Permeable Reactive Barriers

The placement of supplements in the pathway of groundwater flow constitutes a PRB, sometimes referred to as a treatment zone or "biobarrier." With a PRB approach, an active bioremediation zone is created by methods such as backfilling a trench with nutrient-, oxidant-, or reductant-rich materials, or by creating a curtain of active bioremediation zone through direct injection or groundwater recirculation at the toe of a plume. PRBs contain a contaminant plume by treating only groundwater that passes through it.

24.7.2 Effects of the Properties of MTBE and Other Oxygenates on Treatment

Early studies on MTBE contamination of groundwater concluded that the compound was either nonbiodegradable or very resistant to biodegradation. However, more recent research has shown that MTBE can be degraded both aerobically and anaerobically, although anaerobic intrinsic degradation rates are relatively slow. The research has found that there are naturally occurring microbes capable of using MTBE as their sole carbon and energy source. Such microorganisms seem to be widespread, but are present natively in low numbers and take time to reach a sufficiently dense population to sustain MTBE degradation. As a result, cometabolic approaches are often considered for MTBE bioremediation, wherein an organic substrate (electron donor) that is readily degraded is added to the subsurface, resulting in the consequential oxidation of MTBE. Because the ability of microorganisms to cometabolically degrade MTBE is consistently found in strains that are predisposed to catabolize structural analogs of MTBE, suitable cosubstrates include simple branched and even nonbranched alkanes such as propane and butane.58-62

Although detailed comparative evaluations of the aerobic degradation rates of other fuel oxygenates have not been performed to date, the aerobic biodegradation rates of TAME, ETBE, DIPE, TBA, and TAA were observed to be of the same order of magnitude as the aerobic degradation rate of MTBE in one research study using a mixed culture.63 Together with the similarity of product chemical structure, these results suggest that the same or similar enzyme systems and pathways are responsible for the biodegradation of these oxygenates and that the bioremediation of fuel oxygenates other than MTBE therefore has similar constraints.24 Church and Tratnyek63 proposed a degradation pathway of MTBE and other oxygenates as shown in Figure 24.5.

Successful field-scale applications of engineered bioremediation systems have been limited to the aerobic pathway, as opposed to the anaerobic pathway. The advantages of the aerobic pathway include the following:

1. More energy is derived by microorganisms from the aerobic metabolism of MTBE and other fuel oxygenates; consequently, MTBE degrading cultures grow more quickly under aerobic conditions.

2. There are a number of aerobes that are known to use MTBE as a sole carbon and energy source; anaerobic pathways and the types of microorganisms involved are less well documented.

3. Where the terminal electron acceptor is not present initially in sufficient quantity, addition of oxygen for aerobic bioremediation can be as simple as bubbling air into the aquifer; addition of electron acceptors for anaerobic bioremediation is more complex and can foster concerns regarding the toxicity and fate of the added material.

4. Laboratory studies have provided inconsistent results regarding the degree to which MTBE is biodegraded anaerobically to end products and the extent to which other oxygenates are biodegraded under anaerobic conditions.

MTBE

ETBE

ch3 I

TAME

ch3 I

DIPE

CH3 H

Acetaldehyde, Acetate, etc.

FIGURE 24.5 Proposed degradation pathway of MTBE and other oxygenates. (Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04-009, United States Environmental Protection Agency, Washington, DC, May 2004.)

TABLE 24.12

MTBE Biodegradation Mechanisms and Products

TABLE 24.12

MTBE Biodegradation Mechanisms and Products

Electron

Electron

Products of

Zone

Donor

Acceptor

Complete Degradation

Aerobic

MTBE

O2

CO2 and H2O

Anaerobic

Nitrate reducing

MTBE

NO-

N2, NH3, CO2, and H2O

Iron reducing

MTBE

Fe3+

Fe+2, CO2, and H2O

Sulfate reducing

MTBE

SO-2

H2S, CO2, and H2O

Methanogenic

MTBE

CO2

CH4, CO2, and H2O

Source: Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04-009, United States Environmental Protection Agency, Washington, DC, May 2004.

Source: Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04-009, United States Environmental Protection Agency, Washington, DC, May 2004.

However, the many pathways by which MTBE and other oxygenates may be biodegraded anaer-obically have been the subject of recent research and ongoing studies. Table 24.12 highlights the various electron acceptors that are used in anaerobic bioremediation studies and contrasts the products of complete anaerobic degradation with those for aerobic metabolism.

While Table 24.12 lists the products of complete degradation of MTBE, incomplete degradation of MTBE and other oxygenates may also occur under certain conditions. Of specific note, TBA has been shown to be a degradation intermediate that may persist under anaerobic conditions.60 In some cases, this can result in MTBE plumes having a concentration of TBA in excess of the concentration of MTBE.58 A paper published by Schmidt et al.64 provides further information about the role of TBA in microbial biodegradation. Therefore, application of bioremediation approaches to MTBE and other oxygenates often have considered the complete pathway to end products and the possible stall of the bioremediation process at intermediates along that pathway.

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