wastewater, and various processes such as filtration, gravity separation, flotation,
biodegradation, or photocatalysis have been developed (Forgacs et al. 2004; Gupta
and Suhas 2009; Rafatullah et al. 2010; Alventosa-de Lara et al. 2012). Due to its
high efficiency and wide applicability, adsorption is the most employed process for
removing water-soluble dyes, and activated carbons are known to be efficient
adsorbents for the removal of dyes (Forgacs et al. 2004; Mahmoud et al. 2013).
However, activated carbons suffer from low adsorption capacity, high-cost production, and regeneration (Cai et al. 2017). Therefore, other adsorbents such as clays and
zeolites have been developed (Forgacs et al. 2004; Martorell et al. 2017). But, with
this process the dyes are simply transferred from water to another medium causing
secondary pollution. Other techniques such as filtration, gravity separation, and
flotation have high operating costs and could release toxic secondary pollutants
into the ecosystem. Consequently, they are not able to treat the water to meet
standards for reuse filtration or coagulation. Biodegradation of synthetic dyes have
also been developed (Demarche et al. 2012; Martorell et al. 2017). This process is
quite inexpensive, and the end products of complete mineralization are not toxic.
However, many of the synthetic dyes are chemically stable and resistant to microbiological attack. In addition, when the discharges have low biodegradable portions,
for example, waste discharges of chemical industries, biological treatments are
inefficient or insufficient (Oller et al. 2011). The development of an effective process
to depollute water is therefore still of great and continuous interest. To reach this
goal, the heterogeneous photocatalysis appears a promising technology, and it is one
of the most studied for the decomposition of dyes in water phase over the few past
decades (Julkapli et al. 2014; Cambié et al. 2016; Karthikeyan et al. 2017; de Lima
et al. 2017; Sharma and Feng 2017; Cai et al. 2017). Indeed this technique has some
advantages: (1) complete degradation of organic pollutants to CO 2 and water, (2) no
solid wastes disposal problem, and (3) only mild temperature and “pressure” conditions are necessary. Photocatalytic degradation usually occurs at room temperature
and pressure and may be more cost-effective than other conventional techniques
such as activated carbon adsorption and chemical scrubbers, because the semiconductor catalysts are inexpensive and capable of oxidizing most organic compounds
effectively. However, for most photocatalyst the activity still needs to be enhanced in
particular in the visible domain.
Most of the considered techniques for water treatment require the use of materials
for adsorption of the dye prior to its elimination. Thanks to their properties related to
their small size such as large surface area, more active surface available, quantum
effect, and so on, nanomaterials like nanoparticles, nanotubes, or ordered
mesoporous materials are excellent candidates to be integrated into the processes
of dyes removal from water (Cai et al. 2017). For example, Lee et al. have shown that
the ordered mesoporous silica MCM-41 may be an effective absorbent for basic dyes
removal from aqueous solution (Lee et al. 2007). In addition, the nature and the
strength of the interaction between the host molecules and the adsorbent can be
tuned by functionalizing the surface of these supports (Qin et al. 2009; Donia et al.
2009). Qin et al. have modified MCM-41 by introducing ammonium group,
according to the post-synthesis way, for the adsorption of anionic dyes (Qin et al.
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
37
biodegradation, or photocatalysis have been developed (Forgacs et al. 2004; Gupta
and Suhas 2009; Rafatullah et al. 2010; Alventosa-de Lara et al. 2012). Due to its
high efficiency and wide applicability, adsorption is the most employed process for
removing water-soluble dyes, and activated carbons are known to be efficient
adsorbents for the removal of dyes (Forgacs et al. 2004; Mahmoud et al. 2013).
However, activated carbons suffer from low adsorption capacity, high-cost production, and regeneration (Cai et al. 2017). Therefore, other adsorbents such as clays and
zeolites have been developed (Forgacs et al. 2004; Martorell et al. 2017). But, with
this process the dyes are simply transferred from water to another medium causing
secondary pollution. Other techniques such as filtration, gravity separation, and
flotation have high operating costs and could release toxic secondary pollutants
into the ecosystem. Consequently, they are not able to treat the water to meet
standards for reuse filtration or coagulation. Biodegradation of synthetic dyes have
also been developed (Demarche et al. 2012; Martorell et al. 2017). This process is
quite inexpensive, and the end products of complete mineralization are not toxic.
However, many of the synthetic dyes are chemically stable and resistant to microbiological attack. In addition, when the discharges have low biodegradable portions,
for example, waste discharges of chemical industries, biological treatments are
inefficient or insufficient (Oller et al. 2011). The development of an effective process
to depollute water is therefore still of great and continuous interest. To reach this
goal, the heterogeneous photocatalysis appears a promising technology, and it is one
of the most studied for the decomposition of dyes in water phase over the few past
decades (Julkapli et al. 2014; Cambié et al. 2016; Karthikeyan et al. 2017; de Lima
et al. 2017; Sharma and Feng 2017; Cai et al. 2017). Indeed this technique has some
advantages: (1) complete degradation of organic pollutants to CO 2 and water, (2) no
solid wastes disposal problem, and (3) only mild temperature and “pressure” conditions are necessary. Photocatalytic degradation usually occurs at room temperature
and pressure and may be more cost-effective than other conventional techniques
such as activated carbon adsorption and chemical scrubbers, because the semiconductor catalysts are inexpensive and capable of oxidizing most organic compounds
effectively. However, for most photocatalyst the activity still needs to be enhanced in
particular in the visible domain.
Most of the considered techniques for water treatment require the use of materials
for adsorption of the dye prior to its elimination. Thanks to their properties related to
their small size such as large surface area, more active surface available, quantum
effect, and so on, nanomaterials like nanoparticles, nanotubes, or ordered
mesoporous materials are excellent candidates to be integrated into the processes
of dyes removal from water (Cai et al. 2017). For example, Lee et al. have shown that
the ordered mesoporous silica MCM-41 may be an effective absorbent for basic dyes
removal from aqueous solution (Lee et al. 2007). In addition, the nature and the
strength of the interaction between the host molecules and the adsorbent can be
tuned by functionalizing the surface of these supports (Qin et al. 2009; Donia et al.
2009). Qin et al. have modified MCM-41 by introducing ammonium group,
according to the post-synthesis way, for the adsorption of anionic dyes (Qin et al.
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
37