خبراء يحذرون من مخاطر مادة MTBE المضافة لوقود السيارات

المهندس أمجد قاسم

كاتب علمي 

 

          أكد عدد من خبراء جمعية حماية البيئة الأردنية على المخاطر الناجمة عن استخدام مادة MTBE (Methyl tert-butyl ether )  المضافة إلى جازولين السيارات ، كبديل لرابع ايثيل الرصاص المستخدمة سابقا ، حيث تم التوقف تماما عن استخدام الرصاص مؤخرا في كافة أنواع وقود السيارات .

          و أكد الدكتور سيد خطاري الأستاذ في كلية الزراعة في الجامعة الأردنية ، أن مادة MTBE تشكل خطرا حقيقيا على المياه الجوفية وعلى كافة عناصر البيئة وعلى الإنسان ، حيث ثبت أن هذه المادة تذوب في الماء بشكل كبير وذات ثبات بنائي كبير بحيث يتعذر تحللها بيولوجيا .

          يذكر هنا أن مادة MTBE التي تضاف حاليا إلى وقود السيارات كمادة محسنة لعملية الاحتراق داخل محركات السيارات ، قد توقفت الكثير من دول العالم عن استخدامها بسبب أضرارها الصحية في حال اختلاطها مع المياه الجوفية ، وبالرغم من الجوانب الإيجابية لهذه المادة والمتمثلة في تخفيف التلوث البيئي وتحسين عملية الاحتراق بشكل كبير في المحركات ، إلا أنها مادة عضوية تذوب في الماء بشكل كبير كما أنها قد تصل إلى الإنسان عن طريق التنفس.

          وعن مخاطر هذه المادة يوضح الدكتور مصالحة أن الكثير من التقارير أكدت أن التعرض لمادة MTBE يؤدي إلى الإصابة بأعراض مرضية حادة كالصداع والغثيان والدوخة وصعوبة التنفس والربو كما أن لها تأثيرات مسرطنة عند التعرض لجرعات عالية من هذه المادة .

          وفي اجتماع عقد مؤخرا في عمان ، أكد عدد من الخبراء الأردنيين عن كل من وزارة البيئة ووزارة الطاقة ومصفاة البترول وبعض الجمعيات البيئية المختصة وعدد من الأكاديميين ، على المخاطر الكبيرة لهذه المادة بسبب عدم تحللها البيولوجي وأيضا حاجتها إلى مواد خاصة بالإطفاء في حال اشتعالها ، كما أكدوا على المخاطر الصحية الناجمة عن التعرض المباشر لها ، وأوصوا بضرورة التوقف عن استخدامها كليا واستبدالها بمواد أخرى آمنة كالايثانول أو غيره من المواد التي ثبت علميا عدم وجود آثار سلبية لها على الإنسان والبيئة .

          تاليا مزيد من المعلومات حول مادة MTBE ، تركيبها ، خصائصها الفيزيائية ، إنتاجها ، استعمالاتها ، خطرها على البيئة وعلى الإنسان ، درجة السمية ، وغيرها من المعلومات :

من موقع Wikipedia, the free encyclopedia

ومن موقع EPA.gov

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Methyl tert-butyl ether

From Wikipedia, the free encyclopedia

Methyl tert-butyl ether, also known as methyl tertiary butyl ether and MTBE, is a chemical compound with molecular formula C5H12O. MTBE is a volatile, flammable and colorless liquid that is immiscible, yet reasonably soluble in water. MTBE has a minty odour vaguely reminiscent of diethyl ether, leading to unpleasant taste and odour in water. MTBE is a gasoline additive, used as an oxygenate and to raise the octane number, although its use has declined in the United States in response to environmental and health concerns. It has been found to easily pollute large quantities of groundwater when gasoline with MTBE is spilled or leaked at gas stations. MTBE is also used in organic chemistry as a relatively inexpensive solvent with properties comparable to diethyl ether but with a higher boiling point and lower solubility in water. It is also used medically to dissolve gallstones.

 

Production

MTBE is manufactured via the chemical reaction of methanol and isobutylene. Methanol is derived from natural gas, and isobutylene is made from crude oil or natural gas, thus MTBE, as used in motor gasoline, is a fossil fuel. In the United States, it was produced in very large quantities (more than 200,000 barrels per day in the United States in 1999) during its use as a fuel additive. Due to widespread releases of MTBE-containing gasoline from Underground Storage Tanks all over the US, various jurisdictions banned the use of MTBE and production was reduced. MTBE contamination in drinking water aquifers is a serious concern in many states (the most famous cases are Lake Tahoe and Santa Monica). By late 2006, most American gasoline retailers had ceased using MTBE as an oxygenate, and accordingly, US production had declined. Similarly, lack of growth or even decline of MTBE production has been seen in Western Europe. This is not due to environmental concerns; on the contrary, it is because the alternative ethanol-derived ether ETBE has been given more favorable tax treatment. Nevertheless, in other parts of the world, which account for about a half of 2004 production, the use of MTBE will continue and even grow.

Physical properties

MTBE forms azeotropes with water (52.6 °C) and methanol (51.3 °C).

Uses

MTBE is almost exclusively used as a fuel component in motor gasoline. It is one of a group of chemicals commonly known as oxygenates because they raise the oxygen content of gasoline.

As anti-knocking agent

In the US it has been used in gasoline at low levels since 1979 to replace tetra-ethyl lead to increase its octane rating and help prevent engine knocking. Oxygen helps gasoline burn more completely, reducing tailpipe emissions from pre-1984 motor vehicles. In more modern vehicles, the emissions reduction is negligible. In one respect, the oxygen dilutes or displaces gasoline components such as aromatics (e.g., benzene) and sulfur. In another, oxygen optimizes the oxidation during combustion. Most refiners have chosen to use MTBE over other oxygenates primarily for its blending characteristics and for economic reasons. It is produced from natural gas, which is less expensive than oil.

Since 1992, MTBE has been used at higher concentrations in some gasoline to fulfill the oxygenate requirements set by the United States Congress in Clean Air Act amendments; however, since 1999, in California and other locations MTBE has begun to be phased out because of groundwater contamination (California Air Resources Board, 2004). Due to its higher solubility in water MTBE moves more quickly than other fuel components (California Air Resources Board, 2004). The Energy Policy Act of 2005 reduces the federal requirement for oxygen content in reformulated gasoline.

In 1995 high levels of MTBE were unexpectedly discovered in the water wells of Santa Monica, California, and the U.S. Geological Survey reported detections. Subsequent U.S. findings indicate tens of thousands of contaminated sites in water wells distributed across the country. As per toxicity alone, MTBE is not classified as a hazard for the environment. The maximum contaminant level of MTBE in drinking water has not yet been established by the EPA. The leakage problem is partially attributed to the lack of effective regulations for underground storage tanks, but spillage from overfilling remains an important upset scenario. As an ingredient in unleaded gasoline, MTBE is the most soluble part. When dissolved in groundwater, MTBE will lead the contaminant plume with the remaining components such as benzene, toluene, etc. to follow. Thus the discovery of MTBE in public groundwater wells indicates that the contaminant source was a gasoline release. Its criticism and subsequent decreased usage, some claim, is more a product of its easy detectability (taste) in extremely low concentrations (ppb) than its toxicity, as benzene is much more toxic but remains a fuel additive. The MTBE concentrations used in the EU (usually 1.0–1.6%) and allowed (maximum 5%) in Europe are lower than in California.

Alternatives

Other compounds are available as oxygenate additives for gasoline, for example ethanol and related ethers, e.g. tert-amyl methyl ether (TAME). Reasons for using MTBE include economic considerations, as some of the production is obtained by adding methanol to isobutylene produced as a by-product of other processes. However, most MTBE facilities have to manufacture the methanol and isobutylene required to produce MTBE.

Ethanol has been advertised as a safe alternative by the agricultural interest groups in the USA and Europe. Its lack of toxicity is not different from MTBE, but as a polar solvent, it drives off nonpolar hydrocarbons from the gasoline, a problem that MTBE does not cause. Volatile hydrocarbons from gasoline are known carcinogens and the main contributor to pollution.

Advocates of both sides of the debate in the United States sometimes claim that gasoline manufacturers have been forced to add MTBE to gasoline by law. It might be more correct to say they have been induced to do so, although any oxygenate would fulfill the law.

In 2003, California was the first state to start replacing the MTBE with ethanol. Several other states started switching soon afterward.

Higher quality gasoline is also an alternative, i.e. so that additives such as MTBE are unnecessary. Iso-octane itself is used. MTBE plants can be retrofitted to produce iso-octane from isobutylene. Iso-octane is the ideal gasoline, being the standard reference for the octane rating.

In the long run, diesel fuel is also an alternative, although it requires a major switchover to diesel-run cars. There are several varieties of biodiesel; both oxygen-containing esters and oxygen-free alkyl biodiesels are available.

As a solvent

As a solvent, MTBE possesses one distinct advantage over most ethers- it has a much lower tendency to form explosive organic peroxides. Opened bottles of diethyl ether or THF can build up dangerous levels of these peroxides in months, whereas samples of MTBE are usually safe for years (but they should still be tested periodically). For this reason (as well as its higher boiling point), it is used as a solvent extensively in industry, where safety concerns and regulations make working with diethyl ether, THF, or other ethers much more difficult and expensive. However, despite the popularity of MTBE in industrial settings, it is rarely used as a solvent in academia. Research volumes are much smaller, leading to lower risks from other ethers, and in addition the use of MTBE as a solvent is very rare in literature synthetic procedures.

As a chemical reagent

Being an ether, MTBE is a Lewis base. However, unlike other ethers such as diethyl ether or THF, it does not coordinate well enough with magnesium to be used for making Grignard reagents. The tert-butyl group is easily cleaved off under strongly acidic conditions (forming a moderately stable carbocation), particularly if heated (isobutylene is lost), something which can limit the use of MTBE as a solvent.

Persistence and pervasiveness in the environment

The main environmental risk of MTBE is that it gives water an unpleasant taste at even very low concentrations, and thus can easily render large quantities of groundwater non-potable. MTBE is often introduced into water-supply aquifers by leaking underground storage tanks (USTs) at gasoline stations. Although USTs are much better constructed now than in the 1980s, accidental releases still take place because of the very large number of USTs. The high solubility and persistence of MTBE cause it to travel faster and farther than many other components of gasoline when released into an aquifer. It is also released when gasoline containing MTBE is spilled onto the ground. Because it is water soluble, it easily moves through soil, polluting both surface and groundwater.

MTBE has widespread occurrences in the aquifers of North America, where the majority of groundwater chemistry data has been acquired. As one regional example, the San Francisco Bay Area Regional Water Quality Control Board has indicated MTBE is one of the groundwater pollutants of most widespread concern in this metropolitan region.

MTBE is biodegradable to CO2 and water under aerobic conditions with the correct bacteria. However, the natural occurrence of these bacteria with the ability to break ether bonds is not high, and it appears that most strains of MTBE-oxidizing bacteria are slow-growing bacteria with low biomass production per unit MTBE oxidized. In the proper type of bioreactor, such as a fluidized bed bioreactor, MTBE can be rapidly and economically removed from water to undetectable levels.

Health risks

The IARC, a cancer research agency of the World Health Organization, maintains MTBE is not classifiable as a human carcinogen. MTBE can be tasted in water at concentrations of 5 – 15 µg/l.

MTBE is not classified as a human carcinogen at low exposure levels by the International Agency for Research on Cancer (IARC). However, exposure to large doses of MTBE carries significant non-cancer-related health risks. The effects of the prolonged presence of this alcohol derivative are not fully understood.

As of 2007, researchers have limited data about the health effects of ingestion of MTBE. The Environmental Protection Agency (EPA) has concluded that available data are not adequate to quantify health risks of MTBE at low exposure levels in drinking water, but that the data support the conclusion that MTBE is a potential human carcinogen at high doses.

More about MTBE from EPA.gov Site

Is MTBE harmful to humans?

The majority of the human health-related research conducted to date on MTBE has focused on effects associated with the inhalation of the chemical. When research animals inhaled high concentrations of MTBE, some developed cancers or experienced other non-cancerous health effects To date, independent expert review groups who have assessed MTBE inhalation health risks e.g., Interagency Assessment of Oxygenated Fuels) have not concluded that the use of MTBE-oxygenated gasoline poses an imminent threat to public health. However, researchers have limited data about what the health effects may be if a person swallows (ingests) MTBE. EPA's Office of Water has concluded that available data are not adequate to estimate potential health risks of MTBE at low exposure levels in drinking water but that the data support the conclusion that MTBE is a potential human carcinogen at high doses. Recent work by EPA and other researchers is expected to help determine more precisely the potential for health effects from MTBE in drinking water.

EPA reviewed available health effects information on MTBE in its 1997 Drinking Water Advisory guidance and decided that there was insufficient information available to allow EPA to establish quantitative estimates for health risks and as such would not set health advisory limits. The drinking water advisory document indicates that there is little likelihood that MTBE in drinking water will cause adverse health effects at concentrations between 20 and 40 ppb or below.

Has EPA set a drinking water health standard for MTBE?

EPA has not set a national standard for MTBE, although some states have set their own limits. EPA will issue a secondary drinking water standard, based on taste and odor, by late Fall 2000. This taste and odor standard will serve as a guideline that states may adopt. In December 1997, EPA issued a Drinking Water Advisory that states concentrations of MTBE in the range of 20 to 40 ppb of water or below will probably not cause unpleasant taste and odor for most people, recognizing that human sensitivity to taste and odor varies widely. The advisory is a guidance document that recommends keeping concentrations below that range. EPA also reviewed the available information on health effects in the 1997 advisory and stated that there is little likelihood that MTBE concentrations between 20 and 40 ppb in drinking water would cause negative health effects.

EPA is continuing to study both the potential health effects and the occurrence of MTBE, and it is on a list of contaminants (Contaminant Candidate List) for which EPA is considering setting health standards. As a means of gathering occurrence information, beginning in 2001, EPA will require all large drinking water systems and a representative sample of small systems to monitor and report the presence of MTBE (Unregulated Contaminant Monitoring Regulation).

How do I know if I have MTBE in my water?

It is possible your water would taste and/or smell like turpentine if MTBE is present at levels around or above 20-40 ppb (some people may detect it at even lower levels). Though you cannot currently purchase a home testing kit, you can determine if your water contains MTBE the following ways. If your drinking water is supplied by a public water system, you can contact the system directly and ask whether they monitor for MTBE and what levels, if any, have been detected.

How does MTBE get in drinking water sources?

There are opportunities for MTBE to leak into the environment (and potentially get in drinking water sources) wherever gasoline is stored, and there are opportunities for it to be spilled whenever fuel is transported or transferred. While federal and state programs minimize the potential for leaks and spills, no system is foolproof.

Contamination of drinking water sources can occur from leaking underground and above ground fuel storage tanks, pipelinees, refueling spills, automobile accidents damaging the fuel tank, consumer disposal of "old" gasoline", emissions from older marine engines, and to a lesser degree, storm water runoff, and precipitation mixed with MTBE in the air .

What happens when MTBE gets into the environment?

Because MTBE dissolves easily in water and does not "cling" to soil very well, it migrates faster and farther in the ground than other gasoline components, thus making it more likely to contaminate public water systems and private drinking water wells. MTBE does not degrade (breakdown) easily and is difficult and costly to remove from ground water.

How long will MTBE remain in water?

MTBE is generally more resistant to natural biodegradation than other gasoline components. Some monitoring wells have shown little overall reduction in MTBE concentration over several years which suggests that MTBE is relatively persistent in ground water.