Environmental Engineering Reference
In-Depth Information
The effect of the rhizosphere on MTBE was studied by
Ramaswami et al. (2003). As was seen in previously
referenced reports, the potential for MTBE to taken up and
volatilized from shoots and leaves is a controlling factor in
the fate of MTBE. Less is known about the fate in the
rhizosphere, other than that MTBE fractions in some roots
were high (Ma et al. 2004). While MTBE is recalcitrant
under anoxic conditions, where it degrades to TBA (Bradley
et al. 2002), MTBE has been shown to be less recalcitrant
than previously thought, and can undergo complete oxida-
tion to CO
2
under aerobic conditions (Bradley et al. 1999,
2001; Landmeyer et al. 2001). Hence, because the rhizo-
sphere is essentially aerobic, it would be anticipated that
MTBE would be degraded. The compromising factor is
that water flow rates might be faster than contaminant bio-
degradation rates.
Ramaswami et al. (2003), however, did not observe
MTBE degradation in aerobic treatments of MTBE
and rhizospheric bacteria. They show results for DO
concentrations in lab microcosms with no soil and for rhizo-
sphere soils with MTBE-acclimated soils. Although they
report no MTBE degradation in the rhizosphere treatment,
the time course lasted only 48 h and the DO immediately
decreased from 8 to 3 mg/L in 1 day; as such, the aerobic
condition necessary to support aerobic MTBE metabolism
was electron-acceptor limited. They increased the time
course to 10 days and still reported little MTBE aerobic
biodegradation. However, the DO concentrations are not
shown, and the frequency of oxygen addition is not reported.
It is likely that these microcosms did not suggest aerobic
MTBE-degrading bacteria in the rhizosphere simply because
of a lack of oxygen.
Rentz et al. (2003) investigated the possibility of increas-
ing the DO content around roots in high-BOD contaminated
sediments and its effect on the growth of plants that might be
planted at such sites to remediate the contaminated soils.
This is significant, because many of the plants that could be
planted at contaminated sites may not have the gas-transport
structures, or aerenchyma, to transport O
2
. Additionally, as
was shown at the site in Texas discussed in Chap. 12, poplar
trees also can decrease the DO in shallow groundwater by
the release of labile organic matter. Hence, poplar trees can
be a source or sink for DO in contaminated aquifers and
vadose zones.
Ma et al. (2004) investigated the fate of MTBE after
exposure to hybrid poplar trees such as DN-34. In the labo-
ratory, they determined the degree of partitioning of gas-
phase MTBE when added to vials that contained samples of
DN-34, such as cuttings, leafs, and roots. The experiment
was then performed again but water replaced the air in the
vials. They reported that MTBE partitioned to a greater
extent to roots than to leaves or cuttings, and much more
so than between water and cuttings. However, even these
partition coefficients are low, due to MTBE's high water
solubility near 50,000 mg/L.
They also grew cuttings in MTBE solutions after previous
rooting and shoot growth in uncontaminated solution. They
reported that growth of the cuttings was not inhibited during
the 7-d exposure to MTBE in the solution. However, tran-
spiration rates decreased significantly, between 36% and
59%, after MTBE was added compared to rates prior to
MTBE addition. MTBE was detected in the diffusion traps
in all the trees that grew in MTBE-spiked solutions
(Fig.
13.11
). The traps positioned at lower elevations
contained more MTBE than traps located higher on the
cuttings. They report that between 12% and 47% of the
14
Fig. 13.11
Fate of
C-MTBE added to cuttings in diffusion traps
(Modified from Ma et al. 2004). One inch is equivalent to 2.54 cm.
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