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BIO-DEGRADATION

The most effective way to remediate organic contamination is bio-degradation, which is facilitated by native bacteria. The activity of these bacteria is controlled by the concentration of contaminants and by the presence of electron acceptors, which allows the bacteria to oxidize or reduce the contaminants. The two main classes of organic pollutants are Petroleum type Fuel Hydrocarbons and Solvents, and the Chlorinated Solvents. These contaminate classes are most effectively bio-degraded by two different processes.

Bio-degradation of Petroleum Hydrocarbons

The most effective petroleum hydrocarbon degraders (eaters) are the aerobic (oxygen using) bacteria. Usually, the factor controlling the rate that these bacteria degrade the gasoline is the amount of available dissolved oxygen.

A much slower degradation process starts when the dissolved or free oxygen is used up. The plume begins to become anaerobic and the bacteria commence to reduce nitrate, ferric iron, and sulfate to further degrade the hydrocarbons. The reduction of these compounds produces oxygen, which is then used by the bacteria as the electron acceptor for hydrocarbon reaction. Eventually, as these compounds are used, the bacteria begin methogenesis, in which the hydrocarbons are slowly converted to methane.

The primary oxygen based electron acceptors used by the bacteria to degrade Petroleum Hydrocarbons are:

Dissolved Oxygen, O₂
Nitrate, NO₃^-
Ferric iron Fe³⁺ but it is insoluble, so the reaction product Fe²⁺ is measured.
Sulfate, SO₄^2-

As the easier or less stable electron acceptor is consumed the bacteria begin to use the next easier acceptor. This leads to hydrocarbon plumes in which the areas with high concentrations of dissolved hydrocarbons are void of the electron acceptors or are shield by a carbon dioxide blanket

The most effective (it gives the bacteria the most energy in the hydrocarbon reaction) is oxygen; therefore, it is usually most effective to add oxygen to a hydrocarbon groundwater plume in order to enhance bio-degradation. This is often accomplished simply by removing the carbon dioxide allowing oxygen to fill the generated voids.

Bio-degradation of Chlorinated Solvents

PCE has been shown to degrade under anaerobic conditions by reductive dechlorination. During reductive dechlorination, anaerobic microorganisms substitute hydrogen for chlorine, with the chlorine acting as the electron acceptor. The reaction product of the dechlorination of PCE is Trichloroethene (TCE). TCE is further dechlorinated to cis 1,2 Dichloroethene (DCE), which is then reduced to vinyl chloride (VC). Vinyl chloride reduces to ethylene. The microorganisms that reduce PCE and TCE are most active under strongly anaerobic conditions. The energy that is released by dechlorination is small compared to the reactions that involve oxygen; therefore, until the oxygen based electron acceptors are depleted the dechlorinaters are at a disadvantage. This makes it necessary to deplete the aquifer of the oxygen based electron acceptors: dissolved oxygen (O₂₎, nitrate (NO₃), sulfate, (SO₄), and Ferric Iron.