Page 121 - Materials Chemistry, Second Edition

102 Life Cycle Assessment of Wastewater Treatment

decomposition of azo compounds, such as C.I. Reactive Red 22 and Methyl Orange,
was investigated (Okitsu et al., 2005). The results showed that the removal of both
dyes was rapid (>90% in 30 min). When the scope of the dyes was broader (Methyl
Orange, o-Methyl Red, and p-Methyl Red; Joseph et al., 2000), it was observed that
the reaction rates for the o-Methyl Red were between 30% and 40% faster than for
the other compounds. Apparently, there is a strong influence from a carboxylic group
in the ortho position on the azo group.
In the pursuit of better process efficiency and color removal, it is a frequent strategy
to combine different AOPs. Chung and Kim (2011) evaluated the efficiency of combined
ozonation with three AOPs (O /H O , O /UV, and O /H O /UV) for the removal of dye
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from synthetic effluents. The conclusion of this work is that although all the processes
eliminated color within a very short operational time, the O /H O /UV processes pre-
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sented the best results in terms of COD removal and increased biodegradability.
Similarly, the treatment of a textile industry effluent by different AOPs (O , O /
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UV, H O /UV, O /H O /UV, Fe /H O ) was investigated (Azbar et al., 2004). The
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results showed that AOPs provide higher color removal than conventional chemical
precipitation. Among the different AOPs, the O /H O /UV combination showed the
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best results (99% COD elimination and 96% color removal). Moreover, the Fenton
process also showed good results (96 % COD removal and 94% color removal) but
proved to be more economical. With the aim of investigating the relevance of the
different AOP technologies, a search for bibliographic references published between
2012 and 2017 was conducted (Figure 6.2). Regarding the different types of dyes,
azo dyes are by far the most studied, with 81% of the bibliographical references;
carbonyl dyes accounted for 14%, which corresponds to the sum of two individual-
ized searches for anthraquinone dyes and indigo dyes; and finally, phthalocyanine
dyes accounted for 5%. Among the different AOPs used to remove dyes, the most
studied, with 51% of the bibliographical references, is the Fenton process, followed
by photocatalysis (11%) and ozonation (9%).
6.3 PROCESS INTENSIFICATION USING NANOSTRUCTURED
MATERIALS FOR DYE REMOVAL
In the last two decades, the increasing interest in nanosciences and materials with
enhanced functionality has culminated especially in the preparation of nanomate-
rials with a characteristic surface area and size, between 10 and 100 nm. Several
approaches have been proposed for their use in the removal of a wide range of pollut-
ants, comprising heavy metals, algae, organics, bacteria, viruses, nutrients, cyanide,
and antibiotics (Bethi et al., 2016). In this regard, heterogeneous catalysis based on
the use of nanoparticles seems to be a promising technology in the treatment of dif-
ferent types of industrial textile effluents using AOPs. In this section, two strategies
of AOPs based on the use of nanoparticles were assessed for the removal of diverse
types of dyes present in wastewater: (1) a photochemical process based on TiO
2
nanoparticles and (2) heterogeneous Fenton catalytic degradation based on magne-
tite nanoparticles. In addition, a review of recent studies related to the application of
nanoparticles for the treatment of dye-containing effluents was conducted.