2009). Authors have evidenced that the functionalized MCM-41 material has a high
affinity for the considered dyes and that the electrostatic interaction was responsible
of the colorants adsorption. Among the various nanomaterials, titania-based ones are
of particular interest. In fact, TiO 2 is known to be chemically stable, nontoxic, low
cost, and reusable. Even more, the main advantages of titania for water treatment
concern its photocatalytic properties. Therefore TiO 2 is the most studied and used
photocatalyst nowadays (Gaya and Abdullah 2008; Chong et al. 2010; Lazar et al.
2012; Qiu et al. 2012; Lin et al. 2012; Ajmal et al. 2014). As it will be illustrated in
this chapter, titanium dioxide represents a good photocatalyst for wastewater treatment. The adsorption and the photocatalytic efficiencies are favored by an increase
of the surface area, and from this point of view, porous TiO 2 are excellent candidates
to be considered in the water treatment technologies. This chapter will be dedicated
to the use of porous titania-based nanomaterials for the photocatalytic degradation of
dyes contained in water treatment, since this process is the most promising.
2.2 Dye Classifications
Synthetic dyes exhibit considerable structural diversity (Fig. 2.1). The main dyes
employed in the industry are azo, diazo, anthraquinone, sulfur, indigoid,
triphenylmethyl, and phthalocyanine derivatives (Forgacs et al. 2004) (Robinson
et al. 2001). Among the wide variety of colorants available, azo dyes are the most
used and represent over 50% of the colorants used in the industry (Forgacs et al.
2004). Azo dyes can be basic, acid, or sulfur. Azoïc ones are characterized by the
presence of a double bond between two nitrogen atoms (–N¼N–) where at least one
of the two nitrogen atoms is bonded to an aromatic group. In addition to the –N¼N–
and aromatics functions, dye molecules can possess several other organic functions
such as carboxyl, hydroxyl, amino, and sulfoxyl groups. Because of the presence of
such organic functions, most of dyes exhibit amphoteric features. The charge of the
dye is strongly dependent on the pH. Indeed, the deprotonation of acidic groups lead
to the formation of an anionic moiety (Robinson et al. 2001). On the contrary, the
protonation of amino functions lead to cationic dye. The dye can also be nonionic,
dependently on the pH (Suteu and Malutan 2013). Nonionic dyes refer to dispersed
dyes, which do not ionize in water (Robinson et al. 2001).
The chemical structure of dyes can be very different from one to another, and it is
not straightforward to classify them only according to one parameter. Based on their
general structure, dyes can be classified as anionic, nonionic, and cationic dyes
(Robinson et al. 2001). A classification based on their source (natural, synthetic,
animal, plants) and on their chemical structure (azo or non-azo dyes) was proposed
by Ajmal et al. 2014 (Fig. 2.2).
38
B. Lebeau et al.
affinity for the considered dyes and that the electrostatic interaction was responsible
of the colorants adsorption. Among the various nanomaterials, titania-based ones are
of particular interest. In fact, TiO 2 is known to be chemically stable, nontoxic, low
cost, and reusable. Even more, the main advantages of titania for water treatment
concern its photocatalytic properties. Therefore TiO 2 is the most studied and used
photocatalyst nowadays (Gaya and Abdullah 2008; Chong et al. 2010; Lazar et al.
2012; Qiu et al. 2012; Lin et al. 2012; Ajmal et al. 2014). As it will be illustrated in
this chapter, titanium dioxide represents a good photocatalyst for wastewater treatment. The adsorption and the photocatalytic efficiencies are favored by an increase
of the surface area, and from this point of view, porous TiO 2 are excellent candidates
to be considered in the water treatment technologies. This chapter will be dedicated
to the use of porous titania-based nanomaterials for the photocatalytic degradation of
dyes contained in water treatment, since this process is the most promising.
2.2 Dye Classifications
Synthetic dyes exhibit considerable structural diversity (Fig. 2.1). The main dyes
employed in the industry are azo, diazo, anthraquinone, sulfur, indigoid,
triphenylmethyl, and phthalocyanine derivatives (Forgacs et al. 2004) (Robinson
et al. 2001). Among the wide variety of colorants available, azo dyes are the most
used and represent over 50% of the colorants used in the industry (Forgacs et al.
2004). Azo dyes can be basic, acid, or sulfur. Azoïc ones are characterized by the
presence of a double bond between two nitrogen atoms (–N¼N–) where at least one
of the two nitrogen atoms is bonded to an aromatic group. In addition to the –N¼N–
and aromatics functions, dye molecules can possess several other organic functions
such as carboxyl, hydroxyl, amino, and sulfoxyl groups. Because of the presence of
such organic functions, most of dyes exhibit amphoteric features. The charge of the
dye is strongly dependent on the pH. Indeed, the deprotonation of acidic groups lead
to the formation of an anionic moiety (Robinson et al. 2001). On the contrary, the
protonation of amino functions lead to cationic dye. The dye can also be nonionic,
dependently on the pH (Suteu and Malutan 2013). Nonionic dyes refer to dispersed
dyes, which do not ionize in water (Robinson et al. 2001).
The chemical structure of dyes can be very different from one to another, and it is
not straightforward to classify them only according to one parameter. Based on their
general structure, dyes can be classified as anionic, nonionic, and cationic dyes
(Robinson et al. 2001). A classification based on their source (natural, synthetic,
animal, plants) and on their chemical structure (azo or non-azo dyes) was proposed
by Ajmal et al. 2014 (Fig. 2.2).
38
B. Lebeau et al.