Remediation of PAH-Contaminated Soils and Sediments: A ...

Surfactants
Mass transfer of PAH compounds to the aqueous phase in the soil solution can
be a major limiting factor in the bioremediation of PAHs (Volkering et al., 1992).
Compounds like surfactants, cyclodextrins, and vegetable oil may be used to
enhance PAH solubility.
Properly applied surfactants have been shown to improve desorption, apparent
aqueous mobility and bioavailability of hydrophobic organic compounds such as
PAHs (Bragg et al., 1994; Mata-Sandoval et al., 2002). Surfactants are
amphiphilic (possess both hydrophilic and hydrophobic properties) molecules
with a hydrophilic polar head (Gao et al., 2007). They can be classified by the
charge on their polar head as anionic, cationic, nonionic or zwitterionic (Mulligan
et al., 2001). At or above a certain concentration level called critical micelle
concentration, the hydrophobic parts of the surfactants will tend to associate
together to form a micelle (an aggregate of surfactant molecules dispersed in a
liquid colloid) with a hydrophobic core (Santharam et al., 1997). Surfactants
solubilize hydrophobic contaminants by partitioning them into the hydrophobic
core of the micelle. If the concentration of surfactant exceeds the critical micelle
concentration, solubility of hydrophobic compounds can increase by an order of
magnitude over normal aqueous solubility (Edwards et al., 1991; Mulligan et al.,
2001; Gao et al., 2007). The critical micelle concentration of a specific surfactant
depends on temperature, ionic strength and surfactant chemistry.
� In mixed pollutant systems, the extent of solubilization will differ from
those in single solutes. For example, Guha et al. (1999) found that
naphthalene solubilized by surfactants increased the solubilization of other
PAHs such as phenanthrene.
� Surfactants are thought to be able to solubilize sorbed organic compounds
in a soil-water system only after critical micelle concentration is attained
(Laha and Luthy, 1991; Gao et al., 2007).
� Several interactions affect the solubilization of hydrophobic compounds by
surfactants. These include the micellular phase-organic interactions,
surfactants monomer-organic interactions in the aqueous phase, and the
interactions of surfactants and organic compounds with the solid phase
(Mata-Sandoval et al., 2002).
� Soils with predominantly fine particles are generally found to reduce
desorption efficiency of surfactants (Mulligan et al., 2001). Organic matter
content and clay mineralogy also affect surfactant performance, but the
effect differs depending on the type of surfactant used (Rodriquez-Cruz et
al., 2005).
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