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The feasibility of phytoremediation of groundwater contamination with methyl-tert-butyl ether (MTBE)
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was examined experimentally using a six-channel soil system with or without plants. Two bacterial strains
capable of degrading MTBE were each added to two out of six channels. A solution of 0.84 mM MTBE was
continuously fed into each channel at 1 L/day until a stable MTBE concentration level in the groundwater was
established; then the feeding was switched back to distilled water. The channel groundwater effluent MTBE
concentration and the soil gas MTBE fluxes were monitored from the beginning of the MTBE solution feeding
until no MTBE was detected. Integration of the gas flux data indicated that the four vegetated channels with
introduced bacteria had less MTBE at the soil surface than channel 3 which was vegetated but without any
introduced bacteria. The total mass balance for MTBE showed that the fractions of MTBE that were not
recovered in the planted channels were significantly higher than in the unplanted channel. Analysis of the
experimental data indicates that, due to the presence of the plants, MTBE might have been undergoing enhanced
rhizosphere biodegradation.4615
ABSTRACT
Key words: phytoremediation, MTBE, biodegradation, mass balanceINTRODUCTION
Methyl-tert-butyl ether (MTBE), other ethers such as ethyl-tertiary-butyl ether, tertiary-amyl-
methyl ether, tertiary-amyl-ethyl ether, dimethyl ether, diisopropyl ether, tertiary-butyl alcohol, and
ethanol and methanol have been used as fuel oxygenates since 1988 to improve air quality. Most
refiners have chosen to use MTBE because it can be produced at the refinery (ease of production
and lower cost); it blends easily without separating from gasoline; and the MTBE-gasoline blend can
be transferred through existing pipelines. MTBE was first added to gasoline in the late 1970’s to
replace lead as an anti-knocking agent (NSTC, 1997). In the United States, almost all MTBE is
used in gasoline. As a principal fuel additive, MTBE is added to gasoline in high concentrations
(approximately 15% on a volumetric basis) to increase octane levels and to enhance combus-
tion of gasoline.
Gasoline spills and leaks from pipelines, underground and aboveground storage tanks, and
other transport modes are major sources of MTBE contamination. In addition, uncombusted
gasoline is also spilled from boats and recreational equipment directly to surface waters, which may
be water supply reservoirs (MTBE Fact Sheet, 1998). Several events have raised concern over the
safety of MTBE. In 1996, the city of Santa Monica closed some of its major drinking water wells
after discovering MTBE contamination.
As part of the U. S. Geological Survey’s (USGS) National Water Quality Assessment
program, samples of shallow groundwater were taken from eight urban areas during 1993 to 1994(Squillace et al., 1996). The results show that, of the 60 volatile organic compounds (VOCs)
analyzed, MTBE was the second most frequently detected compound. Having been found in 6.9%
of the storm water samples collected by the USGS in 16 cities and metropolitan areas, MTBE was
the seventh most frequently detected VOC (Squillace et al., 1997, 1998).
Vapor pressure reflects the volatilization potential of a chemical. The vapor pressure for
MTBE at 25o
C is 245 mmHg (about one-third of the atmosphere), which is 2.5 times higher than
that for benzene.
Water solubility is an indication of the extent to which the compound can dissolve into the
water phase. MTBE is very water soluble compared to the BTEX compounds (benzene, toluene,
ethyl benzene, and xylene) and other compounds in gasoline. The solubility of MTBE in water is
about 50g/L, whereas the next most soluble component of gasoline is benzene, which has a solubil-