Abstract
Advanced oxidation processes (AOPs), namely the Fenton oxidation, ozonation, electrochemical oxidation, and photocatalysis, are potential alternative techniques for dye removal from textile effluents. Their inherent ability to completely mineralize pollutants including those recalcitrant to biodegradation and to be compatibly integrated in conventional technologies present grounds for consideration of AOPs as alternative wastewater treatment options. Advanced oxidation involves generation and subsequent reaction of various radicals and reacting species with the target compounds. This chapter discusses the fundamentals and chemistry and efficiencies of the Fenton process, ozonation, electrochemical oxidation, and photocatalysis processes for complete dye removal from wastewater. The reaction mechanisms, performance, and factors affecting efficiency are discussed.
TopIntroduction
The textile, printing and leather industries are known for the use of chemicals, especially, in the wetting processes where bleaching, dyeing and finishing take place. The large volumes of wastewater from these industries present serious environmental problems due to their high concentration of non-biodegradable dyes responsible for the intense colour of the effluents, variations in pH, temperature, high levels of dissolved solids and COD together with presence of toxic heavy metal content such as As, Cr, Cu among others (Sundrarajan et al., 2007). Removal of dyes from wastewater is therefore a subject of interest for academicians, environmentalists and regulatory authorities.
The most widely used traditional wastewater treatment process for textile effluents is chemical precipitation. However, alum coagulants generate large volumes of sludge that present disposal challenges among other inherent limitations. Furthermore, conventional wastewater treatment strategies are incapable to completely remove dye compounds or reduction to molecular levels considered safe for human and aquatic life and can therefore be reintroduced into receiving surface waters. To circumvent these challenges, advanced oxidation processes (AOPs) are emergent techniques based on reaction of active radicals, generated by different methods, with organic contaminants. Advanced oxidation processes include the use of hydrogen peroxide (H2O2), ozone (O3) and UV radiation singly or combined. Table 1 highlights the oxidation potentials of certain chemical oxidizers (Babuponnusami and Muthukumar, 2011).
Table 1. Standard potential of common oxidants
| Oxidant | Oxidation Potential (V) |
| Fluorine (F2) | 3.03 |
| Hydroxyl radical (HO●) | 2.80 |
| Atomic Oxygen (O) | 2.42 |
| Ozone (O3) | 2.07 |
| Hydrogen peroxide (H2O2) | 1.77 |
| Hypochlorite | 1.49 |
| Chlorine (Cl2) | 1.36 |
| Potassium permanganate (KMnO4) | 1.67 |
| Chlorine dioxide (ClO2) | 1.5 |
| Hypochlorous acid (HClO) | 1.49 |
| Oxygen (O2) | 1.23 |
| Bromine (Br2) | 1.09 |
Key Terms in this Chapter
Electrochemical AOPs: These are advanced oxidation processes (AOPs) involving the production of hydroxyl radicals for the mineralization of organic pollutants from electrochemical reactions.
Dye: A natural or synthetic coloring material.
Fenton Process: Fenton processes involve the reactions of peroxides (mostly H 2 O 2 ) with iron ions to form active oxygen moieties that oxidize the target compounds.