7.3.3 Water Remediation Using Titanium Oxide
Liquid effluents containing organic contaminants are more difficult to treat using
conventional processes such as physicochemical and biological. In this regard,
AOPs for the elimination of organic contaminants in an aqueous medium using
TiO 2 photocatalysis have been proved to be potentially advantageous for the purification of wastewater, especially for persistent and non-biodegradable contaminants. As a representative photocatalyst based on TiO 2 , it is highly stable, resistant
to acidic or alkaline conditions, nontoxic, safe, and inexpensive and has a very rich
redox reactivity. Thus, TiO 2 is known as the best material for oxidizing organic
pollutants (Chi et al. 2013). When the photocatalyst is applied in UV-based oxidation, various organic pollutants can be degraded and/or mineralized into CO 2 and
H 2 O due to the production of hydroxyl radicals (ÁOH), which have a strong oxidation
potential (2.8 V).
The high importance of TiO 2 as photocatalyst lies in its ability to oxidize many
organic compounds into innocuous species such as CO 2 and H 2 O, (Carp et al. 2004)
although in some other cases, formaldehyde may be the one of the products of the
degradation, being, therefore, a target of study (Khanmohammadi et al. 2014). The
photodegradation properties of titanium oxide are originated from its suitable electronic band structure. The redox potential for photogenerated holes from TiO 2 is
+2.53 V versus the standard hydrogen electrode (SHE) which is positive enough to
produce oxygen or ÁOH from water. The redox potential for CB electrons is
À0.52 V, which is, in principle, negative enough to evolve hydrogen from water
and to produce superoxide O 2
ÀÁ from absorbed oxygen. The generation of ÁOH
radicals and superoxide O 2
ÀÁ significantly facilitates the decomposition of organic
pollutants (Henderson 2011; Fujishima et al. 2008; Fujishima and Zhang 2006).
Textile dyes, compounds of pharmaceutical or industrial origin, or human domestic
wastes constitute some of the largest groups of organic compounds that represent an
increasing environmental danger. Therefore, this is an interesting topic of study, not
only using titanium oxide as semiconductor photocatalyst, but also especially when
the degradation process can be activated by visible light (Candal and de la Cruz
Martínez 2015). Thus, in recent years a wide variety of hybrid systems based on
titanium oxides have been studied for these purposes (Gunti et al. 2017).
Pharmaceutical Compounds
In the environment, pharmaceutical compounds such as nonsteroidal antiinflammatory
drugs (NSAIDs), hormones, antibiotics, etc. have been detected in many areas:
hospital wastewaters, (Kummerer 2001; Wen et al. 2004) in and out sewage treatment
plant effluents, (Carballa et al. 2004) surface water as rivers and lakes, (Boyd et al.
2003) marine waters, (Weigel et al. 2004) and in soil matrices (Scheytt et al. 2006).
Pharmaceutical compounds with ng L
À1 and mg L
À1 concentrations levels have rarely
exceeded the mg/L levels (Chen et al. 2010b; Stumpf et al. 1998; Ternes 1998;
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
227
Liquid effluents containing organic contaminants are more difficult to treat using
conventional processes such as physicochemical and biological. In this regard,
AOPs for the elimination of organic contaminants in an aqueous medium using
TiO 2 photocatalysis have been proved to be potentially advantageous for the purification of wastewater, especially for persistent and non-biodegradable contaminants. As a representative photocatalyst based on TiO 2 , it is highly stable, resistant
to acidic or alkaline conditions, nontoxic, safe, and inexpensive and has a very rich
redox reactivity. Thus, TiO 2 is known as the best material for oxidizing organic
pollutants (Chi et al. 2013). When the photocatalyst is applied in UV-based oxidation, various organic pollutants can be degraded and/or mineralized into CO 2 and
H 2 O due to the production of hydroxyl radicals (ÁOH), which have a strong oxidation
potential (2.8 V).
The high importance of TiO 2 as photocatalyst lies in its ability to oxidize many
organic compounds into innocuous species such as CO 2 and H 2 O, (Carp et al. 2004)
although in some other cases, formaldehyde may be the one of the products of the
degradation, being, therefore, a target of study (Khanmohammadi et al. 2014). The
photodegradation properties of titanium oxide are originated from its suitable electronic band structure. The redox potential for photogenerated holes from TiO 2 is
+2.53 V versus the standard hydrogen electrode (SHE) which is positive enough to
produce oxygen or ÁOH from water. The redox potential for CB electrons is
À0.52 V, which is, in principle, negative enough to evolve hydrogen from water
and to produce superoxide O 2
ÀÁ from absorbed oxygen. The generation of ÁOH
radicals and superoxide O 2
ÀÁ significantly facilitates the decomposition of organic
pollutants (Henderson 2011; Fujishima et al. 2008; Fujishima and Zhang 2006).
Textile dyes, compounds of pharmaceutical or industrial origin, or human domestic
wastes constitute some of the largest groups of organic compounds that represent an
increasing environmental danger. Therefore, this is an interesting topic of study, not
only using titanium oxide as semiconductor photocatalyst, but also especially when
the degradation process can be activated by visible light (Candal and de la Cruz
Martínez 2015). Thus, in recent years a wide variety of hybrid systems based on
titanium oxides have been studied for these purposes (Gunti et al. 2017).
Pharmaceutical Compounds
In the environment, pharmaceutical compounds such as nonsteroidal antiinflammatory
drugs (NSAIDs), hormones, antibiotics, etc. have been detected in many areas:
hospital wastewaters, (Kummerer 2001; Wen et al. 2004) in and out sewage treatment
plant effluents, (Carballa et al. 2004) surface water as rivers and lakes, (Boyd et al.
2003) marine waters, (Weigel et al. 2004) and in soil matrices (Scheytt et al. 2006).
Pharmaceutical compounds with ng L
À1 and mg L
À1 concentrations levels have rarely
exceeded the mg/L levels (Chen et al. 2010b; Stumpf et al. 1998; Ternes 1998;
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
227