Biosurfactants and Bioemulsifiers from Marine Sources - Biosurfactants: Research Trends and Applications - page 136

83%-98%, 80%-85%, and 57%-73%, respectively. Addition of rhamnolipids into
the biodegradation system significantly increased the microbial growth by promot-
ing the efficient utilization of hydrocarbons.
Biosurfactants and microbially enhanced oil recovery (MEOR) : BSs have extensive
potential application in the petroleum industry such as emulsifiers, demulsifiers, and
oil recovery agents. MEOR is a technique that either uses a crude preparation of BS
or sterilized BS containing culture broth to liberate crude oil from a binding sub-
strate (Marchant and Banat, 2012b). For example, Banat et al. (1991) carried out a
crude oil sludge tank cleanup and oil recovery process in which BS-containing ster-
ilized culture broth was used to clean up oil sludge from an oil storage tank. After
5 days of treatment involving energy addition and circulation to enhance the process
of emulsification followed by a deemulsification, 91% of crude oil present in the oil
storage tank was recovered. Hydrocarbon analysis for the recovered crude showed a
100% hydrocarbon content. This result indicated that MEOR process doesn't require
live microorganisms or pure BSs and that sterilized BS-containing broth is sufficient
to mobilize and recover significant amount of oil from oil sludge deposits.
Biosurfactants and heavy metal remediation : Microbial BSs are known for their
metal-complexing activities that have been reported to be effective in the remedia-
tion of heavy metal-contaminated environments (Mulligan et al., 2001; Singh and
Cameotra, 2004). The mechanisms behind metal binding are (1) anionic BSs create
complexes with metals in a nonionic form by ionic bonds. These bonds are stronger
than the metal's bonds with the soil/sediment and metal-BS complexes are desorbed
from the soil matrix to the solution due to the lowering of the interfacial tension and
(2) the cationic BSs can replace the same charged metal ions by competition for some
but not all negatively charged surfaces (ion exchange). Metal ions can be removed
from soil surfaces by the BS micelles. The polar head groups of micelles bind metals
that mobilize the metals in water (Mulligan and Gibbs, 2004).
Applications of marine BSs in heavy metal remediation have been reported by
many researchers. Das et al. (2009b) studied the bacterial cells ( B. circulans ) and
BS-mediated cadmium and lead metal binding and suggested that there was no cell-
mediated metal binding, but that an increase in metal binding was observed with an
increase in BS concentration from 0.5 × CMC to 5 × (CMC of the BS was 40 mg/L).
The percentage removal at 0.5 × CMC was 76.6%, 53.18%, 56.63%, and 42.74%,
29.72%, 23.19% for lead and cadmium, respectively, while the percentage removal
was increased to 100%, 95.75%, 87.69%, and 97.66%, 88.43%, 86.35% for lead and
cadmium, respectively at 5.0 × CMC of BS. A complete removal of the metals was
seen at 10 × CMC.
Gnanamani et al. (2010) reported the chromium reduction and trivalent chro-
mium tolerance behavior of marine Bacillus sp. MTCC5514 through its extracellular
enzyme reductase and BS production. The isolate reduces 10-2000 mg/L of hexava-
lent chromium to trivalent chromium within 24-96 h, and the release of extracellular
chromium reductase was responsible for the metal reduction. The role of the BS in
this metal reduction process is to entrap the trivalent chromium in the micelle of BSs,
which prevents microbial cells from exposure toward trivalent chromium. It was
concluded that extracellular chromium reductase and BS mediate the remediation