that the addition of model hydroquinones and chemically reduced DOM increased reduction rates of free CNAAzB, whereas no enhancement in reactivity was observed with the addition of model quinones or DOM. Bound CNAAzB was also reduced by model hydroquinones and reduced DOM-but not by model quinones and untreated DOM-in the abiotic model systems, indicating that Fe(II)/Fe(III) oxides do not function as a bulk reductant for the reduction of ES. Addition of model quinones or untreated DOM to the biotic models systems with sediment increased reduction rates of bound CNAAzB, which correlated well with the dissolved organic carbon content. In natural sediment slurries, reduction rates of bound CNAAzB correlated well with parameters for organic carbon (OC) content of both sediments and supernatants. Our results support a scenario in which reducible organic contaminants will compete with iron oxides for the electron flow generated by the microbially mediated oxidation of organic carbon and subsequent reduction of quinone functional groups associated with DOM.