Enhancement of Remediation Technologies with Biosurfactants - Biosurfactants: Research Trends and Applications - page 242

saturates (81.6%), and then the aromatics (63.8%). A 120 day biodegradation period
further reduced the contaminants to 5470 mg/kg, a 67.3% removal. Therefore, this
ex situ two-stage process was an effective treatment.
The effect of rhamnolipid on phenol degradation by laccase was investigated (Liu
et al. 2012). It was determined that 98% removal of phenol (up to 400 mg/L) could be
achieved in 24 h in the presence of 318 μM dirhamnolipid. Removal was optimal at
pH 6 and 50°C. This was up to 6.4-fold better than the control at the same conditions,
which indicates potential for enhanced treatment of phenol in water.
Southam et al. (2001) studied the effect of biosurfactants on waste hydrocar-
bon degradation. The bacteria must adsorb first onto the surfactant-oil interface of
25-50 nm in thickness. Approximately 1% of the biosurfactant is needed to emulsify
the oil. Growth on the oil occurred by microbial uptake of the nanometer-sized oil
droplets. This study was enabled by using a transmission electron microscope. More
of this type of research is required to determine the mechanism of hydrocarbon
metabolism and biosurfactant applications.
PAHs
Polycyclic or polynuclear aromatic hydrocarbons (PAHs) are components of creosote,
produced during petroleum refining, coke production, and wood preservation (Park
et al., 1990). Many are suspected to be carcinogens. A general form for PAHs is
C 4n+2 H 2n+4 , where n is the number of rings. A four-ring PAH would be C 18 H 12 . As the
ring number increases, the degradation of the compounds becomes more difficult
due to decreasing volatility and solubility and increased sorption to soil. PAHs are
degraded one ring at a time like single-ring aromatics.
Researchers (Vipulanandan and Ren, 2000) compared the solubilization of the
PAH, naphthalene, by a rhamnolipid, sodium dodecyl sulfate (SDS), an anionic
surfactant, and Triton X-100, a nonionic surfactant. Although the biosurfactant
increased the solubility of naphthalene by 30 times, the biodegradation of naph-
thalene (30 mg/L) took 40 days in the presence of biosurfactant (10 g/L) compared
to 100 h for Triton X-100 (10 g/L). The biosurfactant was used as a carbon source
instead of the naphthalene decreasing its efficiency, which did not occur in the case
of Triton X-100. However, naphthalene was not biodegraded in the presence of SDS.
Deschênes et al. (1994) also showed that the rhamnolipids from the UG2 strain
were more effective than SDS (up to five times) in a bioslurry. The biosurfactant
enhanced the solubilization of four-ring PAHs more significantly than three-ring
PAHs. SDS showed higher levels of toxicity compared to the biosurfactant as the
surfactant concentration increased above 100 mg/kg. Higher molecular weight PAHs
were not biodegraded even in the presence of the surfactants.
Providenti et al. (1995) showed that the effect of UG2 biosurfactants on phenan-
threne mineralization in soil slurries led to decreased lag times and increased degra-
dation. Further experiments were carried out by Dean et al. (2001) who investigated
either rhamnolipid or a biosurfactant-producing strain of P. aeruginosa ATCC 9027
addition. Results were mixed and difficult to interpret as one strain (strain R) showed
enhanced biodegradation when the surfactant was added but the other (strain P5-2)
did not. Co-addition of the two strains showed enhanced mineralization of phenan-
threne by strain R only. However, there seemed to be some interaction between the