The properties of MTBE and other oxygenates, including water solubility, vapor pressure, soil adsorption coefficient, retardation factor, and Henry's law constant, affect their fate and transport in the environment relative to other contaminants. These same properties also affect the selection and design of remediation technologies used to address soil and water contaminated with oxygenates. In general, the same types of treatment technologies have been applied for the treatment of BTEX and MTBE; however, design and operating conditions for MTBE may not be the same as for treatment of BTEX. For example, carbon-based adsorption materials that work well for BTEX may not be effective in the removal of MTBE.
Properties of Fuel Oxygenates and Other Fuel Constituents
Solubility into Pressure
Pure Phase H20 from at 25°C, at Henry's Law
Solubility Gasoline 20°C (mm Constant
TBA DIPE ETBE
Taste Th reshold
2039-9000 at 20°C 26,000 12,000
5500 3393 5241 25,000
804 745 3300 1018 1365 2400 1210 1220 57,000
(Dimensionless) Ab Bc
49-56 544 121.6
0.88 94 500
100 NA 112
NA NA 47
Point Density Octane in Water Threshold
Source: Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04—009, United States Environmental Protection Agency, Washington, DC,
b Soil condition A: Organic fraction/^ = 0.001 mg/mg, bulk density = 1.75 kg/L, porosity = 0.25. 0 Soil condition B:/„ = 0.004mg/mg, bulk density = 1.75 kg/L, porosity = 0.25.
Physical property magnitude relative to benzene 0.01 0.1 1 10
Solubility data not available
H Solubility into H2O from gasoline (mg/L) ^log Koc (log 1/kg) D Vapor pressure HI Henry's law constant O Retardation factor (soil condition A)
FIGURE 24.2 Physical properties of fuel oxygenates relative to benzene. (Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04-009, United States Environmental Protection Agency, Washington, DC, May 2004.)
MTBE TBA Ethanol
100 161 175 534.8 804 2400 3300 5500 25,000 57,000 Solubility in water (mg/L)
FIGURE 24.3 Relative solubility and Henry's law constants for selected fuel oxygenates. (Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04-009, United States Environmental Protection Agency, Washington, DC, May 2004.)
BTEX is the contaminant group most often targeted for treatment at gasoline spill sites, and in many cases the treatment systems have been specifically designed to reduce concentrations of benzene. In some cases, a treatment system designed to remove benzene can also remove the oxygenate contamination. However, because of the differences in the physical properties of oxygenates relative to benzene, certain oxygenates might not be effectively treated by a system designed to treat benzene. Also, because the physical properties of individual oxygenates also differ from one another, a treatment system designed to treat one oxygenate may not effectively treat another oxygenate. An overview of the effects of the physical properties of oxygenates on the effectiveness of various remediation technologies is included in the following sections. Primary considerations related to the treatment of oxygenates include
1. The vapor pressures of most oxygenates, with the exception of ethanol, TBA, and TAME, can result in them being more readily volatilized from soil using certain technologies, such as SVE or MPE.25 26
2. The relatively low Henry's constants (the ratio of a compound's concentration in air relative to its concentration in water) of oxygenates can result in them being more difficult to strip from contaminated groundwater via air sparging or air stripping as part of a pump-and-treat remedy.
3. The presence of an ether bond or hydroxyl group in oxygenates results in these compounds being significantly less likely to partition to organic matter (Koc), such as in the use of granular activated carbon (GAC) in pump-and-treat remedies.
4. Because they can be chemically oxidized or biologically degraded, chemical oxidation and biodegradation technologies (both in situ and ex situ) can be effective in the treatment of oxygenates.
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