Advanced chemical oxidation: Its present role and potential future in hazardous waste treatment

Chemical oxidation reactions involving hydroxyl radicals have been extremely effective in the destruction of organic pollutants. These advanced chemical oxidation processes (AOP) generally use a combination of oxidation agents (such as H2O2 or O3), irradiation (such as uv or ultrasound), and catalysts (such as metal ions or photocatalysts) as a means to generate hydroxyl radical. The hydroxyl radical is one of the strongest inorganic oxidants next to elemental fluorine. The hydroxyl radical is stable over a wide pH range, up to pH 10. The hydroxyl radical reacts with organic by three major mechanisms: hydroxy addition, hydrogen abstraction, and electron transfer. Several AOP systems are reviewed first. The merits as well as limitations of these systems are discussed. The potential of AOP for future hazardous wastes treatment is then demonstrated by four AOP systems, H2O2/Fe2+, TiO2/uv/O2, H2O2, and TiO2/uv/H2O2, exemplifiedby chlorophenols. A reaction scheme can be generalized for the oxidation of halogenated phenols by advanced chemical oxidation, specifically, ones involving hydroxyl radicals. Upon the attack of a halogenated phenol, ArXnOH, by a hydroxyl radical, OH·, a free radical, Ar(OH)2Xn·, is formed. This free radical can undergo two reaction paths: (1)hydroxylation without dechlorination (Type A) and (2) hydroxylation with dechlorination (Type B). It has been observed that mono-halogenated phenols (n=1) only follow Type A path: dichlorophenols (n=2) and trichlorophenols (n=3) can have both Type A and Type B reaction pathways; tetrahalogenated (n=4) and pentahalogenated (n=5) phenols only follow Type B reaction pathway.

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