Enhancement of Remediation Technologies with Biosurfactants - Biosurfactants: Research Trends and Applications - page 252

Kilic et al. (2011) examined the removal of chromium from tannery sludge by
saponin and hydrogen peroxide. The saponin removed 24% of the Cr, while the
hydrogen peroxide was able to extract 70% and oxidize Cr(VI) to Cr(III). The
organic content was the major inhibitor of the removal of Cr by the saponin and,
therefore, pretreatment may be required.
Mercury has been recovered by foam fractionation using surfactin (Chen et al.,
2011). Air is bubbled into the solution to separate the metal contaminant. Various
parameters were evaluated. A surfactin concentration in the order of 10 × CMC, low
Hg concentration (2 mg/L), and high pH (8-9) were optimal. Results were superior
to the chemical surfactants, SDS and Tween 80.
Various bacterial strains were screened for biosurfactant production from
agro-based substrates and the ability to remove heavy metals (Hazra et al., 2010).
Pseudomonas aeruginosa AB4 was isolated and its glycolipid biosurfactant was pro-
duced from renewable nonedible seed cakes. Preliminary tests also indicated that Pb
and Cd could be chelated by the biosurfactant.
In Situ Soil Flushing Studies by Rhamnolipids
PAHs and PCP
To simulate in situ flushing conditions, soil column experiments were performed
(Noordman et al., 1998). A concentration of 500 mg/L of rhamnolipids removed
two to five times more phenanthrene than the control. Noordman et al. (2000),
then, investigated adsorption of the biosurfactant to the soil as they must not adsorb
strongly to the soil in in situ situations. The unitless retardation factor, R, used in
models for transport estimation was determined: R = 1 + (ρ/ɛ)k d , where ρ is the soil
densit y, ɛ is the soil porosity and k d is the soil-water partition coefficient. R was
between 2 for naphthalene with silica and 700 for phenanthrene with octadecyl-
derivatized silica. The addition of 500 mg/L of rhamnolipid decreased R by eight-
fold. To limit interfacial hydrophobic adsorption of surfactant aggregates such as
hemimicelles to the soil, biosurfactant concentrations higher than the CMC should
be used. Adsorption was due to interfacial phenomena not partitioning into soil
organic matter. The transport of the more hydrophobic compounds was facilitated
while the less hydrophobic ones were retarded due to the sorption of biosurfactant
admicelles (micelles adsorbed to the soil surface). The interactions could include
ion exchange reactions with the anionic carboxylate portion of the rhamnolipid,
complexation at the surface and hydrogen bonding of the rhamnose head groups
(Somasundaran and Krishnakumar, 1997). Herman et al. (1995) determined that
low concentrations of rhamnolipids below the CMC enhanced mineralization of
entrapped hydrocarbons while concentrations above the CMC enhanced the mobil-
ity of the hydrocarbons.
Attempts are also being made to determine nonaqueous-phase liquid (NAPL)
micelle-aqueous partition coefficients with rhamnolipids to understand equilib-
rium solubilization behavior in surfactant-enhanced soil remediation (McCray
et al., 2001). Deviation from this ideal Raoult's law behavior occurred depending on
the hydrophobicity of the compounds and the NAPL-phase mole fraction. Micelle-
water partition coefficients were found to be nonlinear according to the NAPL-phase