Focus and Controversy: For Health and Environmental Protection, MTBE Should Be Banned or Restricted from Use in Gasoline

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Preface Methyl tert-butyl ether (MTBE) is a colorless, transparent ether with a high octane rating. Currently, MTBE is primarily used in oil blending to increase octane number, with a typical addition level of around 10%–12%. As a gasoline antiknock agent, MTBE has the advantage of significantly increasing octane rating, but an increasing number of studies indicate that it poses potential threats to the environment and human health. Following the U.S. Environmental Protection Agency’s classification of MTBE as a carcinogen, several countries in North America and Europe have successively introduced policies to prohibit or restrict the use of MTBE in gasoline. 1 MTBE’s anti-knock properties and its contribution to RON: The octane number (RON) according to the Research Method is 117, while the Octane Number on a Motor Test (MON) is 101. Adding 10% (v%) of MTBE can increase the RON of gasoline by approximately 1.8 to 2.0 units. Unlike common ethers such as diethyl ether, the unique molecular structure of MTBE gives it good chemical stability when used as a gasoline additive, preventing the formation of peroxides ； It is miscible with gasoline in any proportion ； The red vapor pressure of the fuel increases significantly after addition; therefore, it is used as a high-octane component in lead-free gasoline and as a blending component in oxygenated gasoline. As an ether oxide, MTBE can undergo chemical reactions with the unsaturated hydrocarbons in gasoline during combustion to form epoxides. This reduces the concentration of peroxides generated throughout the combustion process, prevents the formation of multiple flame centers, and weakens the spread of active combustion cores to unburned areas, thereby improving the anti-knock properties of gasoline. 2 Toxicity of MTBE: The acute toxicity of MTBE is greater than that of diethyl ether; its **index is lower. The **concentration (AC50) and lethal concentration (LC50) for mice are 1.0 and 1.6 mmol/L respectively. Studies by Li Suyun and others have shown that after acute exposure to MTBE, the cerebral cortex, thalamus, hypothalamus, and cerebellar cortex are the areas affected by the acute toxic effects; the relative gray value of Fos protein expression increases gradually as the dose of MTBE exposure rises. Single-cell gel electrophoresis experiments in molecular genetic toxicology have shown that MTBE and its metabolites can cause DNA damage or single-strand breaks in human leukemia cells, as well as DNA damage in nucleated cells in human peripheral blood. Zhou Wei and colleagues used immunohistochemistry and dot blotting to find that MTBE can alter the cell cycle of NIH/3T3 cells, and it induces high expression of the proto-oncogene c-myc in rat liver tissue. This induction of cell proliferation and inhibition of apoptosis are among the possible mechanisms underlying its carcinogenicity in animals. In China, Li Dongmei and others cultured mouse germ cells in vitro and exposed them to MTBE concentrations of 100 ppb, 10 ppm, 1000 ppm, and 3000 ppm for 6 hours, 12 hours, and 18 hours respectively. Using FDA and PI staining, flow cytometry, and the MTT assay, they analyzed the toxicity of MTBE on cells and DNA. The results showed that 3000 ppm of MTBE can directly exert toxic effects on mouse germ cells. 3 Effects of MTBE on the environment and human health 3.1 Effects on the environment Leaks from gasoline storage tanks and pipelines are the main sources of MTBE pollution ； Furthermore, MTBE that has diffused into the atmosphere and been released onto land also re-enters groundwater through rainfall and river runoff. Due to these leakage reasons, MTBE eventually ends up in the groundwater. Between 1993 and 1994, the water quality investigation team of the U.S. Geological Survey’s South Dakota office inspected 482 wells in cities across the United States and their surrounding areas, and MTBE was detected in 13% of them ； The investigation team from the Connecticut branch compiled records of 16,717 water samples from various communities in 10 states in the Northeastern United States and the adjacent Mid-Atlantic region, and found that 9% of them contained MTBE. Numerous studies have shown that when MTBE is released into the atmosphere in gaseous form, it can undergo photooxidation reactions, with a half-life in the air of approximately 4 days. However, since MTBE is highly soluble in water (4.3 g/100 g at 20°C) and not easily adsorbed by soil particles, it enters groundwater more rapidly than gasoline and other components, and spreads in all directions through diffusion. In December 1997, the U.S. Environmental Protection Agency (EPA) stated that MTBE levels in drinking water of 20–40 μg/L or higher can cause unpleasant tastes and odors; even at low concentrations, it can give water an unpleasant smell, making MTBE a pervasive contaminant of groundwater. Groundwater pollution is characterized by its stealthiness and irreversibility. When humans consume groundwater contaminated with harmful or toxic substances, the impact on human health is a chronic, long-term effect that is not easily detectable; moreover, once groundwater is polluted, it is very difficult to clean up and restore. MTBE does not volatilize or get extracted easily from water; contaminated groundwater can penetrate several hundred meters over a period of ten years with little to no degradation, taking even longer than benzene to degrade. Generally, biodegradation is the most effective option for the treatment of pollutants, but the biodegradation of MTBE differs significantly from that of common gasoline pollutants such as benzene, toluene, ethylbenzene, and xylene. Its low organic carbon partition coefficient limits its adsorption in natural water bodies. Its high water solubility enables MTBE to migrate rapidly in groundwater and surface water, thereby reducing the time available for microbial degradation and further increasing the difficulty of its degradation. Liu Shejiang investigated the aerobic degradation patterns of methyl tert-butyl ether (MTBE) using mixed microorganisms that had been self-enriched and domesticated. It was found that the growth of microorganisms using MTBE as a carbon source was extremely slow, resulting in poor biodegradability of MTBE; the highest degradation rate under the studied conditions was 48.88%, and no further degradation of the intermediate product tert-butanol (TBA) was observed within a degradation time of about 200 hours. Under both aerobic and anaerobic conditions, MTBE remains stable in water. MTBE leaks are generally not detected simultaneously with BTEX (benzene, toluene, ethylbenzene, xylene). The underwater portion of an MTBE plume originating from a point source is relatively large; the concentration of MTBE at the edges of the plume is very low, but it increases over time. Studies have shown that MTBE can move rapidly like a tracer, and this cumulative effect of water pollution along with its diffusion in the soil further exacerbates the negative impacts on environmental persistence. 3.2 Effects on human health MTBE is primarily absorbed through the respiratory tract, but it can also be absorbed through the skin and digestive tract. In 1992, Alaska in the United States reported the health hazards of MTBE to humans; residents complained that MTBE caused symptoms such as headaches, dizziness, eye irritation, and nausea. Subsequently, residents in cities such as Wisconsin and New Jersey also experienced similar discomfort symptoms. Studies on American volunteer subjects have shown that after exposure to MTBE, all subjects exhibited irritation symptoms to varying degrees, with slight effects on the central nervous system. The study also found that in adults who consumed water contaminated with MTBE (76 μL/m3) over a long period of time (5–8 years), the rate of lymphocyte apoptosis was significantly higher compared to the control group. In recent years, domestic researchers have gradually begun to conduct health risk assessments of MTBE exposure among professionals. Li Qin and her colleagues used the health risk assessment models recommended by the U.S. Environmental Protection Agency (EPA) to measure MTBE concentrations at the breathing zone height in 9 urban gas stations and 3 suburban gas stations in Nanning. The results showed that the MTBE concentration in the breathing zones of various gas stations ranged from 2.95 to 1,530 μg/m3, with the average concentration being the highest in urban gas station areas, reaching 1,530 μg/m3 ； The health risk to gas station workers due to MTBE through inhalation ranges from 4.45×10-10 to 2.29×10-5, with the carcinogenic risk being higher than the non-carcinogenic risk. The health risks associated with inhalation exposure to MTBE for some gas station workers have exceeded the acceptable level for the general population (10^-6). Numerous studies have also confirmed that MTBE has a strong carcinogenic effect on animals. Chronic toxicity tests on mice using MTBE showed that as the concentration of MTBE increased, the likelihood of liver cell canceration in these mice increased as well. Although there is currently insufficient experimental data to prove that MTBE is carcinogenic to humans as well, MTBE does cause respiratory difficulties and asthma, as well as allergic symptoms such as dizziness, headaches, insomnia, swollen eyes, and rashes, thus having a significant impact on human health. 4. Defects of MTBE in petroleum applications: Gasoline standards specify that the oxygen content in gasoline should not exceed 2.7%, but MTBE is an ether compound with a relatively high oxygen content of 18.2%; therefore, the proportion of MTBE that can be added to gasoline is limited by this oxygen content. Adding more will lead to an excessive oxygen content, accelerate the oxidation of gasoline and reduce the induction period, as well as increase the amount of gum. Furthermore, in engines that use closed-loop control and operate at the theoretical air-fuel ratio, if too much MTBE is added to the gasoline, the excess oxygen will interfere with the operation of the oxygen sensor-based closed-loop control system. This causes the air-fuel mixture in the engine to become slightly leaner than the theoretical ratio, resulting in a reduced conversion efficiency of the three-way catalytic converter and increased NOX emissions. In lean-burn engines as well, oxygen enrichment can degrade operational stability, leading to increased emissions. Research by a Japanese research institution shows that when the MTBE content in gasoline exceeds 7%, nitrogen oxides emitted by vehicles increase significantly. In Japan, the amount of MTBE added to **standard lead-free gasoline is kept at 7% or less. Another issue associated with MTBE as a gasoline anti-knock agent is its transportation and storage. Due to the low boiling points and high volatility of ether compounds, the loss rate of MTBE during transportation and storage is much higher than that of gasoline, thereby affecting the quality of gasoline to some extent. On the one hand, it not only increases the cost of additives but also affects the octane rating during the storage period of gasoline. On the other hand, the volatile nature of MTBE pollutes the atmospheric environment in storage areas. 5 Prospects for the Use of MTBE 5.1 Abroad: In the 1990s, California in the United States banned the use of MTBE as a gasoline antiknock agent at the end of 2002, after traces of MTBE were detected in drinking water. States such as Arizona, Connecticut, and New York also banned MTBE in 2003. In 2005, the United States passed the Energy Policy Act of 2005, which established Renewable Fuel Standards (RFS), requiring ethanol to replace MTBE as an oxygenate in gasoline. The U.S. Environmental Protection Agency (EPA) enacted a law under the Toxic Substances Control Act prohibiting the use of MTBE in gasoline. In 2003, New Hampshire in the United States filed charges against ExxonMobil, alleging that the company’s gasoline additive methyl tert-butyl ether (MTBE), which was used to reduce air pollution in the 1970s and 1980s, had contaminated the state’s groundwater. In April 2013, a jury in New Hampshire, United States, ruled that ExxonMobil was liable for this civil lawsuit totaling $236.4 million. On June 20, 2014, according to Bloomberg, Pennsylvania filed lawsuits against companies such as ExxonMobil, Chevron, BP, and Westlake Petroleum over groundwater contamination caused by MTBE resulting from leaks in underground tanks in the last century, as well as the costs associated with cleanup efforts. Following the U.S. EPA’s classification of MTBE as a potential carcinogen, various countries began to pay attention to its hazards and impacts on the environment and human health. In 2004, the Netherlands, applying the precautionary principle, proposed that the MTBE content in drinking water should not exceed 1 μg/L. Germany set a level of 15 μg/L for MTBE as a criterion for assessing groundwater contamination, while Switzerland used a level of 2 μg/L of MTBE in groundwater as a benchmark for detecting gasoline-derived compounds in such water. Finland adopted Decree No. 214/2007, which sets a threshold of 0.1 mg/kg for MTBE in the context of soil contamination and remediation, while also establishing strict limits of a minimum of 5 mg/kg and a maximum of 50 mg/kg. Denmark was among the first countries to phase out the use of MTBE as a additive in gasoline, and in 2007 a law was enacted requiring that the MTBE content in drinking water not exceed 5μg/L. Since 2009, almost all gasoline producers in Canada have stopped producing and selling gasoline with new MTBE formulations. Mexico, France, and other countries have also begun to reduce and restrict the use of MTBE, with similar trends observed in Spain, Germany, and Italy. On the other hand, the incentives for renewable fuels have also accelerated the reduction in MTBE production, while Europe has sped up the conversion of MTBE plants to produce ETBE (ethyl tert-butyl ether). Between 2005 and 2006, multiple MTBE production facilities in Europe operated by large companies such as Oxene, Total, Norsk Hydro, Fortum, and Miro switched to producing ETBE. 5.2 In the late 1990s in China, the General Administration for Quality Supervision, Inspection and Quarantine issued the **standard for lead-free gasoline for vehicles**. The average amount of MTBE added to gasoline in China was around 0.4%, but by 2008 this average increased to 3.1%. In recent years, due to the continuous improvement of gasoline standards, particularly with regard to sulfur content, the National V gasoline standard specifies a limit of no more than 10 mg/kg. As refineries use different processes to reduce sulfur content, this results in varying degrees of loss of gasoline octane rating. To compensate for or increase the RON of gasoline, the amount of MTBE added is increased. 6 Recommendations 6.1 Restricting and banning the use of MTBE in gasoline. In Europe and the United States, MTBE is considered an indicator for determining pollution levels; the California Water Resources Control Board’s 2012 “Low-Risk Closure Policy for Underground Storage Tanks” still requires that MTBE testing be conducted on soil and groundwater at sites where such tanks leak, before those sites can be deemed closed. Although China has not yet conducted a hazard assessment of MTBE, the experience and lessons learned from the United States—where MTBE was used as a gasoline anti-knock agent for 20 years before it was banned and ethanol fuel was subsequently developed again—should prompt us to re-evaluate MTBE. Based on the comprehensive standards for vehicle gasoline, it is recommended to limit the amount of MTBE added in Standard IV, and to stop using MTBE as an anti-knock agent in gasoline under Standard V. 6.2 As the C4 conversion to the isooctane MTBE industry faces challenges, the C4 produced as a by-product of FCC in domestic refineries also needs new outlets. Companies such as Axens North America and CDTech possess the technology for modifying MTBE production facilities, enabling them to convert the tetrahydrocarbon feedstocks used in MTBE production into isooctene, which is then added to gasoline; or to hydrogenate isooctene into isooctane, which, as a high-octane compound, is an ideal component for blending into gasoline. 6.3 Development of new green high-octane anti-knock agents To protect the environment, countries around the world are imposing increasingly stringent requirements on the quality of gasoline, and domestic standards for vehicle gasoline emissions are also becoming more strict. Therefore, the development of new green high-octane anti-knock agents represents a future development trend. In addition to having high octane rating and not compromising the performance of gasoline, new anti-knock agents should also exhibit good compatibility with various types of gasoline, while contributing to the improvement and purification of gasoline quality. Among them, rare earth polymers and organic ash-free compounds could both become new directions for research and development.