Environmental Engineering Reference
In-Depth Information
25.7.6 Use of Photosensitizers
Dyes having visible light response and redox properties are used as sensitizers on the TiO
2
surface. The reactions occurring can be represented as follows (Malato et al., 2009):
dye + hϑ → dye*
TiO
* − →
+
−
dye
dye
+
e
2
dye
+
+ e
−
→ dye
The dye gets activated under visible light and transfers the electrons to the semiconductor.
Dyes such as thionine, toluidine blue, and methylene blue are used as photosensitizers.
25.8 Photoreactors Used in Water Treatment
Photoreactors can be divided in two groups—suspended and immobilized reactors
depending on whether the catalyst used is in slurry form or in immobilized form. In case
of slurry reactors, posttreatment is used to recover the catalyst. Commonly used methods
are iltration, coagulation, locculation, or centrifugation. Hence, slurry reactors become
expensive. However, slurry reactors possess more photocatalytical eficiency compared
with the immobilized form, due to increased surface area to volume ratio. Also, the cata-
lyst is reused after the treatment. In the case of immobilized catalyst reactors, the catalyst
is coated on the inner walls of the reactor. Selection of a suitable substrate is important for
ixed-bed reactors.
The following factors should be considered while designing a photoreactor: concentra-
tion of the contaminant to be treated, low rate of water, catalyst to be used, and electron
acceptor. In case of solar photocatalysis, the collector can be a concentrating type or a non-
concentrating type.
25.8.1 Practical Applications of Solar Photocatalysis
Photocatalytic treatment systems have been successfully implemented in many industries.
Some of the examples are given in this section. Solar photocatalysis can remove hazard-
ous compounds, emerging contaminants, and recalcitrant compounds from water. The
reactors generally use titanium dioxide. Hence, modiications are made in the design of
reactors to utilize the UV radiation in the sunlight since normal TiO
2
can be photoactivated
only under UV radiation.
It has been reported that parabolic concentrating-type reactors were used for solar photo-
catalysis. It was used for removal of volatile organic compounds from groundwater (Mehos
et al., 1992), pesticide removal (Hermann et al., 1998; Malato et al., 1998), and heavy metal
removal (Prairie et al., 1992). However, Robert and Malato (2002) reported that there are
some disadvantages in the parabolic concentrating-type reactors. For example, they cannot
make use of the UV radiation in diffuse form. They also reported that compound parabolic
collector technology (CPC) collectors show excellent performance under solar radiation.
