Chemical oxidation in wastewater treatment is a process in which undesirable chemical species are converted through oxidation (such as redox reactions) to chemical species that are neither harmful nor objectionable. It modifies the structure of pollutants in wastewater through the addition of an oxidizing agent. During chemical oxidation, one or more electrons transfer from the oxidant to the targeted pollutant, causing its destruction. Both organic matters, including microorganisms and inorganic substances, can be subject to oxidation. Examples of organic matters for chemical oxidation include humic acids, phenols, amines, and bacteria; common inorganic substances that are toxic and/or objectionable may include ions such as Fe2+, Mn2+, S2~, CN~, and SO32~. A handful of oxidizing agents are capable of oxidizing the undesirable substances adequately: oxygen, ozone, UV/H2O2, Fe2+/H2O2 (Fenton's reaction), potassium permanganate, chlorine, chlorine dioxide, and zero-valent iron nanoparticles.
Chemical oxidation applications in wastewater are used throughout entire wastewater management operations—from collection of wastewater streams, to treatment, to disposal. In collection facilities, chemicals such as Cl2, FeCl3, and O3 or H2O2 are employed in controlling slime growth and corrosion; in treatment operations of wastewater, chemicals are used in a number of places often as supplemental procedures for a major wastewater treatment unit operation. For example, Cl2 and O3 are applied to reduce BOD and ammonia; but by and large, chemical oxidation procedures are often used to counter the problems of a biological, microbial, or nos-trilic (odorous) nature. When the wastewater effluent from a treatment plant is ready for disposal, the effluent is often treated with Cl2, H2O2, or O3 to ward off bacteria and odor. In potable water treatment (including membrane-based processes but with the addition of ammonia to neutralize the residual Cl2 or H2O2 in order to protect membranes), chemical oxidation is used for disinfection with Cl2 (mainly in the U.S. and a number of other nations) or O3/H2O2 (mainly in west Europe). Chlorine has a better residual antimicrobial potency than O3 or H2O2, but it could produce harmful substances, as described below.
One common (and inexpensive) method of chemical oxidation, referred to as alkaline chlorination, uses chlorine (usually in the form of sodium hypochlorite) under alkaline conditions to destroy undesirables such as cyanide and some pesticides. However, using alkaline chlorination for chemical oxidation of pollutants may generate toxic chlorinated organic compounds, including chloroform, bromodichloromethane, and dibro-mochloromethane, as by-products. Adjustments to the design and operating parameters may alleviate this problem, or an additional treatment step (e.g., steam stripping, air stripping, or activated carbon adsorption) may be required to remove these by-products.
Oxidation by hydrogen peroxide alone is not effective for converting high concentrations of certain refractory contaminants, such as highly chlorinated aromatic compounds and inorganic compounds (e.g., cyanides), because of low rates of reaction at reasonable H2O2 concentrations. Transition metal salts (e.g., iron salts), ozone, and UV-light can activate H2O2 to form hydroxyl radicals that are strong oxidants:
ozone and hydrogen peroxide O3 + H2O2 b OH + O2 + HO2-iron salts and hydrogen peroxide Fe2+ + H2O2 b Fe3+ + OH + OH-UV-light and hydrogen peroxide H2O2[+UV] b 2OH
The oxidation processes utilizing activation of H2O2 by iron salts are referred to as Fenton's reagent.
In general, oxidation processes that are based on the generation of radical intermediates are termed advanced oxidation techniques. Hydroxyl radicals (oxidation potential: 2.8 V) are stronger oxidants than ozone and H2O2. Hydroxyl radicals nonspecifically oxidize target compounds at high reaction rates of the order of 109 M^1 s_1.
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