Environmental Engineering Reference
In-Depth Information
5
4.5
4
3.5
3
2.5
Saline
50% Culture Supernatant
100% Culture Supernatant
2
1.5
1
0.5
0
Compound
Figure 7.18
Concentration of PAHs in aqueous extracts of SMWT soil with saline
(control) and with the addition of
P. aeruginosa
strain 64 culture supernatant.
making them bioavailable to naturally occurring PAH degraders. Surfactants
may sorb onto soil particles and form structures such as hemimicelles and
admicelles even when present at concentrations well below the CMC; thus,
sorbed surfactants may play a role in facilitating transport of sorbed sub-
strates (Volkering et al., 1998). Alternatively, the rhamnolipid could be acting
on the autochthonous bacterial directly, by modifying the cell surfaces of the
bacteria, rendering them more hydrophobic and thus encouraging bacte-
ria-PAH interactions. Rhamnolipids produced by
P. aeruginosa
can modify
the cell surface hydrophobicity of other bacteria and affect biodegradation
rates (enhancing for some bacteria and inhibiting for other bacteria) (Zhang
and Miller, 1994). Zhang and Miller (1994) also reported that the bioavail-
ability of octadecane in the presence of rhamnolipid biosurfactant was con-
trolled by both aqueous dispersion of octadecane and cell hydrophobicity.
7.4.1.2 Biosurfactant production
The results of the PAH desorption from two different contaminated site soils
under water extraction are presented in Figure 7.18 and Figure 7.19. The
biosurfactant caused increases in the amount of PAHs that could be extracted
from soil with water, indicating that they would consequently be more
available to degrading microorganisms. Similar results were obtained for
P.
aeruginosa
strain 64 supernatant used to extract the PAH-contaminated soil
from the POPILE site (Figure 7.19).
These increases generally correlate well with the amount of surfactant
added, particularly for the lighter PAHs. Results for the higher-molecu-
lar-weight PAHs suggest that a threshold concentration of the surfactant
