of the metal ion (Lestan et al. 2008). Thus, it is possible to get very stable complexes
whose ligand flexibility enables both for high formation and dissociation kinetics
that are required for fast and effective metal extraction and then ligand regeneration.
Thus, chelating agents stabilize easily MTEs in solution in the form of ions, pulling
precipitation equilibria toward dissolution. EDTA has been largely assessed as it
offers among the best cost-to-performance ratios (Nowack et al. 2006). Nevertheless,
it is so powerful that it frees a portion of the strongly bounded fraction of contaminants, then redistributed between the mobilized and the available fractions (Udovic
and Lestan 2009; Zhang et al. 2010a, b). Through the number of e-donor atoms in
their backbone, the diversity of EDTA-derived chelating agents offers the ability to
modulate both the binding constant of the metal complexes and their dissociation
rate, their selectivity toward MTEs and to obtain a strong activity at low concentration. These ligands may also release anionic contaminants, like high oxidation state
MTEs and metalloids, through the dissolution of their counter ions showing a high
activity potential (Mandiwana 2008). Many recent studies have focused on the
biodegradability and the selectivity enhancements of chelating agents in order to
overcome the deficiencies of EDTA and parent compounds (Svenson et al. 1989;
Nowack et al. 2001; Nowack 2002). The most suitable chelating agents currently
available seem to be the iminodisuccinic acid (IDSA) and the ethylene diamine N,
N
0 -disuccinic acid (EDDS) (Tandy et al. 2004, 2006; Hauser et al. 2005; Ahmed
Mohamed et al. 2013; Ferraro et al. 2016).
As a general rule, adding aqueous NOM to contaminated matrices generates an
increase or a decrease of contaminant leaching depending on its solubility (Molson
et al. 2002; van Stempvoort et al. 2002; Moreno-Jimenez et al. 2013). The low
solubility of HA is strongly reduced in presence of MTE cations. Besides, an
important mobilization of As in the liquid and gaseous phases was generally
observed (Beesley et al. 2014). It was also observed that for polycontaminated
soils, the HOC removal was enhanced by the synergistic mobilization of metal by
chelating agents such as EDTA (Subramaniam et al. 2004; Ehsan et al. 2007; RiveroHuguet and Marschall 2011). Such phenomena can be explained by the high affinity
of these organic contaminants for NOM (Ranc et al. 2016). In contrast with strong
chelating agents such as EDTA or EDDS, the use of humic substances enabled a
reduction of leached concentrations without destabilizing the strongly bounded
contaminants. However, the reduction in some MTEs in the solid matrix or in As
leachability may be insufficient to meet the regulatory criteria (Tsang et al. 2013).
The dissolution mechanism of inorganic and organic contaminants from solid
materials involves several successive elementary steps: (1) the transfer of contaminants between binding sites, (2) the chemical reactions, (3) the desorption of
contaminants, and (4) the mass transport into the bulk solution. When dissolution
rates for soils are slow, the rate controlling mechanisms are either the mass transfer
of contaminants within the solid phase or their surface process-controlled detachment. In these cases, the dissolution kinetics follow a zero-order rate law when the
steady state conditions at the surface happen:
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
29
whose ligand flexibility enables both for high formation and dissociation kinetics
that are required for fast and effective metal extraction and then ligand regeneration.
Thus, chelating agents stabilize easily MTEs in solution in the form of ions, pulling
precipitation equilibria toward dissolution. EDTA has been largely assessed as it
offers among the best cost-to-performance ratios (Nowack et al. 2006). Nevertheless,
it is so powerful that it frees a portion of the strongly bounded fraction of contaminants, then redistributed between the mobilized and the available fractions (Udovic
and Lestan 2009; Zhang et al. 2010a, b). Through the number of e-donor atoms in
their backbone, the diversity of EDTA-derived chelating agents offers the ability to
modulate both the binding constant of the metal complexes and their dissociation
rate, their selectivity toward MTEs and to obtain a strong activity at low concentration. These ligands may also release anionic contaminants, like high oxidation state
MTEs and metalloids, through the dissolution of their counter ions showing a high
activity potential (Mandiwana 2008). Many recent studies have focused on the
biodegradability and the selectivity enhancements of chelating agents in order to
overcome the deficiencies of EDTA and parent compounds (Svenson et al. 1989;
Nowack et al. 2001; Nowack 2002). The most suitable chelating agents currently
available seem to be the iminodisuccinic acid (IDSA) and the ethylene diamine N,
N
0 -disuccinic acid (EDDS) (Tandy et al. 2004, 2006; Hauser et al. 2005; Ahmed
Mohamed et al. 2013; Ferraro et al. 2016).
As a general rule, adding aqueous NOM to contaminated matrices generates an
increase or a decrease of contaminant leaching depending on its solubility (Molson
et al. 2002; van Stempvoort et al. 2002; Moreno-Jimenez et al. 2013). The low
solubility of HA is strongly reduced in presence of MTE cations. Besides, an
important mobilization of As in the liquid and gaseous phases was generally
observed (Beesley et al. 2014). It was also observed that for polycontaminated
soils, the HOC removal was enhanced by the synergistic mobilization of metal by
chelating agents such as EDTA (Subramaniam et al. 2004; Ehsan et al. 2007; RiveroHuguet and Marschall 2011). Such phenomena can be explained by the high affinity
of these organic contaminants for NOM (Ranc et al. 2016). In contrast with strong
chelating agents such as EDTA or EDDS, the use of humic substances enabled a
reduction of leached concentrations without destabilizing the strongly bounded
contaminants. However, the reduction in some MTEs in the solid matrix or in As
leachability may be insufficient to meet the regulatory criteria (Tsang et al. 2013).
The dissolution mechanism of inorganic and organic contaminants from solid
materials involves several successive elementary steps: (1) the transfer of contaminants between binding sites, (2) the chemical reactions, (3) the desorption of
contaminants, and (4) the mass transport into the bulk solution. When dissolution
rates for soils are slow, the rate controlling mechanisms are either the mass transfer
of contaminants within the solid phase or their surface process-controlled detachment. In these cases, the dissolution kinetics follow a zero-order rate law when the
steady state conditions at the surface happen:
1 Contaminant Mobilization from Polluted Soils: Behavior and Reuse of Leaching. . .
29