MTBE and other oxygenates typically enter the environment blended with gasoline or other refined fuel products. However, these oxygenates migrate differently within the environment because of the differences in physical properties between oxygenates and the other components of gasoline, such as BTEX, of which benzene is typically the most common contaminant of concern. Table 24.2 contains a summary of some properties that influence the migration of MTBE and other oxygenates in the environment. These physical properties also influence the treatability of MTBE and other oxygenates.
Fuel oxygenates generally exhibit the following physical properties relative to benzene:
1. Greater tendency to partition into the vapor phase from the nonaqueous phase (vapor pressure) (with the exception of TBA, ethanol, and TAME).
2. Greater solubility in water.
3. Lesser tendency to partition to organic matter in soil (soil adsorption coefficient).
4. Lesser retardation factor (slowing of migration with groundwater due to sorption to aquifer matrix).
5. Lesser tendency to partition into the vapor phase from the aqueous phase (Henry's law constant).
Because of their relatively higher vapor pressure, ether-based oxygenates in fuel will tend to volatilize from releases (nonaqueous phase) exposed to the open air more rapidly than benzene. Alcohol-based oxygenates will volatilize less rapidly than benzene. However, once fuel oxygenates enter the subsurface and become dissolved in groundwater (aqueous phase), they are significantly less volatile (lower Henry's constant) than benzene. Oxygenates are many times more soluble than benzene; the concentrations of MTBE in groundwater as high as 1,000,000 pg/L are not uncommon. Also, because MTBE dissolved in groundwater will partition (as a function of its soil adsorption coefficient) to the organic matter in the surrounding soil less readily than benzene, a dissolved MTBE plume typically migrates faster than a dissolved benzene plume (lower retardation factor). As a result, MTBE contamination can result in a relatively larger groundwater plume, compared with plumes originating from gasoline constituents.24,25
While all fuel oxygenates are more water soluble than other gasoline components (benzene), variations in molecular structures result in a range of physical properties and affect the way each of them migrate in the environment. Figure 24.2 summarizes the physical properties discussed above for each of the common fuel oxygenates relative to benzene, which is most often the chemical of concern in gasoline. While the physical properties for each oxygenate are different, there are similarities among the alcohols and ethers. For example, as shown in Figure 24.3, alcohol-based oxygenates have a relatively greater water solubility and a much lower Henry's constant than the ether-based oxygenates. Other properties such as the soil partition coefficient, vapor pressure, and retardation factor do not adhere to these same groupings.
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