364
materials, mainly in the form of weakly cross-linked gels, can be used as complexing agents to interact with many dyes, e.g., acid, direct, mordant, and reactive dye
molecules. The performance in terms of adsorption capacity, evaluated using batch
experiments, depended mainly on the range of dye concentrations used in the experiments. Hydroxypropyl-β-cyclodextrin gels had a lower adsorption capacity than
β-cyclodextrin gels. No correlation was observed between the performance of the
gels and their respective degree of cross-linking. The presence of additives such as
NaCl could improve the complexation of the dye, while sodium dodecyl sulfate had
the opposite effect. Like Professors Solms and Wiedenhof, we explained these early
results mainly by the formation of inclusion complexes and thus by the presence in
the materials of cyclodextrin molecules’ cavities. We used the notion of complexation by chemisorption and assumed that, in this mechanism, no covalent bonding
occurred between the cyclodextrin and the dye. The reaction was a dissociationassociation equilibrium, as in the case of the formation of inclusion complexes
involving native cyclodextrin molecules in solution, in accordance with the conclusions published by Professor Szejtli (1982, 1988). The cross-linking did not change
this property (Shao et al. 1996).
Two years later, in collaboration with an Italian institute, Stazione Sperimentale
per i Combustibili, we proposed several materials with different cyclodextrin contents, ranging from 20% to 80% w/w (Crini et al. 1998a, b; Vecchi et al. 1998). We
have modified the protocol of Professor Solms by increasing the amount of epichlorohydrin to obtain mechanically stable materials but with different mobility in terms
of swelling properties and cyclodextrin content. The results demonstrated that ECP
materials (particles of irregular shape or regular beads) could also be used as adsorbents to efficiently remove organic contaminants from contaminated water, whatever the quantity of cyclodextrin present in the gels (Crini et al. 1998b; Vecchi et al.
1998). ECP materials were able to interact with contaminants such as chlorophenols, nitrophenols, naphthols, and benzoic acids, in complex solutions, particularly
those with hydrophilic properties. They were effective not only at trace levels of
contaminants but also at high concentrations. Kinetics of contaminant adsorption
were rapid: 2 h was sufficient for reaching the maximum adsorption capacity. The
adsorption was much greater in the case of organic molecules which presented compatible size, steric arrangement, and hydrophobicity with the β-cyclodextrin molecules such as β-naphthol, p-nitrophenol, and 4-tert-butylbenzoic acid. However,
small molecules such as phenol, known to be too small for the cyclodextrin cavity,
were also complexed by the materials (Crini et al. 1998b; Vecchi et al. 1998; Bertini
et al. 1999). Comparison with conventional adsorbents such as activated carbons
and organic resins showed that ECP gels and beads were more selective and led to
better results in terms of elimination, especially at trace levels. A more surprising
and interesting result also showed that a high proportion of cyclodextrin was not
necessary to have useful performance in terms of pollutant removal (Bertini et al.
1999). In the mid-2000s, Professor Isasi and Professor Christopher H. Evans
(Ryerson University, Ontario) reported similar conclusions (Orprecio and Evans
2003; Romo et al. 2004, 2006; Zohrehvand and Evans 2005; García-Zubiri
et al. 2006).
G. Crini
materials, mainly in the form of weakly cross-linked gels, can be used as complexing agents to interact with many dyes, e.g., acid, direct, mordant, and reactive dye
molecules. The performance in terms of adsorption capacity, evaluated using batch
experiments, depended mainly on the range of dye concentrations used in the experiments. Hydroxypropyl-β-cyclodextrin gels had a lower adsorption capacity than
β-cyclodextrin gels. No correlation was observed between the performance of the
gels and their respective degree of cross-linking. The presence of additives such as
NaCl could improve the complexation of the dye, while sodium dodecyl sulfate had
the opposite effect. Like Professors Solms and Wiedenhof, we explained these early
results mainly by the formation of inclusion complexes and thus by the presence in
the materials of cyclodextrin molecules’ cavities. We used the notion of complexation by chemisorption and assumed that, in this mechanism, no covalent bonding
occurred between the cyclodextrin and the dye. The reaction was a dissociationassociation equilibrium, as in the case of the formation of inclusion complexes
involving native cyclodextrin molecules in solution, in accordance with the conclusions published by Professor Szejtli (1982, 1988). The cross-linking did not change
this property (Shao et al. 1996).
Two years later, in collaboration with an Italian institute, Stazione Sperimentale
per i Combustibili, we proposed several materials with different cyclodextrin contents, ranging from 20% to 80% w/w (Crini et al. 1998a, b; Vecchi et al. 1998). We
have modified the protocol of Professor Solms by increasing the amount of epichlorohydrin to obtain mechanically stable materials but with different mobility in terms
of swelling properties and cyclodextrin content. The results demonstrated that ECP
materials (particles of irregular shape or regular beads) could also be used as adsorbents to efficiently remove organic contaminants from contaminated water, whatever the quantity of cyclodextrin present in the gels (Crini et al. 1998b; Vecchi et al.
1998). ECP materials were able to interact with contaminants such as chlorophenols, nitrophenols, naphthols, and benzoic acids, in complex solutions, particularly
those with hydrophilic properties. They were effective not only at trace levels of
contaminants but also at high concentrations. Kinetics of contaminant adsorption
were rapid: 2 h was sufficient for reaching the maximum adsorption capacity. The
adsorption was much greater in the case of organic molecules which presented compatible size, steric arrangement, and hydrophobicity with the β-cyclodextrin molecules such as β-naphthol, p-nitrophenol, and 4-tert-butylbenzoic acid. However,
small molecules such as phenol, known to be too small for the cyclodextrin cavity,
were also complexed by the materials (Crini et al. 1998b; Vecchi et al. 1998; Bertini
et al. 1999). Comparison with conventional adsorbents such as activated carbons
and organic resins showed that ECP gels and beads were more selective and led to
better results in terms of elimination, especially at trace levels. A more surprising
and interesting result also showed that a high proportion of cyclodextrin was not
necessary to have useful performance in terms of pollutant removal (Bertini et al.
1999). In the mid-2000s, Professor Isasi and Professor Christopher H. Evans
(Ryerson University, Ontario) reported similar conclusions (Orprecio and Evans
2003; Romo et al. 2004, 2006; Zohrehvand and Evans 2005; García-Zubiri
et al. 2006).
G. Crini