Methyl Tertiary Butyl Ether

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Methyl tertiary butyl ether

Methyl tertiary butyl ether (MTBE) is a flammable, volatile, and colorless liquid fuel additive that is manufactured by the chemical reaction of methanol and isobutylene. It is one of a group of chemicals referred to as oxygenates, because they raise the level of oxygen in gasoline . Oxygen helps gasoline burn more completely, thus reducing emissions.

MTBE has a strong odor similar to a general anesthetic. Humans can detect it by smell at very low concentrations: 53 parts per billion (ppb) in air and 20 to 40 ppb in water. MTBE is very soluble in water and more soluble than other gasoline constituents. It is also persistent and resistant to degradation.

As a gasoline additive, it is used to increase the octane level (replacing the use of lead additives) and to reduce vehicular emissions of carbon monoxide (CO) and ozone-forming pollutants. It was first used in the United States in the 1970s, with its use increasing during the 1990s, when the Clean Air Act Amendments of 1990 created the Oxygenated Fuel Program. Areas of the country with severe air pollution problems are required to add oxygenates to gasoline during winters to reduce vehicular emissions of CO. Although regulatory requirements do not specify the types of oxygenates required, MTBE and ethanol are the main oxygenates used. Ethanol is the primary oxygenate used in the Winter Oxyfuel Program. Areas with problems meeting ozone standards require the use of reformulated gasoline (RFG) year round to reduce ozone-forming pollutants. RFG is oxygenated gasoline (with a minimum of 2% oxygen by weight) that is specially blended to have fewer polluting compounds than conventional gasoline. As the addition of ethanol increases the vapor pressure of gasoline such that it is difficult for ethanol-containing gasoline to comply with summertime volatility requirements, MTBE has been the preferred oxygenate for use during the summer. In 1970, MTBE was the 39th-highest produced organic chemical in the United States; by 1998, it was the 4th-highest produced organic chemical. In 1999 over 200,000 barrels of MTBE per day were produced.

The use of RFG has improved the quality of air in the United States. The U.S. Environmental Protection Agency estimates that smog-forming pollutants are being reduced annually bey at least 105 million tons and toxic chemicals by at least 24 million tons.

However, the success of the air quality program that utilizes MTBE to reduce emissions has been offset by contamination of ground and surface waters with MTBE. MTBE has been detected in soil , surface water, and ground water throughout the United States. Point sources of MTBE include releases from underground and above-ground storage tanks and pipelines due to leaks, overfilling, and faulty construction. Non-point sources of MTBE include urban runoff , precipitation, vehicle accidents, and motorized water craft. MTBE may also be released to the environment through gasoline fumes during vehicle refueling. A study by the U.S. Geological Survey in 1993–1994 showed that MTBE was the second most frequently detected volatile organic chemical of 60 chemicals in samples collected from shallow ground water in eight urban areas. Another report in 1999 by the U.S. Environmental Protection Agency's Blue Ribbon Panel on Oxygenates in Gasoline indicated that between five and ten percent of drinking water wells in areas with high use of MTBE had detectable levels of MTBE, while one percent had levels higher than 20 micrograms per liter. During the summer of 1996, the City of Santa Monica in California stopped pumping ground water from two of its well fields because of persistent and increasing levels of MTBE. These two wells provided about 50% of the city's drinking water supply.

MTBE has been shown to have the potential to produce adverse effects associated with central nervous system depression, including headaches, dizziness, nausea, and disorientation. These effects are reversible if exposure is discontinued. Evidence from animal studies suggests that MTBE is a potential human carcinogen . When it enters the human body through inhalation or absorption through the skin, it may metabolize into two compounds, tertiary butyl alcohol and formaldehyde, that are carcinogenic in animals and are also classified as potential human carcinogens.

In 1997 the U.S. Environmental Protection Agency issued a Drinking Water Advisory for MTBE. The Advisory recommended that the levels of MTBE in drinking water by limited to 20 to 40 micrograms per liter. This limit was set to assure consumer acceptance with regards to taste and odor and to provide an adequate margin of safety from toxic effects. Based on this Advisory, several states have set their own drinking water standards for MTBE. A nation-wide standard for MTBE under the Safe Drinking Water Act is not expected until after February 2005, due to lack of information regarding health effects and occurrence data. In March of 2000, the U.S. Environmental Protection Agency formally began regulatory action to eliminate or phase down MTBE, issuing an Advance Notice of Proposed Rulemaking under Section 6 of the Toxic Substances Control Act, which gives the U.S. Environmental Protection Agency authority to ban, phase out, limit or control the manufacture of any chemical substance deemed to pose an unreasonable risk to the public or the environment. The State of California is considering a ban on the use of MTBE by the end of 2003. As of 2002, bans on MTBE are also under consideration in at least 16 other states. Research is being conducted to develop alternative oxygenates so if MTBE is banned, air quality benefits of the use of oxygenated fuels will be preserved.

Because of its physical properties, during a release MTBE migrates rapidly through the soil column. Upon reaching the ground water, MTBE moves at the same velocity as water and faster and farther than other gasoline constituents. As the gasoline contaminant plume degrades over time, benzene , toluene , ethyl benzene, and xylene (BTEX) decrease in concentration more than MTBE, which is more resistant to degradation. Eventually MTBE may be the only contaminant remaining from the release.

The extent of MTBE contamination in the subsurface has led to the investigation of treatment technologies. If MTBE in soil is not dissolved in water, it can be removed by such treatment methods as soil vapor extraction. However, the high solubility of MTBE in water and its resistance to degradation makes it difficult and time consuming to remove from ground and surface waters using many common remedial technologies. Promising technologies for removal of MTBE that are being studied include chemical oxidation using ultraviolet light and hydrogen peroxide, phytoremediation using deep-rooted trees, and cometabolic degradation processes.

[Judith L. Sims ]