Environmental Engineering Reference
In-Depth Information
are able to oxidize organic pollutants mainly by hydrogen abstraction or by electro-
phylic addition to double bonds that generates organic free radicals (R
•
). These free
radicals can react with oxygen molecules forming peroxy-radicals and initiate oxida-
tive degradation chain reactions that may lead to the complete mineralization of the
organics. AOP-generated free radicals involved in the degradation process may be
produced by photochemical and non-photochemical procedures as has been widely
reported previously (Quiroz et al., 2011).
In particular, photochemical AOPs have generated great interest in the last decade
since these procedures have led to the use of renewable sources of energy to promote the
chemical procedures involved. Solar radiation has been identified as a potential source
for driving photochemical AOPs with interesting potential for real applications, specif-
ically for water detoxification and disinfection (Orozco et al., 2008; Bandala et al.,
2008a,b). The most studied technological approaches to water disinfection using solar
radiation are homogeneous and heterogeneous photocatalysis. Both processes have
been widely tested at the laboratory, bench and pilot-plant scale for water detoxifica-
tion and disinfection with interesting results that will be discussed in later sections of
this chapter.
13.2 SOLAR RADIATION COLLECTION FOR AOPs APPLICATIONS
Solar driven AOPs possess interesting advantages when compared with artificial light
promoted AOPs reactions. Solar radiation availability, reduced cost, and simplicity
are the most commonly cited. Nevertheless, use of solar radiation also has some chal-
lenges that must be faced being solar radiation collection probably the most significant.
Despite the availability of free solar radiation almost everywhere around the world, its
use requires an efficient and cost effective optical system that focuses and uniformly
distributes the radiation of a surface. This optical system is known as a solar radiation
collector.
The first reported attempts to collect solar radiation for the promotion of AOPs
were in the early 90's at Sandia National Laboratories (USA). These early efforts used
parabolic trough collectors, usually employed for solar thermal applications. The initial
project objectives were not achieved at that time since the high concentration optical
system required an expensive sun tracking system for optimal operation.
After initial use of high concentration solar systems for driving AOPs, interest
shifted toward the use of low or non-concentrating, i.e. non imaging systems, since
it was identified that non-tracking systems may be able to promote AOPs without
the disadvantages of high concentrating, i.e. tracking, collectors. Since the first use
of the non-tracking, low concentration solar collectors for AOP applications, a wide
variety of different solar collection geometries have been tested. Some of the impor-
tant geometries are shown in cross section in Figure 13.2.1. The main differences,
advantages and disadvantages of these different geometries have been discussed in
the past and the discussion continues today. Some of the main findings have been
summarized by Bandala et al. (2004). These authors reported that reactors based on
non-imaging collectors have attracted interest (Blanco et al., 1994) since these reactors
share some of the advantages of tracking parabolic troughs and non-concentrating
reactors (Malato et al., 1997), which has been confirmed by several studies comparing
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