Environmental Engineering Reference
In-Depth Information
that even trapped electrons and holes can rapidly recombine on the surface of a par-
ticle (Equation 13.4.2). The recombination process can be partially avoided through
the capture of the electron by pre-adsorbed molecular oxygen, forming a superoxide
radical (Equation 13.4.4).
13.4.1 Degradation of organic pollutants by solar driven
heterogeneous photocatalysis
Degradation of organic pollutants using photocatalytic processes is probably the
most investigated AOP over the last three decades. It has been widely tested for
the degradation of mono aromatics (i.e. benzene, dimethoxybenzenes, halobenzenes,
nitrobenzene, chlorophenols, nitrophenols, benzamide, and aniline) and, conse-
quently, these pollutants appear as model compounds in a wide variety of scientific
papers. In addition to these, several other types of molecules have been investi-
gated as substrates for photocatalytic degradation. Some of the most frequently
reported are water-miscible solvents (i.e. ethanol, alkoxyethanol), haloaliphatics
(i.e. trichloroethylene, tetrachloromethane), pesticides and surfactants, among many
others.
Despite the wide variety of reports on the application of heterogeneous photo-
catalytic processes for water and air treatment, relatively few reports are available
dealing with the application of solar radiation to drive these processes. Solar driven
heterogeneous photocatalytic processes (SDHPC) have been, however, applied to the
treatment of organic contaminants. Some of the most important applications are
described below.
13.4.1.1 Pesticide degradation
Strongly colored compounds can be removed from surface or wastewater by con-
ventional water treatment processes (i.e. adsorption), however these phase change
processes have to be discouraged anytime they result in solid matrix waste. It is highly
desirable to degrade dyes by oxidation to avoid the risk of contamination. Solar driven
heterogeneous photocatalytic processes have been extensively applied for pesticide
degradation. Quiroz et al. (2011) have recently published an interesting review about
the different applications of SDHPC for the removal of these compounds. To men-
tion some examples, chlorinated insecticides (Bandala et al., 2002; Hincapié et al.,
2005); triazine derivatives (Parra et al., 2004); carbamate derivatives (Arancibia et al.,
2002); haloacetic acid derivatives (Terashima et al., 2006) and organophosphorus
(Pichat et al., 2007), among many others, have been successfully removed from water
(Bandala and Torres, 2008d).
13.4.1.2 Dye degradation
As mentioned early for homogeneous photocatalytic processes, SDHPC degradation
of dyes and pigments has been extensively tested. The most recent published works
dealing with SDHPC dye degradation focus mainly on azo dyes (Sajjad et al., 2010;
Chung and Chen, 2009; Malato et al., 2009). However, a few reports dealing with
SDHPC removal of other dye types have been published.
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