Glycolipids produced by Rhodococcus species H13-A
and
a representative synthetic surfactant Tween-80
(polyoxyethylene sorbitan monooleate) were used to
demonstrate enhanced substrate “solubility”
(aqueous-plus-micellar phase) in the presence of surfactants.
Nascent
concentrations of naphthalene and its methyl-substituted
derivatives in crude oil were used as representative
polycyclic aromatic hydrocarbons for the study. Both
biosurfactant glycolipids from H13-A and Tween-80 lowered
the surface tension of aqueous solutions from 72 to ∼30
dyn/cm. The two-ring aromatics showed a substantial
increase
in their apparent solubilities in the presence of
surfactants; the increase being significantly greater for
the biosurfactant as compared to the synthetic surfactant.
The aqueous phase solubility enhancement was greater
for the highly substituted derivatives as compared to the
lesser
substituted compounds. Higher toxicity levels, as
seen
by the lower EC50 values, of the surfactant mixtures
indicated
enhanced partitioning of the petroleum contaminants in
the aqueous phase. Higher initial EC50 values for the
biosurfactant meant that they exhibit lesser aqueous toxicity
as compared to the synthetic surfactant. When
compared
on a toxicity per mass of PAH basis, the end point Tween-80 system was approximately 50% more toxic than the biosurfactant system.
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