Advanced Oxidation Processes AOP

Advanced oxidation processes (AOPs) have been defined by Glaze et al. (1987) as processes which involve the generation of hydroxyl radicals in sufficient quantity to effect water purification . The most common processes are 03 /H202, 03/UV and H202/U V. The development of the AOPs is an attempt to produce the nonselective and rapid OH0 to oxidize pollutants. Each of these processes involves chemistry similar to that discussed above. 

Hydrogen peroxide reacts with ozone when present as anion, H02. The reaction rate of the system ozone/hydrogen peroxide is based on the initial concentration of both oxidants  

The reaction of ozone with the undissociated hydrogen peroxide is negligible (Taube and Bray, 1940)  

The reaction continues along the indirect pathway, described previously and OH-radicals are produced (Staehelin and Hoign6, 1982 Biihler et al 1984). 

Comparison of the initial reaction with H02 (k9 = 2.2 106M s 1) and with OH (k[ = 70 M 1 s 1) shows that in the 0,/H202 -system the initiation step by OH is negligible. Whenever the concentration of hydrogen peroxide is above 10 7 M and the pH-value less than 12, H02 has a greater effect than OH has on the decomposition rate of ozone in water. 

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Fig. 2. Steps in advanced oxidation process (AOPs) involving o2one, hydrogen peroxide, and uv light of 254 nm. ( D) represents the doublet state ( ) represents quantum yield, and the other numbers associated with the reaction arrows are rate constants in units of (Af-s). Dashed arrows indicate...

Chemical processes include reduction and oxidation. Conventional chemical (coagulation-flocculation) and advanced oxidation processes (AOPs), such as chemical oxidation (ozonation, Fenton oxidation, Fe2+/H202), ultrasonic chemical oxidation, photocatalysis oxidation (UV/H2Q2, UV/O3, and W/O3/H2O2),... 

During the last two decades an increasing interest in the application of Advanced Oxidation Processes (AOPs) is observed. These processes are applied for the oxidation of toxic organic pollutants present in wastewater or in surface water that is used for the production of drinking water. The process of oxidation is mainly based on the oxidative destruction by radicals of which the hydroxyl radical (OH ) is the most powerful one. Most important AOPs are ... 

Advanced oxidation processes (AOPs) are a range of water treatments which involve the in situ formation of radicals, particularly hydroxyl radicals, in sufficient quantity to affect chemical or biological contaminants. These include ultrasonic and ultraviolet irradiation but they are sometimes ineffective for the remediation of water which contains a mixture of organic and inorganic compounds. Chemical oxidants can be used to add additional oxidising power to such processes and ozone in conjunction with ultrasound is one such option [31]. 

Andreozzi, R Vaprio, V Insola, A Marotta, R. Advanced oxidation processes (AOP) for water purifieation and recovery. Catalysis Today, 1999 53,51-59. 

TABLE 1 Cost Comparison of ECOCHOICE to Other Advanced Oxidation Processes (AOPs)... 

As shown in Table 3-1, ozone can destroy other disinfectants. This should be avoided by dosing them not ahead of ozonation stages, but rather at the end of the total treatment before the distribution of water to the supply-net. A special case is the reaction of ozone with H202 (correctly with the species H02 ), which is used as an advanced oxidation process (AOP) for intensified formation of hydroxyl radicals and their oxidative attack on persistent organic target compounds (persistent against ozone in the direct reaction mechanism) (see Chapter A 2). 

Any oxidation process in which hydroxyl radical is the dominant species is defined as an advanced oxidation process (AOP). For any oxidation reaction, two factors determine the rate of reaction. First, if a reaction has a high free energy or high electrical potential, the reaction is very likely to occur and it is considered to be thermodynamically favorable. The oxidation potentials for common oxidants suitable for environmental applications are listed in Table 4.1. 

Hydrogen abstraction by RH02 could also participate in the process of initiating a chain of thermal oxidation reactions (pathy). In aqueous systems, cations will further react by solvolysis, and superoxide anion will readily disproportionate to yield H202 (path i). This is in contrast to the fate of superoxide anions in ozonation advanced oxidation processes (AOPs), where they react primarily with ozone to produce hydroxyl radical. This description of the chemical pathways of UV/H202 oxidation of organics illustrates that, when oxygen is present, the major paths directly or indirectly create more... 

When the effects of UV/ozone treatment on the degradation of protocatechuic acid were compared with the rates of six other advanced oxidation processes (AOPs), the UV/ozone process ranked second behind only the UV /03/H202 process in terms of oxidation kinetics. The combination of UV irradiation and ozone was more effective than either UV or ozone alone in terms of degradation rates of protocatechuic acid. The contribution of different reaction pathways in a combined system will be discussed in detail in Chapter 14. 

In routine analytical laboratories, the use of advanced oxidation processes (AOPs) is an emerging alternative to conventional sample treatments2 for analytical and environmental chemists. AOPs involve the in situ generation of highly potent chemical oxidants, such as the hydroxyl radical (OH ). Several processes have been applied in analytical sample pretreatment homogenous UV irradiation, either by direct irradiation of the sample or photolysis mediated by an appropriate chemical reagent ozone and ultrasonic irradiation. A variety of AOPs ensures compliance of specific treatment requirements with optimum treatment technologies (Table 5.1). 

These processes, included in a special class of oxidation techniques defined as advanced oxidation processes (AOPs), are based on the irradiation of a semiconductor photocatalyst with UV light that leads to the formation of highly reactive hydroxyl radicals. 

Trapido M, Yeressinina J, Munter R. Ozonation and advanced oxidation process (AOP) treatment of phenanthrene in aqueous solutions. Ozone Sci Eng 1994 16 475-486. 

B. Advanced Oxidation Process (AOP) In the Treatment of Chemical Contaminants... 

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