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Abiotic

Abiotic

Abiotic

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Donald L. Macalady

Colorado School of Mines

a wide range of electron transfer processes. NOM in this context refers to the suite of extracellular organic chemicals present in any natural water system. The components of NOM can vary from molecules recently excreted by active microorganisms to facilitate electron transfer process of utility to the microbial community to “abiotic” degradation products from the debris of long inactive living systems. The ability to mediate electron transfer processes seems to be ubiquitous among NOM samples (Macalady and Ranville, 1998; Tratnyek and Macalady, 2000).

NOM has been shown to be an effective mediator of electron transfer in a wide variety of redox processes. Included are the reduction of nitroaromatic compounds, azo compounds, halogenated organic chemicals, metals and metal/organic complexes (see for example, Macalady and Ranville, 1998; Larson and Weber, 1994; Curtis and Reinhard, 1994; Wolfe et al., 1986; Wittbrodt and Palmer, 1996; Skogerbee, 1981; Matthiessen, 1996). NOM can also serve as an electron transfer mediator in microbial processes, e.g. the microbial reduction of solid iron (III) oxyhydroxides (Lovley et al., 1996). The role of NOM in mediating the transfer of electrons in the opposite direction has received less attention, but the oxidation of sulfide and ferrous iron is clearly enhanced in the presence of oxidized NOM. In fact, one of the few redox processes that is apparently not mediated by NOM is the reduction of molecular oxygen (Peiffer, unpublished data).

The mechanism(s) by which NOM facilitates electron transfer processes is only partially understood. Clearly quinone-like functional groups within NOM structures are an important part of this reactivity. However, in certain pH ranges, other functional groups may be important (Dunnivant et al., 1992; Perlinger et al., 1996; Schwarzenbach et al., 1990; Gantzer and Wackett 1991; Schindler et al., 1976).

Attempts to determine the role of NOM in specific ground-water redox processes are related to both the solution phase and particulate NOM fractions in the aquifer matrix. The potential roles of NOM include direct participation as an electron transfer mediator and an indirect role as a transport inhibitor (particulate NOM) or facilitator (dissolved and/or colloidal NOM). Specific examples of such processes serve to illustrate the importance of NOM in such considerations.

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Curtis, G. P. and Reinhard, M., 1994. Reductive dehalogenation of hexachlorethane, carbon

tetrachloride, and bromoform by anthrahydroquinone disulfonate and humic acid. Environmental Science and Technology, v. 28, pp. 2393-2401.

Dunnivant, F. M., Schwarzenbach, R. P. and Macalady, D. L. 1992. Reduction of substituted nitrobenzenes in aqueous solutions containing natural organic matter. Environmental Science and Technology, v. 26, pp. 2133-2141.

Eary, L. E. and Schramke, J. A. 1990. Rates of inorganic oxidation reactions involving dissolved oxygen. In Chemical Modeling of Aqueous Systems, II, Chapter 30, eds. D. C. Melchior and R. L. Bassett, American Chemical Society, Washington, D.C., pp. 379-396. Gantzer, C.J. and Wackett, L.P., 1991. Reductive

dechlorination catalyzed by bacterial transition-metal coenzymes. Environmental Science and Technology, v. 25, pp. 715-722.

Larson, R. A. and Weber, E. J. 1994. Reaction Mechanisms in Environmental Organic Chemistry. Lewis Publishers, Chelsea, Michigan, pp. 169-273. Lovley, D. R., Coates, J. D., Blunt Harris, E. L.,

Phillips, E. J. P. and Woodward, J. C. 1996. Humic substances as electron acceptors for microbial respiration. Nature, v. 382, pp. 445-448.

Lyman, W. J., Bodek, I., Reehl, W. F., Rosenblatt, D. H. 1987. Electron transfer reactions. In Methods for Estimating Physicochemical Properties of Inorganic Chemicals of Environmental Concern, Final Report, Chapter 2, U.S. Army Medical Research and Development Command, Contract DAMD 17-83-C-3274.

Macalady, D. L. and Ranville, J. F. 1998. The chemistry and geochemistry of natural organic matter. In Perspectives in Environmental Chemistry, Chapter 5, ed. D. L. Macalady, Oxford University Press, New York, pp. 94-137.

Matthiessen, A. 1996. Kinetic aspects of the reduction of mercury ions by humic substances. Fresenius Journal of Analytical Chemistry, v. 354, pp. 747-749. Perlinger, J. A., Angst, W., and Schwarzenbach, R. P. 1996. Kinetics of the reduction of hexachloroethane by juglone in solutions containing hydrogen sulfide. Environmental Science and Technology, v. 30, pp. 3408-3417.

Peiffer, S., Walton-Day, K., Macalady, D. L., 1999. The interaction of natural organic matter with iron in a wetland receiving acid mine drainage. Aquatic Geochemistry, v. 5, pp. 207-223.

Schindler, J. E., Williams, D. J., Zimmerman, A. P. 1976. Investigation of extracellular electron transport by humic acids. In Environmental Biogeochemistry, Vol. 1., ed. J. O. Nriagu, Ann Arbor Science, Ann Arbor, Michigan, pp. 109-115.

Schwarzenbach, R. P., Stierli, R., Lanz, K. and Zeyer, J. 1990. Quinone and iron porphyrin mediated reduction of nitroaromatic compounds in homogeneous aqueous solution. Environmental Science and Technology, v. 24, pp. 1566-1574. Skogerboe, R. K. 1981. Reduction of ionic species by

fulvic acid. Analytical Chemistry, v. 53, pp. 228-232. Tratnyek, P. G. and Macalady, D. L. 2001. Oxidation- reduction reactions in aquatic systems. In Estimation of Chemical Properties for the Environmental and Health Sciences: A Handbook of Methods, eds. B. Boethling and D. Mackay, Ann Arbor Press, Ann Arbor, Michigan.

Walton-Day, K., Macalady, D. L., Brooks, M. H., Tate, V. T. 1990. Field methods for measurement of ground water redox chemical parameters. Ground Water Monitoring Review, v. 10, pp. 81-89.

Walton-Day, K. 1991. Hydrology and Geochemistry of a Natural Wetland Affected by Acid Mine Drainage, St. Kevin Gulch, Lake County, Colorado. Ph. D. Thesis, Colorado School of Mines, Golden, CO, 300 p.

White, A. F., Peterson, M. L., Solbau, R. D. 1990. Measurement and interpretation of low levels of dissolved oxygen in ground water. Ground Water, v. 28, pp. 584-589.

Wittbrodt, P. R. and Palmer, C. D. 1996. Effect of temperature, ionic strength, background electrolytes, and Fe(III) on the reduction of hexavalent chromium by soil humic substances. Environmental Science and Technology, v. 30, pp. 2470-2477.

Wolfe, N. L., Kitchens, B. E., Macalady, D. L., and Grundl, T. J. 1986. Physical and chemical factors that influence the anaerobic degradation of methyl parathion in sediment systems. Environmental Toxicology and Chemistry, v. 5, pp. 1019-1026.

The most important oxidation reduction process involved in the destruction of chlorinated organic compounds in ground water is biological sequential reductive dechlorination (Vogel and McCarty, 1985). In some cases this process does not provide benefit to the organisms that carry it out, the process is entirely accidental, and may be considered a form of co- metabolism. In other cases the biological process yields energy to the microorganisms and can support their growth and proliferation (Maymo-Gatell et al., 1995). In this circumstance the process functions as a respiration, and has been termed halorespiration (See discussion in Chaper 6, Wiedemeier et al., 1999). Chlorinated organic compounds may also be destroyed by chemical reaction in aquifers, usually involving direct chemical reaction with sulfide or ferrous iron. Examine Butler and Hayes (1999), and Devlin and Muller (1999) for illustrations of recent research.

In “The Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water”, the U.S. Environmental Protection Agency used a scoring system to identify sites where geochemical conditions were appropriate or not appropriate for biological reductive dechlorination of chlorinated solvents. Weights were given to the concentrations of important electron acceptors, including oxygen, nitrate, and sulfate, to the concentrations of important electron donors including molecular hydrogen, volatile fatty acids, dissolved native organic carbon (TOC), and petroleum derived monoaromatic hydrocarbons (BTEX), and the concentrations of metabolic end products including methane, ferrous iron, sulfide, chloride and carbon dioxide represented as increases in carbon dioxide or alkalinity. Weights were also given to general descriptions of the oxidation/reduction environment such as electrode potential, pH, and temperature. Finally, weight was given to the accumulation of metabolic daughter products including dichloroethylene, dichloroethane, vinyl chloride, ethene, or ethane. Weight was only given when

the daughter products were not originally present in the material that was released to the environment. The weighted scores were totaled, and compared to a table that interpreted the scores as providing either inadequate evidence, limited evidence, adequate evidence, or strong evidence of anaerobic biodegradation of chlorinated organic compounds.

The National Research Council (2000) in the report Natural Attenuation for Groundwater Remediation noted on pages 210 and 211 that

Unfortunately, this scoring system is being widely adopted for uses that the authors never intended. For example, many states are using it to evaluate natural attenuation for all types of chlorinated solvents. Tables of natural attenuation scores are showing up in remedial investigation reports at Superfund sites. Maps and cross sections showing natural attenuation scores are being included in final reports as a key line of evidence. Some regulators are accepting this inappropriate use of scoring.

* The method applies only to chlorinated ethenes.

* The scores emphasize reducing environments more than dehalogenation reactions.

* A reduced geochemical environment does not guarantee that natural attenuation will occur, because geochemical environments can be very reduced without reductive dehalogenation of chlorocarbons occurring (for example, if dehalogenating bacteria are not present).

* The scoring system included items that are of current research interest (for example, hydrogen concentration), but that may have limited practical impact on making remediation decisions.