Biosurfactant Complexation of Metals and Applications for Remediation - Biosurfactants: Research Trends and Applications - page 299

enhanced biodegradation rates (Zhang and Miller, 1992). This was demonstrated
in a study examining the effect of rhamnolipid on the degradation of naphthalene
by a Burkholderia sp. in the presence of Cd. Results showed that a 1:1 molar ratio
of rhamnolipid:Cd reduced cadmium toxicity, while increasing the molar ratio to
10:1 eliminated cadmium toxicity completely (Sandrin et al., 2000). In a second
study, rhamnolipid was shown to enhance the degradation of phenanthrene by a soil
consortium inhibited by the addition of Cd. In this study, phenanthrene mineraliza-
tion reached the same levels as metal-free controls when several pulses of rhamno-
lipid were added. The pulsed addition technique was used to replenish rhamnolipid
depleted through biodegradation (Maslin and Maier, 2000).
Biosurfactants have also been studied for simultaneous removal of organic and
metal contamination from soil. In one study, saponin removed 76% of phenan-
threne and 88% of Cd simultaneously (Song et al., 2008). Interestingly, removal
levels for both phenanthrene and Cd were the same whether they were present indi-
vidually or in combination. This suggests that the two contaminants do not com-
pete with each other. This may be because Cd interacts with the head group of the
surfactant and phenanthrene interacts with the micelle interior (Song et al., 2008).
A second study examined removal of Cd and phenanthrene cocontaminants by four
biosurfactants including surfactin; an iturin and fengycin mixture (lipopeptides
produced by Bacillus sp.); arthrofactin (a lipopeptide produced by Arthrobacter
oxydans ); and flavolipid (now siderolipids) (Lima et al., 2011). Removal ranged
from 79% to 87% and from 66.9% to 71.9% for phenanthrene and Cd, respectively,
depending on the biosurfactant used. The use of an iodide ligand increased Cd
removal from 74% to 99%. The iodide ligand is thought to form a neutral com-
plex with Cd, which can then interact with the interior hydrophobic domain of the
micelle, thereby increasing the amount of Cd associated with each micelle (Lima
et al., 2011).
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There are a variety of approaches for the removal of metals from aqueous solutions.
One is the use of sorbents, which can range from ion exchange resins to clays to micro-
bial biomass. A recent report used rhamnolipid to assist in the process of sorption
of Cu to clay. In this study, the efficiency of Cu sorption by an Na-montmorillonite
clay was increased considerably when modified by rhamnolipid (Ozdemir and
Yapar, 2009). A small amount of added rhamnolipid (2 × 10 −6 M) acted to disperse
the clay particles, thus increasing the total available surface area for Cu sorption.
The sorbed metals were subsequently removed from the clay using a higher concen-
tration rhamnolipid wash treatment (Ozdemir and Yapar, 2009).
Metals can also be removed directly from aqueous solution using biosurfactants.
Once the aqueous solutions are treated with biosurfactants, the metal-surfactant
complex must be removed from the solution. This can be accomplished in two ways:
micellar-enhanced ultrafiltration and ion flotation. Micellar-enhanced ultrafiltration
is a membrane-based separation process that uses anisotropic membranes with small
size pores that do not allow surfactant micelles to pass through. Thus, the micelle-
bound metal in solution is retained on the filter and removed from the bulk aqueous