Environmental Engineering Reference
In-Depth Information
initial dioxygenase system in the cells, which for strain EPA 505 was a
preference of phenanthrene over fluoranthene and fluoranthene over pyrene
(Luning Prak and Pritchard, 2002a).
7.1.3.5.5.2 Biosurfactants.
Microbes produce surface-active agents,
biosurfactants, when grown on insoluble or immiscible compounds (Desai and
Banat, 1997; Lin, 1996; Matsuyama et al., 1986; Neu, 1996; Zajic and Mahomedy,
1984; Zajic and Panchal, 1977). The use of biosurfactants has been proposed as
an alternative to chemical surfactants to enhance the bioavailability of hydro-
phobic contaminants (Hunt et al., 1994; Oberbremer et al., 1990). Biosurfactants
are a structurally diverse group of surface-active agents. They are amphipathic
molecules consisting of a hydrophilic portion and a hydrophobic moiety, usu-
ally in the form of bound fatty acids. They exhibit low interfacial tension (IFT)
and low critical micelle concentration (CMC) values.
The CMC is the amount of surfactant needed to achieve the lowest possible
surface tension. Alternatively, the CMC can be defined as the surfactant con-
centration at which the addition of free monomer will form micellar structures.
In the process of micelle formation, the aggregating surfactant molecules have
the ability to surround slightly soluble compounds, which disperses them into
the aqueous phase (Singer and Finnerty, 1984). Biosurfactants have about a 10-
to 40-fold lower CMC than chemical surfactants (Desai and Banat, 1997). Dis-
tilled water has a surface tension of 73 dyne/cm, and an effective biosurfactant
can lower this value to <30 dyne/cm (Lang and Wagner, 1987). Rhamnolipids
lower the surface tension of water from 72 mN/m to 25 to 30 mN/m
at con-
centrations of 10 to 200 mg/l (Lang and Wullbrandt, 1999).
Biosurfactants can be divided into groups based on their overall struc-
ture: glycolipids, lipopeptides, and high-molecular-weight biopolymers.
Glycolipids contain various sugar moieties (i.e., rhamnose, sophorose, treh-
alose) attached to long-chain fatty acids. Lipopeptides consist of a short
polypeptide (3 to 12 amino acids) attached to a lipid moiety. High-molecu-
lar-weight biopolymers are lipoprotein, lipopolysaccharide-protein com-
plexes, and polysaccharide-protein-fatty acid complexes. Biosurfactants are
produced by a variety of bacteria, including
Pseudomonas
sp.,
Bacillus
sp.,
and
Mycobacterium
(Cooper et al., 1981; Cooper et al., 1989; Patel and Desai,
1997; Yakimov et al, 1995; Zhang and Miller, 1995). They are also produced
by yeasts (Cooper and Paddock, 1984; Hommel et al., 1987) and fungi (Fautz
et al., 1986).
Biosurfactants have several advantages over synthetic surfactants, such
as a lower toxicity for most biosurfactants (Lang and Wagner, 1993), higher
biodegradability (Desai and Banat, 1997), more environmentally friendly
(Georgiou et al., 1990), and good specific activity at pH, temperature, and
salinity extremes (Kretschmer et al., 1982). Rhamnolipid biosurfactant bio-
degradability was demonstrated in the Organization for Economic Cooper-
ation and Development (OECD) 301D “Ready Biodegradability” Test, and
it showed no toxicity to activated sludge in the OECD 209 Test (Maslin and
Maier, 2000).
