366
formation of an inclusion complex due to the β-cyclodextrin molecules and the
physical adsorption in the polymer network. In the mid-2000s, Professor Isasi’s
work also confirmed that the presence of cyclodextrin cavities cannot alone explain
the adsorption results and stressed the importance of the polymer network structure
and thus of the degree of cross-linking (Romo et al. 2004, 2006, 2008; GarcíaZubiri et al. 2006; Vélaz et al. 2007).
As the materials were relatively highly cross-linked, they could be used both in
batch and column studies (Crini et al. 1998b; Vecchi et al. 1998; Bertini et al. 1999).
The method proposed extended the potential applications of these materials because
the use of cyclodextrin cross-linked gels in adsorption columns in general had limitations due to hydrodynamic problems and column fouling. Another advantage that
has been mentioned was the regeneration of adsorbents after use (Vecchi et al. 1998;
Janus et al. 1999). In the 1980s, Professor Szejtli stressed that the reversible nature
of complex formation was essential in the case of water treatment (Szejtli et al.
1978; Szejtli 1980, 1982) since it enabled the ECP materials to be regenerated after
use as first suggested by Professors Solms, Wiedenhof, and Hoffman. Our group has
also confirmed this subsequently (Crini 2003; Crini and Peindy 2006; Crini et al.
2007). The ECP polymers could be easily regenerated, and column adsorption and
desorption tests showed that the contaminants adsorbed on cross-linked polymers
were successfully released by different types of aqueous alcohol solutions. Unlike
for active carbons, the regeneration of these systems is simple and straightforward,
which makes them more attractive (Crini et al. 2007, 2019b).
8.4.3 Pollutant Removal Using Modified
Cyclodextrin Polymers
It is known that ECP polymers without modification had a low affinity for cationic
dyes. An improvement can be obtained by introducing groups such as carboxyl or
amino groups onto ECP materials able to complex target dyes. Some materials were
prepared by reticulation in the presence of carboxymethyl cellulose. Due to the –
OH and –COOH groups in the polymer network, the material was hydrophilic and
easily swollen by water, but above all it had ion-exchange properties. Indeed, the
gels exhibited more specific and higher adsorption of contaminants from water samples than other traditional ECP materials (Crini et al. 2002, 2003; Crini 2003). The
presence of carboxymethyl cellulose also enhanced both accessibility and mobility
of the cyclodextrin in the polymer by promoting the swelling of the material in
water. However, the results confirmed that, despite identical experimental conditions, as for the performance of unmodified materials, the performances of two
batches of modified ECP material may be different, mainly due to the exothermic
nature of the cross-linking reaction, which makes it difficult to maintain the temperature in the reaction medium during the synthesis of the material. This last conclusion had previously been reported by Professor Szejtli (Szejtli et al. 1978; Szejtli
G. Crini
formation of an inclusion complex due to the β-cyclodextrin molecules and the
physical adsorption in the polymer network. In the mid-2000s, Professor Isasi’s
work also confirmed that the presence of cyclodextrin cavities cannot alone explain
the adsorption results and stressed the importance of the polymer network structure
and thus of the degree of cross-linking (Romo et al. 2004, 2006, 2008; GarcíaZubiri et al. 2006; Vélaz et al. 2007).
As the materials were relatively highly cross-linked, they could be used both in
batch and column studies (Crini et al. 1998b; Vecchi et al. 1998; Bertini et al. 1999).
The method proposed extended the potential applications of these materials because
the use of cyclodextrin cross-linked gels in adsorption columns in general had limitations due to hydrodynamic problems and column fouling. Another advantage that
has been mentioned was the regeneration of adsorbents after use (Vecchi et al. 1998;
Janus et al. 1999). In the 1980s, Professor Szejtli stressed that the reversible nature
of complex formation was essential in the case of water treatment (Szejtli et al.
1978; Szejtli 1980, 1982) since it enabled the ECP materials to be regenerated after
use as first suggested by Professors Solms, Wiedenhof, and Hoffman. Our group has
also confirmed this subsequently (Crini 2003; Crini and Peindy 2006; Crini et al.
2007). The ECP polymers could be easily regenerated, and column adsorption and
desorption tests showed that the contaminants adsorbed on cross-linked polymers
were successfully released by different types of aqueous alcohol solutions. Unlike
for active carbons, the regeneration of these systems is simple and straightforward,
which makes them more attractive (Crini et al. 2007, 2019b).
8.4.3 Pollutant Removal Using Modified
Cyclodextrin Polymers
It is known that ECP polymers without modification had a low affinity for cationic
dyes. An improvement can be obtained by introducing groups such as carboxyl or
amino groups onto ECP materials able to complex target dyes. Some materials were
prepared by reticulation in the presence of carboxymethyl cellulose. Due to the –
OH and –COOH groups in the polymer network, the material was hydrophilic and
easily swollen by water, but above all it had ion-exchange properties. Indeed, the
gels exhibited more specific and higher adsorption of contaminants from water samples than other traditional ECP materials (Crini et al. 2002, 2003; Crini 2003). The
presence of carboxymethyl cellulose also enhanced both accessibility and mobility
of the cyclodextrin in the polymer by promoting the swelling of the material in
water. However, the results confirmed that, despite identical experimental conditions, as for the performance of unmodified materials, the performances of two
batches of modified ECP material may be different, mainly due to the exothermic
nature of the cross-linking reaction, which makes it difficult to maintain the temperature in the reaction medium during the synthesis of the material. This last conclusion had previously been reported by Professor Szejtli (Szejtli et al. 1978; Szejtli
G. Crini