Enhancement of Remediation Technologies with Biosurfactants - Biosurfactants: Research Trends and Applications - page 253

mole fraction. Enhancements by the biosurfactant were greater than predicted for
hydrophobic and less than predicted for the more hydrophilic compounds. Empirical
relationships were also developed for multicomponent NAPL and the biosurfactant.
Correlations would then be incorporated into transport models in the future.
The effect of rhamnolipids on the partitioning of the PAHs, naphthalene, fluorene,
phenanthrene, and pyrene from NAPLs has been examined by Garcia-Junco et al.
(2003). Enhanced partitioning of most of the PAHs, with the exception of naphtha-
lene, occurred even with humic acid-smectite clay complexes. The rhamnolipids
sorbed onto the solids typical of those found in the subsurface and increased the
amount of solid-phase PAHs. The equation

dt eq was used, where C eq is the
equilibrium aqueous-phase concentration and C is the PAH concentration in aque-
ous and solid phases. It was determined that at biosurfactant concentrations above
the CMC, k values were lower since there was competition for the PAH between the
micelles and sorbed biosurfactants,
A rhamnolipid produced by Pseudomonas putida was evaluated for the desorp-
tion of phenanthrene from clay-loam soil (Poggi-Varaldo and Rinderknecht-
Seijas, 2003). The biosurfactant (250 mg/L) improved desorption of the PAH linearly
and therefore a linear k (k l ) was determined. Desorption with water (reference) gave
a k l,ref = 139 mL/g and with the biosurfactant, kl, l , = 268 mL/g. Therefore, there was
an almost twofold improvement of the availability enhancement factor (AEF) by the
biosurfactant.
The capability of a rhamnolipid in the form of foam was evaluated for the removal
of pentachlorophenol (PCP) from soil (Mulligan and Eftekhari, 2003). The stability
of the rhamnolipid foam was excellent. When the foam was injected into the con-
taminated soil (1000 mg/kg of PCP), 60% and 61% of the PCP was removed from a
fine sand and sandy-silt soil, respectively. The foam can be injected into the soil at
low pressures, which will avoid problems like soil heaving. The high-quality foams,
generated in this study (99%), contain large bubbles with thin liquid films that can
collapse easily and are less resistant to the soil, resulting in lower soil pressures.

dC

= k(C ) −

Heavy Metals
Metal removal from a contaminated sandy soil (1710 ppm of Cd and 2010 ppm
of Ni) was evaluated by a foam produced by 0.5% rhamnolipid solution, after
20 pore volumes (Mulligan and Wang, 2004). The biosurfactant foam removed
73.2% of Cd and 68.1% of Ni, compared to the biosurfactant liquid solution, where
61.7% Cd and 51.0% Ni were removed. This was superior to Triton X-100 foam,
which removed 64.7% Cd and 57.3% Ni, and liquid Triton X-100, which removed
52.8% Cd and 45.2% Ni. Distilled water removed only 18% of both Cd and Ni. For a
90% foam quality, the average hydraulic conductivity was 4.1 × 10 −4 cm/s, for 95%,
it was 1.5 × 10 −4 cm/s and for 99%, it was 2.9 × 10 −3 cm/s. Increasing foam quality
decreases substantially the hydraulic conductivity. All these values are lower than the
conductivity of water at 0.02 cm/s. This higher viscosity will allow better control of
the surfactant mobility during in situ use. Therefore, rhamnolipid foam may be an
effective and nontoxic method of remediating heavy-metal, hydrocarbon-, or mixed-
contaminated soils. Further efforts will be required to enable its use at field scale.