The Krafft point is the temperature at which the solubility of an ionic surfactant
becomes equal to the CMC value. Nevertheless, some benzene-based surfactants
that degrade as nonylphenols such as Triton X100 are now banned because of their
toxicity for living organisms. Biosurfactants are attractive because of their natural
production and their high biodegradability; however, their very high production cost
is still limiting their use. Their assessment to enhance the removal of HOCs started a
long time ago, remains relevant and their efficiency may be similar to those of
effective synthetic surfactants (Mulligan and Eftekhari 2003; Bezza and Chirwa
2016). Polysaccharide- and betaine-based neutral synthetic surfactants are good
alternatives to biosurfactants, thanks to their high biodegradability and low toxicity.
The used SW/SF surfactant concentrations depend on the characteristics of the
polluted matrix, but they are usually ranging from 0.1 to 5 %w. The commercial
cost for synthetic surfactants is quite high, usually in the range from 500 to 5000 €
ton
À1 . Some surfactants are also able to mobilize simultaneously HOCs, metals, and
metalloids, whereas their action on the soil matrix is soft (Hong et al. 2002a, b;
Mulligan and Wang 2006; Reynier et al. 2013).
1.3.3 Inorganic Contaminants Extraction
As previously stated, the leaching of inorganic contaminants depends much on pH
and Eh conditions. Acid leaching in oxidizing conditions was traditionally
implemented for the mobilization of metallic contaminants as metal solubility is
maximal in these conditions (Quina et al. 2009; van der Sloot and Kosson 2012).
Nevertheless, this strategy is neither selective of MTEs because aluminosilicates and
carbonates become soluble at pH lower than 5 (Bonneau and Souchier 1994), nor
economic because of the high buffer capacity of soils (e.g., carbonates, NOM—
Bisone et al. 2012).
Metal ions are Lewis acids and form complexes with electron-donor atoms such
as O, N, S, P. The hard and soft base theory allows to predict the most favorable
associations. Water molecules in the first coordination sphere of the metal are then
replaced by one or several atoms donors of the ligand. The formation of soluble
complexes enables to mobilize adsorbed metals in solution by lowering the free
metal ions (M) concentrations according to the following equilibria where charges
are omitted:
M ads Ð M þ L Ð ML
ð1:21Þ
where M and L are the metal ion and the ligand, respectively.
Thus, several water molecules might be displaced from the first coordination
sphere of the metal by a ligand according to the stepwise formation constants:
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
27
becomes equal to the CMC value. Nevertheless, some benzene-based surfactants
that degrade as nonylphenols such as Triton X100 are now banned because of their
toxicity for living organisms. Biosurfactants are attractive because of their natural
production and their high biodegradability; however, their very high production cost
is still limiting their use. Their assessment to enhance the removal of HOCs started a
long time ago, remains relevant and their efficiency may be similar to those of
effective synthetic surfactants (Mulligan and Eftekhari 2003; Bezza and Chirwa
2016). Polysaccharide- and betaine-based neutral synthetic surfactants are good
alternatives to biosurfactants, thanks to their high biodegradability and low toxicity.
The used SW/SF surfactant concentrations depend on the characteristics of the
polluted matrix, but they are usually ranging from 0.1 to 5 %w. The commercial
cost for synthetic surfactants is quite high, usually in the range from 500 to 5000 €
ton
À1 . Some surfactants are also able to mobilize simultaneously HOCs, metals, and
metalloids, whereas their action on the soil matrix is soft (Hong et al. 2002a, b;
Mulligan and Wang 2006; Reynier et al. 2013).
1.3.3 Inorganic Contaminants Extraction
As previously stated, the leaching of inorganic contaminants depends much on pH
and Eh conditions. Acid leaching in oxidizing conditions was traditionally
implemented for the mobilization of metallic contaminants as metal solubility is
maximal in these conditions (Quina et al. 2009; van der Sloot and Kosson 2012).
Nevertheless, this strategy is neither selective of MTEs because aluminosilicates and
carbonates become soluble at pH lower than 5 (Bonneau and Souchier 1994), nor
economic because of the high buffer capacity of soils (e.g., carbonates, NOM—
Bisone et al. 2012).
Metal ions are Lewis acids and form complexes with electron-donor atoms such
as O, N, S, P. The hard and soft base theory allows to predict the most favorable
associations. Water molecules in the first coordination sphere of the metal are then
replaced by one or several atoms donors of the ligand. The formation of soluble
complexes enables to mobilize adsorbed metals in solution by lowering the free
metal ions (M) concentrations according to the following equilibria where charges
are omitted:
M ads Ð M þ L Ð ML
ð1:21Þ
where M and L are the metal ion and the ligand, respectively.
Thus, several water molecules might be displaced from the first coordination
sphere of the metal by a ligand according to the stepwise formation constants:
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
27