Environmental Engineering: Fundamentals and Design - Chapter 3

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TOC > Ch. 3 > 3.8 Oxidation - Reduction

# Chapter 3 Chemistry

Noel R. Urban, Judith A. Perlinger and James R. Mihelcic

## Oxidation - Reduction

### Redox Chemistry in Sediments

As introduced subsequently in Section 5.3.1, a series of microbially-mediated redox reactions serves to carry out the mineralization of labile (degradable) organic carbon in the environment. In each of these redox reactions, organic carbon (the electron donor) is oxidized to carbon dioxide and one of several electron acceptors is reduced at an alternate form. The equation below describes the general reaction using a simple carbohydrate, C(H2O), to represent organic matter,

C(H2O) + EA CO2 + RS

where EA is the electron acceptor and RS the electron acceptor's reduced species form.

In the case of the microbially-mediated redox reactions involved in carbon mineralization, the selection of a candidate electron acceptor is made based on thermodynamic principles, i.e. the electron acceptor yielding the most thermodynamically favorable reaction is used in preference to all others. As a result, electron acceptors may be ranked in order of their thermodynamic favorability; a series termed the ecological redox sequence (Sec. 5.3.1).

Thus the mineralization of organic carbon begins with oxygen as the electron acceptor, switching to alternate acceptors as those higher on the ecological redox sequence are depleted, and concludes with methanogenesis where carbon dioxide and intermediates in the breakdown of organic matter serve as the electron acceptors.

While we appropriately view the ecological redox sequence as a continuum in time, it also occurs in certain natural systems, e.g. lake sediments, as a continuum in space. Because the electron acceptors supporting organic carbon mineralization in the sediment originate in the overlying water, they become depleted with depth in the sediment as they are consumed in redox reactions. Thus oxygen is the favored electron acceptor at positions nearest to the sediment-water interface and the remaining electron acceptors become active as one goes deeper in the sediment as long as labile organic carbon is available.

The level to which a particular electron acceptor or acceptors penetrate lake sediments depends on the demand for reaction and thus on the labile organic carbon content of the sediment. In oligotrophic (nutrient-poor, Sec. 8.3.3) lakes, rates of primary production are low and little labile organic carbon reaches the sediment. The attendant rates of electron acceptor demand are thus low and oxygen, for example, penetrates deep into the sediment. In such cases, the labile organic matter may become totally mineralized before the entire ecological redox sequence is exercised and a less thermodynamically favorable reaction such as sulfate reduction may not occur.

Lake Superior is an oligotrophic system having sediments with low labile organic carbon content. As a result, oxygen (the most thermodynamically-favorable electron acceptor) penetrates deep into the sediment. In this figure the limit of oxygen penetration, several centimeters, is evident as a reddish band where ferrous iron present in the deeper anoxic sediments is oxidized to ferric iron as it encounters oxygen and forms a precipitate. Other measurements (inset) demonstrate that oxygen can penetrate to depths in excess of 20 cm in Lakes Superior.

A contrasting condition is observed in eutrophic (nutrient-rich, Sec. 8.3.3) systems where rates of primary production may exceed the capability for mineralization and significant amounts of labile organic carbon may accumulate in the sediment. Here, the entire ecological redox sequence may be observed with depth downward in the sediment.

CLICK EACH GRAPH TO ENLARGE.

Onondaga Lake (Syracuse, New York) is a eutrophic system having sediments with high labile organic carbon content. Here, the entire ecological redox sequence comes into play. Although not evident visually, the distribution of the ecological redox sequence with depth can be detected by measuring concentrations of electron acceptors and reduced species end products in the sediment. In this organic-rich sediment, oxygen and nitrate penetrate less with sulfate reduction and methanogenesis operating several centimeters below. The location of iron and manganese were not measured, but may be inferred from the nitrate and sulfate profiles and a knowledge of the ecological redox sequence.

A variation on this spatial continuum may also occur within sediment particles where electron acceptors. If the rate of diffusion into a particle is sufficiently slow and the electron acceptor demand (richness of the organic matter) sufficiently high, electron acceptors may be serially depleted as they diffuse into and are consumed within an individual particle.

#### Credits

Klump et al. 1982
Lake Superior sediment photo by Marty Auer.
Onondaga Lake sediment by Swiss Federal Institute of Aquatic Science and Technology.
Onondaga Lake profiles courtesy of: Charles T. Driscoll, Jr. (sulfate), Albert Galicinao (oxygen and sulfide), Heidi Brixen (nitrate) and Virginia Breidenbach (methane).

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