Redox reactions, of organic pollutants

Let us now take a brief look at some important redox reactions of organic pollutants that may occur abiotically in the environment. We first note that only a few functional groups are oxidized or reduced abiotically. This contrasts with biologically mediated redox processes by which organic pollutants may be completely mineralized to C02, HzO and so on. Table 14.1 gives some examples of functional groups that may be involved in chemical redox reactions. We discuss some of these reactions in detail later. In Table 14.1 only overall reactions are indicated, and the species that act as a sink or source of the electrons (i.e., the oxidants or reductants, respectively) are not specified. Hence, Table 14.1 gives no information about the actual reaction mechanism that may consist of several reaction steps. 

General Remarks. As is the case for any chemical reaction, the partners in a bimolecular electron-transfer reaction must encounter one another (i.e., to colloide) in order to allow the reaction to proceed. Since the majority of chemical redox reactions of organic pollutants proceed in sequential one-electron steps, we consider a reaction in which one electron is transferred between two reactants P and R, where P is, for example, an organic pollutant and R is a reductant. In a general way, one may express such a one-electron transfer reaction schematically as (Eberson, 1987) ... 

In short, much future research on kinetics of soil chemical processes is needed. Areas worthy of investigation include improved methodologies, increased use of spectroscopic and rapid kinetic techniques to determine mechanisms of reactions on soils and soil constituents, kinetic modeling, kinetics of anion reactions, redox and weathering dynamics, kinetics of ternary exchange phenomena, and rates of organic pollutant reactions in soils and sediments. 

The role of photocatalysis by transition metal complexes in the environment is reviewed, and its influence on composition of the environmental compartments, transport between them, and activation of the environmental self-cleaning behavior is characterized. In description of atmospheric processes, the attention is paid to coordination compounds as photocatalysts of the transfer and redox reactions of nitrogen oxides. In the case of hydrosphere and soils, various mechanisms of organic pollutant photodegradations are presented in which the iron, copper, and chromium complexes play... 

To illustrate how we may now calculate more precisely to what extent a given organic compound may be reduced or oxidized in a given system,.we consider a dilute solution (e.g., 0.1 mM) of 2-hydroxy naphthoquinone [trivial name lawson (LAW), reaction 13 in Table 5] in 5 mAf aqueous hydrogen sulfide at various pH values. As we will see later, since such quinoid compounds undergo reversible redox reactions in natural systems, they may play a pivotal role in the transformation of organic pollutants. Hence, we consider the reversible reaction... 

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