Reducing environments redox processes

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. 

TT heoretical equilibrium models can be established for oxidation-reduc-- tion systems in natural waters in much the same way that acid-base or solubility models have been developed and found useful in interpreting observed concentrations of ions and other materials. To relate the theoretical models for redox processes to observed conditions and processes in the aquatic environment is, however, much more difficult and cannot be done as rigorously. Primarily this situation occurs because true oxidation-reduction equilibrium is not observed in any natural aquatic system this is partly because of the extreme slowness of most oxidation-... 

For example Kurihara and Fendler [258] succeeded in forming colloid platinum particles, Ptin, inside the vesicle cavities. An analogous catalyst was proposed also by Maier and Shafirovich [164, 259-261]. The latter catalyst was prepared via sonification of the lipid in the solution of a platinum complex. During the formation of the vesicles platinum was reduced and the tiny particles of metal platinum were adsorbed onto the membranes. Electron microscopy has shown a size of 10-20 A for these particles. With the Ptin-catalyst the most suitable reductant proved to be a Rh(bpy)3+ complex generated photochemically in the inner cavity of the vesicle (see Fig. 8a). With this reductant the quantum yield for H2 evolution of 3% was achieved. Addition of the oxidant Fe(CN), in the bulk solution outside vesicles has practically no effect on the rate of dihydrogen evolution in the system. Note that the redox potential of the bulk solution remains positive during the H2 evolution in the vesicle inner cavities, i.e. the inner redox reaction does not depend on the redox potential of the environment. Thus redox processes in the inner cavities of the vesicles can proceed independently of the redox potential in the bulk solution. 

Sulfur Speciations and Redox Processes in Reducing Environments... 

The entry of strongly reduced landfill leachate into a pristine, often oxidized, aquifer, leads to the creation of very complex redox environments. Important processes include organic matter biodegradation, biotic and abiotic redox processes, dissolution/precipitation of minerals, complexa-tion, ion exchange, and sorption. The resulting... 

These microbially mediated redox processes utilize electron acceptors and produce reduced species. This will generate more reduced environments as long as there are electron donors available. The microbial population thus strongly affects their environment in the core of the plume. At the boundaries of the plume, complex microbial communities may exist, and steep redox gradients are created when dissolved electron acceptors are consumed. In addition, reoxidation of sulfides or ferrospecies by oxygen diffusing into the plume may increase the concentration of sulfate and ferric iron, which can stimulate sulfate and iron reduction in these zones as observed at Norman Landhll (Cozzarelli et al., 2000). 

Figure 12.15 Distribution of sulfur species versus pH in the system H2S-S(conoid)-H2O-NaCl(0.7 M) for ZS(aq) = 10" M. Reprinted with permission from Am. Chem. Soc. Symp. Ser. 93, J. Boulegue and G. Michard, Sulfur speciations and redox processes in reducing environments. In Chemical modeling in aqueous systems. Copyright 1979 American Chemical Society,...

The S system is one of the main systems for regulating sedimentary redox processes. It participates in a series of diagenesis such as complexation, exchange adsorption, precipitation. The existing forms of S are regulated by environmental pE and pH. S in the East China Sea sediment interstitial waters mainly exists in the form of SO , it amounts to 99% of total S, HS amounts to only 1% of total S, the others are a little H2S, S , etc. Sulfate is reduced to sulfide through microbial processes in anoxic circumstances. SO / HS (S°) is one of the main redox electrode pairs which regulate marine environments. 

The oxidation state of contaminants, such as U and Cr, dictate their fate and transport within the environments. The mobility and, hence, risk associated with U(VI) and Cr(VI) decreases upon reduction due to increased adsorption and precipitation capacity of their reduced states. The fate of each element will depend on the kinetics and thermodynamics of microbial and chemical processes within soils. By impeding the spread of contaminants in the environment, remediation strategies can focus on source areas thereby accelerating clean-up activities at reduced effort and cost. Moreover, enhancing our knowledge of redox processes will improve modeling efforts aimed at predicting the fate of contaminants in surface and subsurface environments. 

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