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AOPs are the technologies being used for the oxidation of a wide range of organic contaminants in polluted water in order to obtain clean water for human consumption such as for drinking and other household use. These methods involve in-situ generation of highly potent chemical oxidants such as the hydroxyl radical (.OH) which bring the contaminant level in water to their permissible limits from toxic level. Glaze et.al., in 1987 defined the AOPs and observed that ozone (O3) along is not capable to oxidize organic contaminants because there should be a generation.OH radical which is possible if certain combinations are used such as O3/UV and O3/hydrogen peroxide (H2O2) (Glaze et al., 1987). AOPs have been applied for the treatment of water contaminated with several pollutants such as antibiotics, herbicides, insecticides, endocrine disrupting chemicals (EDCs), personal care products, pharmaceutical products (PPCPs), effluent organic matters etc (Fast et al., 2017). AOPs can be classified into two broad categories which are discussed as follows.
These methods include four ways to generate.OH radical in absence of light. Three of them use ozone for generating highly reactive hydroxyl radical and the fourth one uses ferrous ion as a catalyst, known as Fenton system. Ozonation can be carried out at high pH value; greater than 8.5, with hydrogen peroxide and in presence of catalyst along with ozone.
Molecular ozone (O3) has been employed commonly for disinfection and oxidation of organic pollutant for purification of drinking water. The decomposition of ozone in water is dependent on pH value. As the pH increases the decomposition rate also increases. The half-life of ozone in water is less than 1 minute at pH 10. The ozone decomposition is initiated by hydroxyl ion, followed by the series of reaction which leads to the formation of.OH, radical which is short-lived and extremely potent oxidizing agent. The other species which are formed during ozone decomposition include superoxide anion radical (O2.-), hydroperoxyl radical (HO2.) and ozonide anion radical O3.-. The reaction shows the formation of two molecules of.OH radical by utilizing three molecules of O3.
In natural water bodies, the chain reaction is suppressed by bicarbonate and carbonate which act as.OH, radical scavengers. The product of the reaction is also radical but they are passive bicarbonate and carbonates radicals which do not react further with ozone or organic compounds thus terminating chain reaction (Gottschalk et al., 2009). Ozonation method is capable of removing 90 % of the contaminant from water (Esplugas et al., 2007) and it is widely used for drinking water treatment because of its oxidation potential and disinfectant properties (Von Gunten, 2003).
Catalytic ozonation can be divided into two categories:
Transition metals ions in water can catalyze the decomposition of O3, generating O2.- which transfer an electron to another molecule of O3 to form O3.- and subsequently leads to the formation of .OH. The decomposition rate of O3 is dependent on the pH and reagent concentrations. The other way of decomposition is the formation of a complex between metal catalyst and the organic molecule which further reacts with O3 leading to oxidation of organic molecule (Gracia et al., 1996; Pines and Reckhow, 2002).
This AOP is catalyzed by a solid catalyst such as iron oxides, titanium oxide, alumina, and their combination. The mechanism and efficiency of ozonation are dependent on several factors such as the type of catalyst, pH of the water and its surface properties. As this catalytic system is heterogeneous so while selecting catalyst, its surface area, density, pore volume, porosity pore size must be considered because these factors affect catalyst efficiency. Metal oxide-supported metal oxide and supported metal catalyze ozonation by different mechanisms. In the supported metal oxide, first, there is adsorption of ozone followed by decomposition to.OH radical (Beltrán et al., 2000). When ozonation is catalyzed by metal on a support, it involves the transfer of an electron from metal to O3 which leads to O3.-formation and subsequent release of.OH radical (Kasprzyk-Hordern et al., 2004).
The combination of O3 and H2O2 is known as peroxone. This process involves the generation of the conjugate base of H2O2 i.e., HO2- which reacts with O3 resulting information.OH radical (Staehelin and Hoigne, 1985).
Fenton process was first applied for oxidation of maleic acid. In this method, ferrous ion and H2O2 are reacted in acidic medium and.OH, radical is formed. The generated.OH, radical reacts with organic pollutants to transform them into less or nontoxic products.
In this oxidation, ultra-violet light is used along with H2O2, O3, Fe+2; to generate highly reactive.OH, radicals. This method is employed when conventional O3 and H2O2 are not able to completely oxidize organic pollutants to CO2 and H2O. In some cases, reaction resulted in the formation of toxic products compared to the initial organic pollutant. UV energy can be helpful in the destruction of organic compounds which are not transforming in absence of UV light. O3 also undergoes photolysis at 254 nm wavelength (Munter, 2001).
O3 has a molar extinction coefficient 3300 M–1 cm–1and strongly absorbs UV light of wavelength 254 nm. Once O3 absorbs UV light in water, it produces H2O2 which further decomposes into.OH (Peyton and Glaze, 1988). UV lamps such as mercury lamp are used to generate energy at this wavelength. Due to the high molar extinction coefficient, this results in quick absorption of UV light by O3 and subsequently more production of .OH. It has been observed that if the water contains high UV radiation absorbing compounds such as phenol, xylenols, 5-methylresorcinol etc then UV light is absorbed by these compounds (Munter et al., 1995; Trapido and Kallas, 2000). However, phenolic compounds are easily oxidized by O3 but complete conversion to CO2 and H2O is not achieved. Organic compounds such as oxalic acid, glyoxal, glyoxylic acid, and formic acid are easily oxidized to CO2 and H2O (Gurol and Vatistas, 1987; Takahashi, 1990).
Peroxide linkage undergoes homolytic cleavage in presence of UV energy to form.OH radical which propagates chain reaction and finally leads to formation O2 and H2O. The efficiency of photolysis of H2O2 is dependent on the absorption of UV energy which ultimately depends on the molar extinction coefficient (19.6 M-1cm-1) which is very less compared to organic pollutants present in water. However quantum yield of photolysis of H2O2 is high.
The decomposition rate of O3 is increased by the addition of H2O2 along with UV light, which results in increased rate of generation of .OH.
The use of Fe+2/Fe+3 and H2O2 with UV/visible radiation is known as Photo-Fenton/Fenton-like system, employed for the removal and mineralization of organic pollutant from the drinking water with improving the rate of removal than Fenton system without radiation. The formation of.OH, radical takes place by photoreduction of Fe+3 to Fe+2. The Fe+2 again reacts with H2O2 (Ruppert et al., 1993).
In acidic medium (at pH=3), Fe(OH)2+ complex is formed which under UV radiation generates Fe+2 and .OH (Pignatello et al., 2006).
In Photocatalytic oxidation, a metal oxide semiconductor, titanium dioxide (TiO2) is used with UV. TiO2 has been found the most effective catalyst and can be developed either in slurry form or immobilized on some support (Belgiorno et al., 2007). The TiO2 absorbs UV radiation that leads to its excitation and produces conduction band electrons and valence band holes. Holes are able to oxidize almost all chemicals due to their extremely positive oxidation potential and react with absorbed species on TiO2 surface, an outline in below equations (Matthews, 1986). Figure 1 represents various AOPs which can be employed for drinking water treatment.
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