Page 120 - Materials Chemistry, Second Edition

Textile Wastewater Treatment by Advanced Oxidation Processes 101

A modified configuration corresponds to the Photo-Fenton process, which can
also produce ∙OH radicals.

−
3+
Fe 2+ + HO 2 → Fe + OH + i OH (6.6)
2
hν
3+
+
Fe + HO→ Fe 2+ + H + i OH (6.7)
2
OH RH+ → HO R+ (6.8)
2
A number of advantages are associated with the Fenton for the elimination of
organic pollutants: no energy necessary to activate H O , low cost and easy opera-
2
2
tion, short reaction time, and no mass transfer limitation due to its homogeneous
catalytic nature. However, this process has some drawbacks that have to be consid-
ered, such as low pH requirements (2–3), consumption of ferrous salts due to inef-
fective regeneration, and the presence of residual iron in the effluent and chemical
sludge production.
The efficiency of Fenton processes for the treatment of textile effluents is exten-
sively reported in the literature. The combination of chemical oxidation using the
Fenton reagent (H O /Fe ) achieved partial removal of color and organic content
2+
2
2
(COD) of a textile industry effluent (Papadopoulos et al., 2007). When Fenton oxi-
dation was coupled to an aerobic sequencing batch reactor (Blanco et al., 2012) and
aerobic biological treatment (Pérez et al., 2002), not only was the removal of COD
and color observed but also efficient pathogen removal.
6.2.4 pHoTocaTalysis
Photocatalysis relies on the capacity of semiconducting materials to act as sensitiz-
ers for light-reduced redox processes due to their electronic structure. A photocata-
lyst is a chemical that produces electron–hole pairs by absorption of light quanta,
causes chemical transformations of the reaction participants that come into contact
with it, and regenerates its chemical composition after each cycle (Fox and Dulay,
1993). Photocatalysts include titanium dioxide (TiO ), zinc oxide (ZnO), zinc sulfide
2
(ZnS), ferric oxide (Fe O ), silicon (Si), tin oxide (SnO ), and cadmium sulfide (CdS),
2
2
3
among others. TiO has been the most widely cited photocatalyst in the literature due
2
to its properties: considerable activity, high stability, lack of environmental impact,
and low cost (Augugliaro et al., 2006).
6.2.5 sonolysis
During acoustic sonication, bubbles are formed, which grow and collapse implo-
sively, producing an unusual chemical and physical phenomenon. This collapse gen-
erates hotspots with transient temperature of about 5000 K and pressures of about
1000 atm. Under such extreme conditions, water molecules dissociate into ∙OH radi-
cals and H atoms. The radical species react with other molecules to induce sono-
chemical degradation (Özdemir et al., 2011). The sonochemical decolorization and