Environments, redox processes

In the reprocessing environment there are many mthenium compounds, some of which are gaseous. Some reprocessing approaches, notably the REDOX process, require a mthenium removal step in the off-gas system. The PUREX process maintains mthenium in one of its nonvolatile states. 

As Table 20.4 shows, several processes can occur in both the near-surface and deep-well environments. For example, neutralization of acidic or alkaline wastes is a straightforward process, and although temperature differences between the two environments may need to be considered, no other factors make the deep-well setting distinctly different. The same holds true for oxidation-reduction (redox) processes. 

Theoretical calculations have been carried out on a number of zinc-containing enzymatic systems. For example, calculations on the mechanism of the Cu/Zn enzyme show the importance of the full protein environment to get an accurate description of the copper redox process, i.e., including the electronic effects of the zinc ion.989 Transition structures at the active site of carbonic anhydrase have been the subject of ab initio calculations, in particular [ZnOHC02]+, [ZnHC03H20]+, and [Zn(NH3)3HC03]+.990... 

The thioester hypothesis can be summed up as follows the formation of thiols was possible, for example, in volcanic environments (either above ground or submarine). Carboxylic acids and their derivatives were either formed in abiotic syntheses or arrived on Earth from outer space. The carboxylic acids reacted under favourable conditions with thiols (i.e., Fe redox processes due to the sun s influence, at optimal temperatures and pH values) to give energy-rich thioesters, from which polymers were formed these in turn (in part) formed membranes. Some of the thioesters then reacted with inorganic phosphate (Pi) to give diphosphate (PPi). Transphosphorylations led to various phosphate esters. AMP and other nucleoside monophosphates reacted with diphosphate to give the nucleoside triphosphates, and thus the RNA world (de Duve, 1998). In contrast to Gilbert s RNA world, the de Duve model represents an RNA world which was either supported by the thioester world, or even only made possible by it. 

Stumm, W., and B. Sulzberger (1991), "The Cycling of Iron in Natural Environments Considerations Based on Laboratory Studies of Heterogeneous Redox Processes", Geochim. Cosmochim. Acta (in print). 

Of more apparent significance in the aquatic environment are redox processes induced or enhanced on absorbance of light by chromophores at metal oxide surfaces in which the metal of the oxide lattice constitutes the cationic partner. Light induced electron transfer within such a chromophore often results in disruption of the oxide lattice. The photoredox-induced dissolution of iron and manganese oxides by such a mechanism has been proposed as a possible means of supply of essential trace-metal nutrients to plants and aquatic organisms (29-31). ... 

Pettine M (2000) Redox processes of chromium in sea water. In Chemical Processes in Marine Environments. 

Stumm, W. Sulzberger, B. (1992) The cyding of iron in natural environments Considerations based on laboratory studies of heterogeneous redox processes. Geochim. Cosmochim. Acta 56 3233—3257 Stumm, W (1992) Chemistry of the solid-water interface. Wiley Sons Inc., New York,... 

The definition of stability of an oxidation state of a given atom is meaningless without referring to its immediate environment. Any ligated metal atom which undergoes a redox process with its ligands at a measurable rate is here defined as being in an unstable state of oxidation. This is somewhat broader than the con-... 

For reactions of these types, the identity of the ligand, its environment within the complex—e.g., the nature of the central metal and the properties and positions of the other ligands—and the character of any external redox reagent are important variables which will influence the thermodynamic feasibility of processes, the products formed under given experimental conditions, and the rates and mechanisms of redox processes. Within this framework some significant problems await solution. 

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. 

The stereochemical specificity of enzymes depends on the existence of at least three different points of interaction, each of which must have a binding or catalytic function. A catalytic site on the molecule is known as an active site or active centre of the enzyme. Such sites constitute only a small proportion of the total volume of the enzyme and are located on or near the surface. The active site is usually a very complex physico-chemical space, creating micro-environments in which the binding and catalytic areas can be found. The forces operating at the active site can involve charge, hydrophobicity, hydrogen-bonding and redox processes. The determinants of specificity are thus very complex but are founded on the primary, secondary and tertiary structures of proteins (see Appendix 5.1). 

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-... 

The need for biological mediation of most redox processes encountered in natural waters means that approaches to equilibrium depend strongly on the activities of the biota. Moreover, quite different oxidation-reduction levels may be established within biotic microenvironments than those prevalent in the over-all environment diffusion or dispersion of products from the microenvironment into the macroenvironment may give an erroneous view of redox conditions in the latter. Also, because many redox processes do not couple with one another readily, it is possible to have several different apparent oxidation-reduction levels in the same locale, depending upon the system that is being used as reference. 

The constituents discussed above may participate in the following redox reactions. No attempt is made to cover every possible redox process in the sediments and in the hydrosphere but only to give an idea of how some different redox reactions may proceed, depending on the environments. 

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. 

In these systems, chirality and sub-molecular motion stem directly from each other, thus showing an even more intimate relationship. Many different kinds of stimuli have been used to provoke such changes in co-conformation including pH-change [41-47], redox processes [41,47-49], the nature of the environment [50], photochemistry [51,52], temperature (entropy-driven shuttling) [53] and reversible covalent chemistry [54]. 

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