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Title: KINETIC CONTROL OF OXIDATION STATE AT THERMODYNAMICALLY BUFFERED POTENTIALS IN SUBSURFACE WATERS

Author
item WASHINGTON, J - USEPA-NERL, ATHENS, GA
item Endale, Dinku
item SAMARKINA, L - USEPA-NERL, ATHENS, GA
item CHAPPELL, K - USEPA-NERL, ATHENS, GA

Submitted to: Geochimica et Cosmochimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2004
Publication Date: 12/1/2004
Citation: Washington, J.W., Endale, D.M., Samarkina, L.P., Chappell, K.E. 2004. Kinetic control at thermo-buffered potentials in subsurface waters. Geochimica et Cosmochimica Acta. 68(23):4831-4842.

Interpretive Summary: Almost all chemical reactions in the environment involve exchange of a hydrogen proton, H+, and/or an electron, e-. These reactions are known as acidity and redox reactions, respectively. Measurement of the hydrogen-proton concentration using a pH meter generally is straightforward, so controls on these reactions are well understood. In contrast, measurement of the electron concentration in the environment is not routine and controls on these reactions are not well understood. It is difficult to understand environmental systems without a clear understanding of controls on electron-exchange reactions. Scientists at the USEPA National Exposure Research Laboratory in Athens GA, and USDA-ARS J. Phil Campbell Sr. Natural Resource Conservation Center in Watkinsville GA, studied the phenomenon in subsurface waters at a spring draining one of the watersheds operated by the USDA-ARS in Watkinsville. The site proved ideal for such study because the management was well documented and clear, which allowed the understanding of system inputs necessary to quantify chemical reactions. Water samples were collected monthly for two years from the spring and analyzed for about 20 chemical constituents with potential to take part in redox reactions. Redox chemical reactions were observed to cluster centered around either dissolved oxygen or organic carbon. The sensitivity of redox reaction rates to reactant concentrations determined this clustering. The investigators also predicted this phenomenon using chemical principles. This knowledge will find wide usage in many scientific disciplines for modeling electron-exchange reactions including environmental chemistry, soil chemistry, geochemistry, environmental engineering and environmental microbiology.

Technical Abstract: Dissolved oxygen (DO) and organic carbon (OC) are among the highest- and lowest-potential reactants, respectively, of redox couples in subsurface waters. When DO and OC are present, other couples are drawn toward potentials determined by them, generating a bimodal clustering. Which cluster a couple is drawn toward to is determined by whether oxidant or reductant is more concentrated. Generally, reactants >10-6M are in a cluster, whereas reactants <10-6M are outliers. As DO or OC decreases, the gap separating clusters diminishes. Composite reactions, NO3- N2 and O2 H2O, are best characterized in multi-steps due to slow reaction of low-concentration intermediates.