**1.3 Environmental impact of oxygenated fuels**

Despite providing better conditions in terms of fuel quality, an environmental impact from oxygenated fuel hydrocarbons related to widespread contamination of groundwater and other natural waters exists. The distribution and storage of crude oil and refined products result in releases of significant amounts of hydrocarbons to the atmosphere, surface waters, soils, and groundwater. Groundwater contamination by crude oil, and other petroleum-based liquids, is a particularly widespread problem. In Mexico, the agency in charge of producing and distributing fuels derived from petroleum distillation, such as gasoline, diesel, fuel oil, diesel, and LP gas, is Petróleos Mexicanos (PEMEX). The retail distribution of gasoline and diesel is carried out by service stations (gas stations). One of the environmental risks that involves the handling of these stations is spills or leaks of fuels, which cause the contamination of the sites where the storage tanks are located [15].

 Unfortunately, these oxygenates have high water solubility and high volatility, causing a high concentration of oxygenated fuel in the environment, air, and water. Another important problem happens when oxygenated fuel is accumulated in the groundwater due to it not absorbing appreciably to soil and undergoing only in slow biodegradation compared to the benzene, toluene, ethylbenzene, and the xylenes (BTEX) in gasoline. The relatively recalcitrant nature of oxygenated fuel to microbial attack makes them persistent, due to them being refractory to the biological treatments. Because of the chemical structure of these, oxygenated fuel hinders their natural biodegradation, which contains a combination of two biorecalcitrant organic functional groups: the ether bond and tertiary carbon atom. These are the reasons why water supplies close to the production sites of MTBE, ETBE, and TAME or near underground petroleum storage tanks and fuelling stations are often contaminated by large amounts of these compounds [16, 17].

There have been extensive occurrences of groundwater contamination by MTBE in the United States because of its prevailing use. In a sampling study of 1208 domestic wells in the United States, MTBE was the most frequently detected fuel oxygenate and the eighth most commonly detected VOC. Perhaps the most publicized case of MTBE contamination of groundwater is the one involving public water supply wells in Santa Monica, California. In August 1995, the city of Santa Monica discovered MTBE in wells used for drinking water supply through routine analytical testing of well water [18, 19].

MTBE has been detected in snow, storm water, surface water (streams, rivers, and reservoirs), groundwater, and drinking water, based on limited surveillance operations conducted in the United States. MTBE concentrations found in storm water ranged from 0.02 to 8.7 μg/l, with a median value of less than 1.0 μg/l. In streams, rivers, and reservoirs, the detection range was 0.2–30 μg/l, and the range of median values in several studies was from 0.24 to 7.75 μg/l [5, 19].

In fact, the US Environmental Protection Agency (USEPA) included MTBE in its Contaminant Candidate List. MTBE in drinking water is carcinogenic for humans and animals. USEPA established a drinking water health advisory of 20–40 μg/l MTBE in December 1997, because it is hazardous to human health (US Environmental Protection Agency, 1997).

Although alternative ether oxygenates are detected less frequently than MTBE, these alternative oxygenates show future groundwater contaminations similar to MTBE if they are not under control.

 The toxicokinetic data on MTBE in people come mainly from controlled studies of healthy adult volunteers and in a population exposed to oxygenated gasoline. MTBE quickly passes into circulation after inhalation exposure. In healthy volunteers exposed to inhalation, MTBE kinetics was linear up to concentrations of 268 mg/m3 (75 ppm). It was measured in the blood and urine of people exposed to tertiary butyl alcohol, metabolic MTBE. The maximum blood concentrations of tertiary butyl alcohol were 17.2–1144 μg/m3 and 7.8–925 μg/m3 , respectively, in people exposed between 5.0 and 178.5 mg/m3 (1.4–50 ppm) of MTBE. Based on a singlebehavior model, rapid (36–90 min) and slow (19 h) components of MTBE half-life were identified (41). Following the introduction of two separate fuel programs in the United States, which require the use of gasoline oxygenation products, consumers in some areas have complained of acute health disorders, such as headaches, irritation of the eyes and nose, cough, nausea, dizziness, and disorientation. The acute experimental toxicity (CL50) of MTBE in fish, amphibians, and crustaceans is greater than 100 mg/l.

 WAO consists of an oxidation in aqueous medium at high temperatures using pure oxygen or high pressure air as an oxidant to maintain the liquid phase. The pressures used and reported in the literature range from 20 to 200 bar and temperature between 150 and 350°C, making this process highly expensive for industrial application. The use of catalysts allows reducing the temperature and pressure conditions for oxidation and even increasing the selectivity toward CO2. That is why we employ catalytic wet air oxidation, instead of WAO. CWAO of MTBE and oxygenated fuels of gasoline as ETBE and TAME is a nonconventional treatment for degradation of organic compounds in aqueous medium. Our research group developed a wide study in order to evaluate several catalysts and to know what the best are for the efficiency of oxidation process and the total mineralization of pollutants into CO2 and H2O.
