Environmental Engineering Reference
In-Depth Information
a biosurfactant, particularly if the microbe will grow on one of the contam-
inants or degradation metabolites in the soil to be treated (Arino et al., 1998).
The effect of biosurfactants present at concentrations below their CMCs on
the degradation of PAHs has received little attention. Different hydrocar-
bon-degrading bacterial strains that produce biosurfactants were examined.
Our attention has focused on
Pseudomonas aeruginosa
strain 64, which pro-
duces rhamnolipid biosurfactant on water-soluble and water-immiscible
growth substrates. In this study, biosurfactant application involved
whole-cell cultures because of economic considerations for biosurfactant
addition.
It is known that introducing organisms into contaminated soil is mechan-
ically difficult and often results in poor survival if the organisms are added
from liquid culture (O'Reilly and Crawford, 1989). If bioaugmentation pro-
tocols are to be successful, it is critical to establish a protocol for augmenting
bacteria into established soil communities in a way that maintains their
viability. One approach for improving the effectiveness of bioaugmentation
is to introduce the inoculum on an inert carrier. Immobilization or encapsu-
lation has been shown to enhance survival and activity of the inoculum in
bioremediation scenarios (Lin and Wang, 1991; Oh et al., 2000; O'Reilly and
Crawford, 1989; Rhee et al., 1996; Weir et al., 1995). These studies used a
variety of carriers including alginate, polyurethane foam, and inert granular
formulations. We have had considerable success using vermiculite, an inor-
ganic material that is environmentally safe. Since dry vermiculite adsorbs
several times its weight in liquid, it is possible to add complex microbial
growth medium to sterilized vermiculite and have it soak into the interstitial
spaces. Inoculation with organisms then allows the cells to grow “into” the
vermiculite, where they are shielded from predation and direct competition
with indigenous organisms in the soil and where they can slowly grow and
“shed” cells into the surrounding soil. This prevents the introduced bacteria
from being outcompeted initially and increases the survival rate. Previous
studies in the laboratory have demonstrated that vermiculite carrying
S.
paucimobilis
strain EPA505 cells remained viable in soils for months after
inoculation (Pritchard et al., 1995, 1999). In addition, studies with EPA 505
have shown that its ability to mineralize fluoranthene in soil is not affected
by immobilization on vermiculite (Liu et al., 1995; Pritchard et al., 1995).
Benefits of this procedure include:
•
Increased shelf life of microbial cultures so that they are available
when needed
•
Increased microbial survival in the soil
•
Increased aeration of the soil by the inert carrier material
•
A possible means for slowly releasing added nutrients to the soil
system
Thus, vermiculite appears to be an excellent biocarrier, ensuring some
degree of staying power for the inoculum.
