16593 12 tủ tài liệu bách khoa
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16593 12 tủ tài liệu bách khoa
_____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro 16593 12 tủ tài liệu bách khoaolina Chapel Hill. North CarolinaDavid A. Reckhow, Ph.D.Professor of Civil and Environmental Engineering University of MassachusettsAmh erst. Massach use I IsChemical oxidation processes play several important roles in the treatment of drinking waler. Chemical oxidants are used for the oxidation of 16593 12 tủ tài liệu bách khoareduced inorganic species, such as ferrous iron, l'c(ll); manganous manganese. Mn(ll); and sulfide. S(-Ll); and hazardous synthetic organic compounds16593 12 tủ tài liệu bách khoa
such as trichloroethylene (TCF.) and atrazine. Oxidants can also be used to destroy taste- and odor-causing compounds and eliminate color. Tn addition_____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro 16593 12 tủ tài liệu bách khoa, they can be used to control nuisance aquatic growths, such as algae, in prelrealmenl basins, and may be used as primary' disinfectants to meet Cl (disinfectant concentration times contact time) requirements (see Chapter 14). These oxidants are often added al the head of the treatment plant, prior 16593 12 tủ tài liệu bách khoato or at the rapid mix basin, but they can also be employed after clarification, prior to filtration, after a substantial portion of the oxidant deman16593 12 tủ tài liệu bách khoa
d has been removed.The most common chemical oxidants used in water treatment arc chlorine, ozone.chlorine dioxide, and permanganate. Ozone is sometime_____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro 16593 12 tủ tài liệu bách khoahnologies arc also available.Free chlorine has traditionally been the oxidant (and disinfectant) of choice in the United States, but concerns about the formation of potentially harmful halogenated disinfection by-products (DBPs) produced by reactions between free chlorine and natural organic materia 16593 12 tủ tài liệu bách khoal (NOM). exacerbated in some cases by the presence of bromide, have caused many water systems to adopt alternative chemical oxidants (and disinfectant16593 12 tủ tài liệu bách khoa
s) to lower halogenated ĐBP formation.These other oxidants may also react with NOM and bromide to various degrees, depending upon12.2CHAPTER TWELVEthe_____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro 16593 12 tủ tài liệu bách khoaroblems in the treatment plant or distribution system.This chapter reviews thermodynamic and kinetic principles associated with the use of chemical oxidants in general, the types and properties of the chemical oxidants used in water treatment, specific applications of oxidation processes for the tre 16593 12 tủ tài liệu bách khoaatment of drinking water, and the formation and control of oxidation and disinfection by-products. Comparisons among the different oxidant choices are16593 12 tủ tài liệu bách khoa
presented where information is available.PRINCIPLES OF OXIDATIONThermodynamic ConsiderationsThermodynamics establishes the bounds or constraints for _____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro 16593 12 tủ tài liệu bách khoaalpies and entropies of reaction. Despite its limitations, the domain of thermodynamics is where one must begin the task of characterizing and understanding oxidation reactions. In this section, the most basic thermodynamic concepts relating to oxidation reactions will be presented. l or a more comp 16593 12 tủ tài liệu bách khoarehensive treatment of the subject, there are many excellent textbooks that can be consulted (e.g.. Stumm and Morgan. 1996: Pankow. 1991).Electrochemi16593 12 tủ tài liệu bách khoa
cal Potentials. Oxidation reactions are often viewed as reactions involving the exchange of electrons. Since acids are frequently defined as proton do_____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro 16593 12 tủ tài liệu bách khoample. Many oxidants actually donate an electron-poor clement or chemical group, rather than simply accept a lone electron. Nevertheless, it’s useful to treat all oxidation reactions as simple electron transfers for the purpose of balancing equations and performing thermodynamic calculations.Thermody 16593 12 tủ tài liệu bách khoanamic principles can be used to determine if specific oxidation reactions are possible. This generally involves the calculation of some form of reacti16593 12 tủ tài liệu bách khoa
on potential. Although in most cases oxidation equilibria lie very far to one side or the other, it is sometimes instructive to calculate equilibrium _____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro 16593 12 tủ tài liệu bách khoaeactions and their standard half-cell potentials and respectively) are available in tables of thermodynamic constants (a few are listed in Tables 12.1 and 12.2). These may be combined to get the overall standard cell potential (Eq. 12.1).£ỉct = ESx+^(12.1)Much as a pKa describes the tendency of an a 16593 12 tủ tài liệu bách khoacid to give up a hydrogen ion. an electrochemical potential E describes the tendency of an oxidant to take up an electron, or a reductant to give one16593 12 tủ tài liệu bách khoa
up.The standard-state Gibbs Free Energy of reaction ỉsG" is related to the standard electrochemical cell potential by Faraday's constant /’and the num_____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro 16593 12 tủ tài liệu bách khoamentOxidantReduction half-reaction£^d. voltsOzone^Oj(aq) + I r + e~ —> >6O2(aq) + %H2O Hydroxyl radicalOH + H* + e~ -> H2O I lydrogcn peroxide!4H2O2 + IF + —» H2O PermanganateZ»MnO4“ + %H+ + e~ -> !6MnO2(s) + %H2O Chlorine dioxideCIO2 + e~ —» C1O2" I Typochlorous acid'/A IOCI + '/AT+ + e~ —> J4Cr + 16593 12 tủ tài liệu bách khoa'/A I2O Hypochlorite ionZOCl + H’ + e/’Cl ZH.’O Hypobromous acid!4IIOBr + /II+ + e~ —> /?Br + /ILO .Monochloramine’ZNH.Cl + H' +1’- -> Ỉ4C1 + /-NH/ Di16593 12 tủ tài liệu bách khoa
chloraminc/NIICI2 + %H+ + e_—» !4CI~ + /1NIĨ4* Oxygen/4O2(aq) + H' + e /H2O2.08 2.85 1.78 1.68 0.95 1.48 1.64 1.33 1.40 1.34 1.23Sources: Lidc (1995);_____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro 16593 12 tủ tài liệu bách khoa positive will spontaneously proceed in the direction as written (i.c., from left to right), and those with a positive value (or negative E") will proceed in the reverse direction.Consider a generic oxidation reaction:<*AOX T /^rcd *4" ^’14 ox( 1 2.4)where substance A picks up one electron from subs 16593 12 tủ tài liệu bách khoatance B. Tn order to determine which substance is being reduced and which is being oxidized, one must calculate and compare oxidation stales of the re16593 12 tủ tài liệu bách khoa
actant atoms and product atoms.The equilibrium constant K for this reaction defines the concentration quotient for the reactants and products at equil_____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North Caro_____CHAPTER 12____CHEMICAL OXIDATION1Philip c. Singer, Ph.D.Professor. Department of Environmental Scienc es and Engineering University of North CaroGọi ngay
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