Photocatalytic Processes for the Removal of Dye - Green Chemistry for Dyes Removal from Wastewater - page 125

photoreactor in dye degradation, and finally discusses the dependence of
different parameters (pH, photocatalyst loading, initial dye concentration,
electron scavenger, light intensity) on dye degradation.

4.2 Photocatalysis - An Emerging Technology

The heterogeneous photocatalysis process has shown huge potential for
water and wastewater treatment over the last few decades. The process basi-
cally combines semiconductor material with either high energy UV photon
(300-400 nm) or low energy visible photon (400-700 nm). In this process,
the semiconductor photocatalyst generates electron-hole pair (e - /h + ) ater
electronic excitation and finally produces hydroxyl radials (HO · ) in the sys-
tem. Hydroxyl radical is the second highest oxidizing species (Table 4.2) after
fluorine, which reacts non-selectively with most of the organic pollutants in
wastewater and produces carbon dioxide, water, and mineral acid [1,6].
Photocatalysis for water decontamination was first recognized during
the 1980s when photocataltyic mineralization was successfully conducted
for different halogenated hydrocarbons [13,14]. Substantial research for
photocatalytic degradation of various organic compounds, heavy metals,
etc., with both UV and solar radiation was carried out at a later stage [15].

Table 4.2 Oxidation Potentials of Some Oxidants in Descending Order [6].

Species Oxidation Potential (V)

Fluorine 3.03

Hydroxyl radical 2.80

Atomic oxygen 2.42

Ozone 2.07

Hydrogen peroxide 1.78

Perhydroxyl radical 1.70

Permanganate 1.68

Hypobromous acid 1.59

Chlorine dioxide 1.57

Hypochlorous acid 1.49

Hypoiodous acid 1.45

Chlorine 1.36

Bromine 1.09

Iodine 0.54