Environmental Engineering Reference
In-Depth Information
However, once phenanthrene was degraded, pyrene metabolism com-
menced at a faster rate than that seen with no preexposure to phenanthrene.
These results have important implications for land-farming treatment.
If the bioavailability of the PAHs can be enhanced by the addition of sur-
factants, then once the readily degradable PAHs are removed, one can
assume that, for a short period, the microbial communities will be fully
induced and this will be the time in which maximum cometabolism can be
expected. Thus, if organisms with broad cometabolic capabilities are to be
added to soil during land-farming treatment, timing is important. They must
be added during a time early enough in treatment when they will be induced
by growth on low-molecular-weight PAHs, yet not so late that they are
outcompeted for these low-molecular-weight PAHs by the indigenous micro-
bial communities.
7.1.3.5.5 Surfactants to enhance bioavailability.
7.1.3.5.5.1 Chemical surfactants.
One limitation to degradation of
HMW PAHs is their low solubility in water and high affinity for surfaces.
Bioavailability of HMW PAHs in soil has been extensively studied, and it is
clear that slow desorption of the PAHs from soil particles plays a key role
in the ability of bacteria to degrade these PAHs. As a contaminated soil ages,
PAHs tend to move into the deeper recesses of soil particles, soil aggregates,
and the organic matter sorbed to soil particle surfaces (Cornelissen et al.,
1998; Jixin et al., 1998; Pignatello and Xing, 1996; Zhang et al., 1998). Con-
sequently, desorption is usually described as a rapid initial release of PAHs
that are close to the surface and a very slow release of PAHs that are more
deeply sorbed. Although considerable amounts of sorbed PAHs will even-
tually leach out over years, this time frame is usually too long for
shorter-term remediation techniques, such as land-farming treatment
(months), to be effective. If strategic modifications of bioremediation tech-
niques can be made to increase desorption rates over the shorter treatment
term, then the added amount of degradation may mean meeting cleanup
goals in a reasonable time.
Because most microorganisms take in PAHs from the aqueous phase,
their degradation rate is often limited by the mass transfer from the sorbed
or nonaqueous liquid phase into the aqueous phase. One method to enhance
PAH transfer rates into the aqueous phase is to add surfactants to increase
micellar solubilization of the PAHs (Luning Prak and Pritchard, 2002b).
Surfactants have been found to enhance the degradation rates of individual
PAHs in pure and mixed cultures (Grimberg et al., 1996; Guha and Jaffe,
1996; Guha et al., 1998; Liu et al., 1995; Madsen and Kristensen, 1997; Tiehm,
1994; Volkering et al., 1995; Willumsen et al., 1998). Success is controlled by
the type and concentration of surfactant utilized and the type of organisms
tested. Many surfactants can be toxic to the microorganisms used. In the
work of Willumsen et al. (1998), Tween 80 (0.24 m
M
) had a stimulatory effect
on the mineralization of fluoranthene by both
Sphingomonas
and
Mycobacte-
rium
sp., but Triton X-100 (0.48 m
M
) was quite toxic to most PAH degraders,
