Environmental Engineering Reference
In-Depth Information
24
NaNocatalytic WasteWater treatmeNt system
for the removal of toxic orgaNic compouNds
Sodeh Sadjadi
Nuclear Science and Technology Research Institute, Tehran, Iran
24.1
iNtroductioN
Water is a limited resource; the world is facing formidable challenges in meeting the rising demand for clean water as the available
supplies of freshwater are decreasing due to contamination of this natural resource.
Heterogeneous catalysis has received a tremendous amount of interest, both from a scientific and an industrial perspec-
tive. Catalysts are also essential in converting hazardous waste into less harmful products. One area of catalysis that is
developing at a rapid pace is nanocatalysis. Nanoparticles have great potential as water-purifying catalysts and redox-active
media due to their large surface areas and their size- and shape-dependent optical, electronic, and catalytic properties [1]. To
avoid the dangerous accumulation of organic pollutants in the aquatic environment, powerful oxidation methods are devel-
oped for their complete destruction from natural waters and wastewaters. Although different isolation, physical separation,
biological, and chemical methods can be utilized, the most promising techniques are the so-called advanced oxidation
processes based on the in situ generation of the hydroxyl radical ( · OH) as an oxidant of organic matter. These processes
involve catalyzed chemical, photochemical, and electrochemical techniques to bring about chemical degradation of organic
pollutants [2].
This chapter is divided into six sections. Following the introductory section, Sections 24.2-24.6 highlight the results of
selected studies on the use of nanomaterials as catalysts for different advanced oxidation processes.
24.2
photocatalytic oxidatioN
24.2.1
introduction
Photocatalysis may be described as a photoinduced reaction that is accelerated by the presence of a catalyst [3]. When a semi-
conductor photocatalyst is irradiated with photons of energy equal to or greater than its band gap, electrons are excited from the
valence band to the conduction band, leaving positively charged holes in the valence band, thus leading to a charge separation [4].
The photogenerated electrons can react with electron acceptors such as O 2 adsorbed on the catalyst surface or dissolve in water
to give O 2 −· radicals [5]. The photogenerated holes react with OH or H 2 O, oxidizing them into · OH radicals [6]. These active
radicals are responsible for the decomposition of organic compounds (Fig. 24.1a). Photocatalysts also promote a photocatalytic
reaction by acting as mediators for the charge transfer between two adsorbed molecules (Fig. 24.1b). Photocatalysts quench the
excited state either by accepting an electron or transferring the charge to another substrate [7]. The efficiency of photocatalysis
depends on how well one can prevent this charge recombination [8]. There has been considerable interest in the use photocatalysis
 
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