MTBE Facts & Research
 

The History of MTBE
 
Methyl tertiary-butyl ether (MTBE) is a fuel additive made by combining methanol and isobutylene. Since 1979, MTBE has been used in the United States as an octane-enhancing replacement for lead, primarily in mid- and high-grade gasoline at concentrations as high as 8 percent (by volume). Since the middle of the 1980s, it has been widely used throughout the country for this purpose. It is also used as a fuel oxygenate at higher concentrations (11 to 15 percent by volume) as part of the U.S. EPA’s programs to reduce ozone and carbon monoxide levels in the most air-polluted areas of the country.

Health Concerns
 
CERCLA ("Superfund") lists MTBE as a hazardous substance. MTBE is also considered a potential human carcinogen. Although it is know to be less toxic than many other gasoline constituents, concerns have been raised about the potential for acute effects from inhalation and the long-term effects from drinking water contamination.
MTBE Characteristics
 
In comparison to petroleum products, MTBE poses problems when it escapes into the environment through gasoline releases, typically from underground storage tank (UST) systems, aboveground storage facilities, or pipelines. MTBE is capable of traveling rapidly through soil, is much more soluble in water, and is more resistant to biodegradation than most other petroleum constituents. Therefore, it often travels farther than other gasoline constituents, making it more likely to impact drinking water wells. Because of its solubility in water and, consequently, its tendency to form large contaminant plumes, petroleum releases with MTBE are more difficult and costly to remediate than petroleum releases that do not contain MTBE.
 
MTBE vs. Benzene
 
As mentioned above, MTBE can be a difficult constituent to remediate. Benzene is most often the contaminant of concern in gasoline because of its relatively high solubility and known carcinogenicity. Comparing the characteristics of MTBE with benzene helps to show how remediation approaches may differ when MTBE is present.
  • MTBE is approximately 30 times more soluble than benzene in water. Pure MTBE can reach an equilibrium concentration in water of 48,000 mg/L, approximately 5 percent.
  • When moving from the free product liquid phase to the vapor phase, MTBE is three times more volatile than benzene; however…
  • When moving from the dissolved phase to the vapor phase, MTBE is about ten times less volatile than benzene (i.e. its Henry’s Constant is 1/10th that of benzene).
  • MTBE is much less likely than benzene to adsorb to soil or organic carbon.
  • MTBE is more resistant to biodegradation than benzene.

Because of these chemical and physical characteristics, MTBE is very difficult to remediate in groundwater using traditional remediation methods such as pump-and-treat, soil vapor extraction, air sparging, etc.
 
MTBE Research
 

Current MTBE research continues to verify the biodegradability of MTBE under aerobic conditions. Furthermore, this research has identified specific components necessary for accelerated aerobic MTBE remediation, including the need for monooxygenase enzyme activity and high levels of molecular dissolved oxygen. For clarification, the DO-IT™ process includes the specific application of monoxygenase enzymes in combination with high levels of molecular dissolved oxygen, lending significant credibility to field treatment results that have been achieved with the DO-IT™ process. The following information is an overview of an excellent review prepared by Andrew Stocking of Malcolm Pirnie, Oakland, CA.

Enzyme Oxidation Activity: Data from Hyman (1998) indicate that MTBE will be cometabolically oxidized by monooxygenase (MO) enzyme activity which is induced by oxidation of n-alkanes under aerobic conditions. The MO enzyme requires molecular oxygen to oxidize the target chemical; this process is called beta oxidation and is a common mechanism for n-alkane biodegradation. Despite success with n-alkanes, Hyman noted that the presence of toluene inhibited MTBE biodegradation. Glaser (1998) speculates that this is a result of toluene dioxygenase production for BTEX degradation, which is not ultimately used in the biodegradation mechanism for MTBE. Glaser (1998) further suggests that the activity of the dioxygenase enzyme may actually inhibit formation of MO, which in turn inhibits MTBE biodegradation. However, Mahaffey (1998) has observed cometabolic MTBE degradation with microbes grown on ortho-xylene and benzene. While biodegradation of these aromatics could be the result of MO enzyme, these results also suggest that a dioxygenase enzyme could be involved (Mahaffey, 1998). To further clarify the importance of MO in the biodegradation of MTBE, Steffan (1997) analytically confirmed that MO enzyme is utilized by his propanotrophs in cometabolically degrading MTBE. In summary, while the successful aerobic degradation of MTBE appears to be the result of a MO enzyme, there are likely other enzymes which significantly contribute.

Molecular Oxygen Requirements: Regardless of the active enzyme requirements and the specific biodegradation mechanism, cometabolic degradation of MTBE depends strongly on elevated concentrations of surrounding molecular oxygen. The following data strongly suggests that enhanced biodegradation strategies may successfully remove MTBE from the subsurface. Park and Cowen (1997) showed that when dissolved oxygen (DO) concentrations are greater than 2 ppm, the MTBE degradation rate is independent of DO; however, when DO falls below 2 ppm, which is common in the vicinity of a gasoline release, the MTBE degradation rate decreases dramatically. Furthermore, Park and Cowen (1997) concluded that the microbial population capable of degrading MTBE is much more sensitive to DO concentrations than typical organics-degrading microbial populations. Yang et al. (1998) also found that as oxygen availability increased, the MTBE degradation rate increased. This was verified both in the lab (the MTBE degradation rate in an oxygen rich serum bottle was at least twice as fast as that in an oxygen-limited bottle) and the field (the biodegradation activities due to oxygen-enhanced air sparging increased significantly over standard air sparging) (Yang et al., 1998). Deshusses (1998b) similarly concluded that the rate of MTBE uptake by microbes follows Michaelis-Menton kinetics with respect to dissolved oxygen, which suggests that MTBE biodegradation is proportional to DO concentrations. Finally, Salanitro (1998) concludes that “the ability to transport O2 and sustain adequate dissolved O2 levels . . . is critically important to the success of stimulating the aerobic bioremediation of MTBE.

Each of these studies seems to indicate that MO enzyme activity coupled with the introduction of high levels of molecular oxygen into the subsurface will result in successful MTBE biodegradation. If MTBE is biodegraded as the sole carbon and energy source, it is likely that other, more easily degraded compounds are preferentially utilized or competitively inhibit the MTBE biodegradation. Consequently, the introduction of molecular oxygen will facilitate the rapid aerobic degradation of these easily degraded compounds, which will then allow the microbes to biodegrade MTBE. Alternatively, if MTBE is biodegraded cometabolically with other aromatics and/or alkanes, the introduction of MO enzyme activity and molecular oxygen will facilitate enhanced cometabolic biodegradation of MTBE along with these compounds. Under both scenarios, a strategy of enhanced aerobic biodegradation will result in the effective removal of MTBE from the subsurface.
 
References:
 
Deshusses, Marc. 1998a. Treatment of MTBE Vapors in Biotrickling Filters. Santa Clara Valley Water District’s Leaking Underground Storage Tank Oversight Program. October 14-15, 1998.

Deshusses, Marc. 1998b. Personal Communication.

Glaser, John. 1998. Personal Communication.

Hyman, Michael. 1998. Cometabolism of MTBE by Alkane-Utilizing Microorganisms. Presented at the First International Conference on Remediation of Chlorinated and Recalcitrant Compounds. Monterey, California: May 18-21, 1998 

Mahaffey, William. 1998. Personal Communication.

Park, K. and Cowan, R. M. 1997a. Biodegradation of gasoline oxygenates. In Situ and On-Site Bioremediation: Volume 1, Battelle Press.

Park, K. and Cowan, R. M. 1997b. Effects of oxygen and temperature on the biodegradation of MTBE. Preprints of Extended Abstracts 37(1)421-423, Proceedings of the 213th ACS National Meeting, San Francisco, CA.

Salanitro, J. P., C. S. Chou, H. L. Wisniewski, and T. E. Vipond. 1998. Perspectives on MTBE Biodegradation and the Potential for In-Situ Aquifer Bioremediation. Presented at the Southwestern Regional Conference of the National Groundwater Association, Jun 3-4, 1998; Anaheim, California.

Steffan, R. J., McClay, K., Vainberg, S., Condee, C. W. and Zhang, D. 1997. Biodegradation of the gasoline oxygenates methyl tert-butyl ether, ethyl tert-butyl ether, and tert-amyl methyl ether by propane-oxidizing bacteria. Applied and Environmental Microbiology 63(11):4216-4222.

Yang, X., D. Tsao, M. Javanmardian, and H.A. Glasser. 1998. Development of Cost-Effective MTBE In-Situ Treatment Technologies. Presented at the ATV Vintermode om Grundvandsforurening. Belje, Denmark: March 10-11, 1998.