Role Of Advanced Oxidation Technologies For Innovative Water Treatment

The so-called advanced oxidation technologies (AOTs) are among the most emerging chemical oxidation processes and are anticipated to play a crucial role in water treatment as stand-alone processes or posttreatment options in combination with conventional technologies in the near future [1]. AOTs refer to a set of chemical treatment processes designed to decompose organic and inorganic materials in water by oxidation route. The technologies are particularly useful to destroy biologically toxic and non-degradable chemicals such as aromatics, pesticides, and volatile compounds.

As summarized in Table 1, AOTs, as a powerful tool for water and wastewater treatment, include chemical oxidation, Fenton and photo-Fenton processes, ultraviolet (UV)-based processes, photocatalytic redox processes, supercritical water oxidation, sonolysis, and electron beams and

Table 1 Advanced oxidation technologies

Process

Chemicals or equipment used

Chemical oxidation

O3+H2O2

Fenton and photo-Fenton processes

Fe2++H2O2, Fe2++H2O2+UV

UV-based processes

UV+O3, UV+H2O2, uv+o3+h2o2

Photocatalytic redox processes

Semiconductor (TiO2, ZnO)/UV

Supercritical water oxidation

High temperature and pressure

Sonolysis

Ultrasound

Electron beam and g-rays

Beam generator

g-ray irradiation [2]. In conventional oxidation technologies, the role of common oxidants such as chlorine and permanganate is well-known to directly oxidize and thus decompose water contaminants. Meanwhile, AOTs are based on further activation of oxidants, such as ozone, hydrogen peroxide, peroxymonosulfate (PMS), and persulfate, to generate other transient species such as hydroxyl radicals (*OH) and sulfate radicals (SO4) that demonstrate much higher oxidation capability than the oxidant sources. There are many alternative ways of generating such oxidizing species, including TiO2 photocatalysis and sonolysis. The various combinations of these technologies such as UV/H2O2/O3 and Fenton-like reaction are preferred for their high reactivity and efficiency and they are already in use for large-sale industrial water treatment. For example, the well-known system, Fenton reagent leads to the generation of hydroxyl radicals in the presence of iron (Reaction 1):

This technology is currently applied for the treatment of industrial discharges, soil, and groundwater remediation. However, the majority of the other technologies are still in the developmental stage.

AOTs are characterized by a specific oxidation pathway. Extremely reactive hydroxyl radicals are commonly formed during the reaction initiated by the chemicals or equipment used for AOTs. In this chapter, we mostly focus on more common hydroxyl radical-based AOTs while sulfate radicals also play a role as a strong oxidizing species in certain types of AOTs (sulfate radicals will be briefly mentioned in Section 7.2). The hydroxyl radicals readily attack organic contaminants. In sequential reactions, the organic contaminants are transformed to simpler organic molecules that are eventually mineralized to CO2, H2O, and inorganic species (i.e., Cl", NO", SO4"). The general scheme and concept of AOTs are demonstrated in Fig. 1. The mechanism of organic decomposition using AOTs involves a sequential reaction pathway that includes the following steps: (a) generation of hydroxyl radicals, (b) formation of carbon center radicals in organic compounds by hydroxyl radicals attack, (c) transformation of carbon center radicals to peroxyl radicals by the addition of oxygen,

(d) degradation of peroxyl radicals to form simpler organic molecules, and

(e) repetition of the cycle until complete organic mineralization [2]. Due to rapid oxidation reactions, AOTs are characterized by high reaction rates and short treatment times, which makes them promising in water and wastewater treatment.

Figure 1 General scheme and concept of AOTs. Reactive radical species ("OH, O2~) generated from various sources (TiO2/UV, UV/H2O2, O3, UV/O3) attack organic contaminants in water to break down and transform their molecular structure to simple intermediates via chain reactions, and then finally mineralize them to water, carbon dioxide, and inorganic species.

Figure 1 General scheme and concept of AOTs. Reactive radical species ("OH, O2~) generated from various sources (TiO2/UV, UV/H2O2, O3, UV/O3) attack organic contaminants in water to break down and transform their molecular structure to simple intermediates via chain reactions, and then finally mineralize them to water, carbon dioxide, and inorganic species.

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