Characterization, Production, and Applications of Lipopeptides - Biosurfactants: Research Trends and Applications - page 166

Reddy et al. (2009) evaluated surfactin as a stabilizing agent for the synthesis of
silver nanoparticles. The stability of the nanoparticles was determined and found to
be stable for a period of 2 months in the presence of surfactin. pH and temperature
conditions affected particle size. Surfactin as a stabilizing agent is renewable, less
toxic, and biodegradable, and thus an environmentally friendly additive.
In addition, the presence of two negative charges, one on the aspartate and the
other on the glutamate residue of surfactin, enables the binding of various metals
such as calcium, barium, lithium, magnesium, manganese, and rubidium (Thimon
et al., 1992). Eliseev et al. (1991) also showed that a Bacillus species could release oil
at low concentrations of 0.04 mg/L from oil-saturated columns.
A strain of B. subtilis isolated from contaminated sediments (Olivera et al., 2000)
could produce surfactin. A crude form was then added to ship bilge waste to enhance
biodegradation. Although aliphatic and aromatic compounds in a nonsterile envi-
ronment were degraded more quickly in the presence of the biosurfactant, N-C17
pristane and N-C18 phytane degradation were not.
Using a technique called micellar enhanced ultrafiltration, Mulligan et al. (1999)
studied the removal of various concentrations of metals from water by various concen-
trations of surfactin by a 50,000 Da MW cutoff ultrafiltration membrane. Cadmium
and zinc rejection ratios were superior (close to 100%) at pH values of 8.3 and 11,
while copper rejection ratios were the highest at pH 6.7 (about 85%). The addition of
0.4% oil as a cocontaminant slightly decreased the retention of the metals by the mem-
brane. The ultrafiltration membranes also indicated that metals became associated with
the surfactin micelles as the metals remained in the retentate and did not pass through
into the permeate as illustrated in Figure 6.5. The ratio of metals to the surfactin was
determined to be 1.2:1, which was only slightly different from the theoretical value of
1 mol metal: 1 mol surfactin due to the two charges on the surfactin molecule.
Batch soil washing experiments were performed to evaluate the feasibility of
using surfactin from B. subtilis for the removal of heavy metals from a contaminated
soil and sediments (Mulligan et al., 1999). Compared to minimal amounts for the
control, 0.25% surfactin with 1% NaOH removed 25% of the copper and 6% of the
zinc from the soil and 15% of the copper and 6% of the zinc from the sediments.
A series of five washings of the soil with 0.25% surfactin with 1% NaOH removed
70% of the copper and 22% of the zinc. Ultrafiltration, octanol-water partitioning,
and zeta potential measurement determined that surfactin was able to remove the
metals by sorption and complexation at the soil interface, then desorption of the
metal through interfacial tension lowering and fluid forces into solution and finally
micellar complexation (Figure 6.5).
CONCLUSION
Surfactin has very interesting surfactant properties. Potential medical applications are
related to antiinflammatory, antiviral, antibiotic, and antiadhesive activities. However,
the economics are not competitive due to poor yields and the requirement for expen-
sive and complex substrates. Portillo-Rivera et al. (2009) have postulated that bio-
surfactant costs can be as low as $0.50 per liter from molasses sugarcane. Low-cost
purification methods are also needed as downstream costs can account for 60% of