Therefore, surfactants have an effective multi-action on the mobilization of hydrophobic contaminants. On the one hand, they lower interfacial tensions that contribute
to overcome capillary trapping of contaminants in pure phases and that favor HOCs
emulsification and mobilization. On the other hand, for C S ! CMC in soil pores,
they improve the aqueous solubility of contaminant by entrapping them in the form
of soluble complexes.
The average position occupied by the host molecules in micelles depends on their
interactions with surfactants and water. On the basis of experimental measurements,
five main positions described in the literature are shown in Fig. 1.10. The most
saturated HOCs are in position 5. In contrast, aromatic rings such as benzene stand in
positions 2–5 depending on the surfactant structure (Nagarajan et al. 1984), because
of their polarity due to the resonance of π-electrons. Micelles swell until they
become oil in water emulsions as host molecules incorporate into their hydrophobic
lobes. Emulsions are categorized as microemulsions (size <100 nm), miniemulsions (<400 nm), and macro-emulsions (>400 nm). It is reported that surfactant/HOC mass ratios decrease from 15 to 30% for microemulsions to 1–3% for
mini-emulsions (Rosen and Kunjappu 2012). Nevertheless, usually only
microemulsions move quite freely in most of low permeability soil materials. They
are much used in Enhanced Oil Recovery (EOR) as they generate for ultralow
interfacial tensions (about 10
À3 mM m
À1 ) that enhance NAPLs mobility (Rosen
and Kunjappu 2012; Bera and Mandal 2015).
The mechanism for NAPL (or HOCs adsorbed onto a solid surface) dissolution in
micellar aqueous solution involves a rate-limiting step of micelles desorption from
the NAPL/water interface and their diffusion through the boundary layer until they
reach the bulk aqueous phase (Fig. 1.11, Grimberg et al. 1995; Bernardez and
Ghoshal 2008). In addition, some useful information has been reported by Prak
et al. (2000) regarding the selection of surfactants and conditions for SW/SF
applications:
• The loosed fraction of surfactant onto the NAPL surface increases with its
lipophilic character
• The adsorbed surfactant density at the NAPL/water interface lowers as the
surfactant cross-section increases
Fig. 1.10 Scheme of the
different loci for hosts in
surfactant micelles
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
25
to overcome capillary trapping of contaminants in pure phases and that favor HOCs
emulsification and mobilization. On the other hand, for C S ! CMC in soil pores,
they improve the aqueous solubility of contaminant by entrapping them in the form
of soluble complexes.
The average position occupied by the host molecules in micelles depends on their
interactions with surfactants and water. On the basis of experimental measurements,
five main positions described in the literature are shown in Fig. 1.10. The most
saturated HOCs are in position 5. In contrast, aromatic rings such as benzene stand in
positions 2–5 depending on the surfactant structure (Nagarajan et al. 1984), because
of their polarity due to the resonance of π-electrons. Micelles swell until they
become oil in water emulsions as host molecules incorporate into their hydrophobic
lobes. Emulsions are categorized as microemulsions (size <100 nm), miniemulsions (<400 nm), and macro-emulsions (>400 nm). It is reported that surfactant/HOC mass ratios decrease from 15 to 30% for microemulsions to 1–3% for
mini-emulsions (Rosen and Kunjappu 2012). Nevertheless, usually only
microemulsions move quite freely in most of low permeability soil materials. They
are much used in Enhanced Oil Recovery (EOR) as they generate for ultralow
interfacial tensions (about 10
À3 mM m
À1 ) that enhance NAPLs mobility (Rosen
and Kunjappu 2012; Bera and Mandal 2015).
The mechanism for NAPL (or HOCs adsorbed onto a solid surface) dissolution in
micellar aqueous solution involves a rate-limiting step of micelles desorption from
the NAPL/water interface and their diffusion through the boundary layer until they
reach the bulk aqueous phase (Fig. 1.11, Grimberg et al. 1995; Bernardez and
Ghoshal 2008). In addition, some useful information has been reported by Prak
et al. (2000) regarding the selection of surfactants and conditions for SW/SF
applications:
• The loosed fraction of surfactant onto the NAPL surface increases with its
lipophilic character
• The adsorbed surfactant density at the NAPL/water interface lowers as the
surfactant cross-section increases
Fig. 1.10 Scheme of the
different loci for hosts in
surfactant micelles
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
25