However, the high affinity of the resin for Fe(III) may prevent the use of this
technology in iron-rich soils.
1.4.2.2 Precipitation
Neutralization is the action to destabilize a relatively hydrophobic ionic solute in
aqueous solution through the formation of an ion-pair complex. Generally, a Lewis
base reacts with a Lewis acid, e.g., Hg
2+ binds S
2À . In some cases, a simple pH
modification is enough. Hong et al. (2002a) were able to remove exchangeable and
carbonated Cd(II) and Zn(II) from three different soils using Saponine at 3.7%; they
recovered up to 78% surfactant in treated leachates after a metal precipitation stage at
pH 10.7. Alternatively, dissociation in highly acidic medium was used for the EDTA
recovery, before MTEs precipitation in alkaline medium (Di Palma et al. 2005;
Pociecha and Lestan 2012). Despite the high recovery yields achieved, up to 88%,
this strategy requires a prior concentration step and several pH adjustments that
consume important quantities of inorganic reactants and increase the salinity of
concentrates. MTEs have been displaced from chelants and then removed from
soil leachates at pH 9 as oxyhydroxides in addition to As(V) using excess of Mg
(Ehsan et al. 2007; Rivero-Huguet and Marshall 2011; Wen and Marshall 2011; Wen
et al. 2012). It must be noticed that alkaline-earth metal chelates are still very reactive
for metal extraction in soils, but the use of elementary alkaline-earth metals is too
expensive for the regeneration of chelating agents. The low solubility of MTE–
sulfide complexes was used to regenerate chelating agents (Hong et al. 1999, 2002b;
Zeng et al. 2005), but the use of free sulfide ion as reagent is very dangerous.
Efligenir et al. (2013) has shown that the regeneration of polyaminocarboxylate
ligands in MTE-contaminated soil leachates was achieved within few hours of
contact with FeS at pH 5. The resulting iron chelates were further transformed in
the form of reactive Ca complexes in presence of calcium phosphate in order to
remove iron as FePO 4 (s). Despite it being a two-step process, the low reagents cost,
the high efficiencies of reactions, and its low sensitivity to iron-rich soils make it
especially interesting. Diethyldithiocarbamate is also very effective to remove MTEs
from chelates (Xie and Marshall 2001), but its high cost prevents its use from
regenerating most of chelating agents.
Coagulation–flocculation is a two-step process in which charged contaminants in
wastewater are first neutralized, using polyions of opposite charge, before the size of
the colloidal product obtained is increased, using a low solubility and high molecular
weight polymer, in order to enable rapid particles settling. The removal of the
chemical oxygen demand (COD) from the PAH-contaminated soil leachates using
coagulation was studied for anionic, cationic, and nonionic surfactants (LopezVizcaino et al. 2012). Trivalent iron and aluminum salts were used as low-cost
reagents to remove micelles and emulsions. As expected, the separation occurred
efficiently for the anionic surfactant only. A COD removal larger than 90% was
obtained for huge amounts of Al(III), but the remaining surfactant was not quantified. Ahmed Mohamed (2014) has shown that up to 94% of petroleum hydrocarbons
38
N. Fatin-Rouge
technology in iron-rich soils.
1.4.2.2 Precipitation
Neutralization is the action to destabilize a relatively hydrophobic ionic solute in
aqueous solution through the formation of an ion-pair complex. Generally, a Lewis
base reacts with a Lewis acid, e.g., Hg
2+ binds S
2À . In some cases, a simple pH
modification is enough. Hong et al. (2002a) were able to remove exchangeable and
carbonated Cd(II) and Zn(II) from three different soils using Saponine at 3.7%; they
recovered up to 78% surfactant in treated leachates after a metal precipitation stage at
pH 10.7. Alternatively, dissociation in highly acidic medium was used for the EDTA
recovery, before MTEs precipitation in alkaline medium (Di Palma et al. 2005;
Pociecha and Lestan 2012). Despite the high recovery yields achieved, up to 88%,
this strategy requires a prior concentration step and several pH adjustments that
consume important quantities of inorganic reactants and increase the salinity of
concentrates. MTEs have been displaced from chelants and then removed from
soil leachates at pH 9 as oxyhydroxides in addition to As(V) using excess of Mg
(Ehsan et al. 2007; Rivero-Huguet and Marshall 2011; Wen and Marshall 2011; Wen
et al. 2012). It must be noticed that alkaline-earth metal chelates are still very reactive
for metal extraction in soils, but the use of elementary alkaline-earth metals is too
expensive for the regeneration of chelating agents. The low solubility of MTE–
sulfide complexes was used to regenerate chelating agents (Hong et al. 1999, 2002b;
Zeng et al. 2005), but the use of free sulfide ion as reagent is very dangerous.
Efligenir et al. (2013) has shown that the regeneration of polyaminocarboxylate
ligands in MTE-contaminated soil leachates was achieved within few hours of
contact with FeS at pH 5. The resulting iron chelates were further transformed in
the form of reactive Ca complexes in presence of calcium phosphate in order to
remove iron as FePO 4 (s). Despite it being a two-step process, the low reagents cost,
the high efficiencies of reactions, and its low sensitivity to iron-rich soils make it
especially interesting. Diethyldithiocarbamate is also very effective to remove MTEs
from chelates (Xie and Marshall 2001), but its high cost prevents its use from
regenerating most of chelating agents.
Coagulation–flocculation is a two-step process in which charged contaminants in
wastewater are first neutralized, using polyions of opposite charge, before the size of
the colloidal product obtained is increased, using a low solubility and high molecular
weight polymer, in order to enable rapid particles settling. The removal of the
chemical oxygen demand (COD) from the PAH-contaminated soil leachates using
coagulation was studied for anionic, cationic, and nonionic surfactants (LopezVizcaino et al. 2012). Trivalent iron and aluminum salts were used as low-cost
reagents to remove micelles and emulsions. As expected, the separation occurred
efficiently for the anionic surfactant only. A COD removal larger than 90% was
obtained for huge amounts of Al(III), but the remaining surfactant was not quantified. Ahmed Mohamed (2014) has shown that up to 94% of petroleum hydrocarbons
38
N. Fatin-Rouge