Redox intensity

Representative ranges of redox intensity in soil and water... 

Similarly the redox transformations Mn(III,IV) oxides / Mn2+ causes rapid electron cycling at suitable redox intensities. Two differences of Mn and Fe in their redox chemistries are relevant ... 

The free electron activity, pE, which indicates the redox intensity in a system, is defined as... 

Similarly, we can define a convenient parameter for the redox intensity ... 

Redox-sensitive trace elements may undergo changes in their redox states, according to the predominant redox intensities. 

Although the electron activity is a hypothetical phenomenon, p2 is a useful parameter to describe the redox intensity of natural systems and hence the species distribution under prevailing redox conditions (Example 3.10). 

Redox intensity or electron activity in natural waters is usually determined by the balance between those processes which introduce oxygen (e.g. dissolution of atmospheric oxygen, photosynthesis) and those which remove oxygen (e.g. microbial decomposition of organic matter). Often these processes are controlled by the availability of inorganic nutrients such as phosphate and nitrate, e.g. as utilized in the formation of organic matter during photosynthesis (see Section 3.3.4). 

Table 8.8. Sequence of Progressive Reduction of Redox Intensity by Organic Pollutants...

In summary, because many redox reactions in natural waters do not couple with one another readily, different apparent redox levels exist in the same locale thus an electrode or any other indicator system cannot measure a unique h or pc- If tho electrode (or the indicators) reached equilibrium with one of the redox couples, it would indicate the redox intensity of that couple only. A few conditions are necessary to obtain meaningful operational values ... 

The weathering cycle is affected markedly at least locally and regionally by civilization. In local environments the proton, and the electron, e, balances may become upset, and significant variations in pH and redox intensity (pe) result. 

Mam heterogeneous processes such as dissolution of minerals, formation of he solid phase (precipitation, nucleation, crystal growth, and biomineraliza-r.on. redox processes at the solid-water interface (including light-induced reactions), and reductive and oxidative dissolutions are rate-controlled at the surface (and not by transport) (10). Because surfaces can adsorb oxidants and reductants and modify redox intensity, the solid-solution interface can catalyze rumv redox reactions. Surfaces can accelerate many organic reactions such as ester hvdrolysis (11). 

The Concept of pt. The redox intensity, pe, is defined in a manner analogous to... 

The balance between P and R is responsible for regulating the concentration of 02 in the oceans and the atmosphere (27). Thus, biological processes in the sea (probably more than half of the total global photosynthesis takes place in the sea) regulate po2 and in turn the redox intensity of the interface between atmosphere and hydrosphere. 

Redox parameters analogous to those for acid-base chemistry can be defined for all aqueous systems. The redox intensity factor pE is an energy parameter in non-dimensional form that describes the ratio of electron acceptors (oxidants) and donors (reductants) in a redox couple. The redox potential (Ej ) of the system is an alternative and equivalent intensity factor. Table I summarizes the complete thermodynamic analogy between pH and pE. An analogy between acid-base and redox systems can also be made for capacity factors. 

One experimental approach to determine the redox intensity at which each of these inorganic systems function has been to set up a system in which the redox potential is closely controlled and the reduction of the oxidized component of various inorganic redox systems studied. This technique involves the use of stirred soil suspensions that have their redox potential closely controlled at any point in the range indicated in Figure 4.17. A suspension with soil-to-water ratio of 1 4 for mineral soils or 1 10 for organic soils is incubated in a sealed chamber and the desired redox potential is obtained by automatically adding very small amounts of oxygen or appropriate amounts of alternate... 

FIGURE 7.31 Carbon dioxide fixation in salt meadow cordgrass and rice as a function of soil redox intensity (Eh) along the portion of the redox scale. (Modified from Kludze and DeLaune, 1995a.)... 

FIGURE 7.35 Root porosity and radial oxygen loss in Spartina patens gvov/n under various soil redox intensity for 50 days. (Modified from Kludze and DeLaune, 1994.)... 

Kludze, H. K. and R. D. DeLaune. 1995. Gaseous exchange and wetland response to soil redox intensity and capacity. Soil Sci. Soc. Am. J. 59 939-949. 

Lissner, J. L, A. Mendelssohn, and C. J. Anastasium. 2003. A method for cultivating plants under controlled redox intensities in hydroponics. Aquatic Botany. 76 93-108. 

W Stumm. Interpretation and measurement of redox intensity in natural waters. Schweiz Z Hydrol 46 291-296, 1984. 

The reducing capacity of a groundwater is the net amount of reductants available to maintain the redox intensity below a fixed threshold if oxidants, such as O2, are introduced. Repository safety requires that even if O2 is introduced... 

For example, if the amount of ferrous iron minerals present in repository backfill and fracture minerals (represented by FeC03(s) in Fig. 1(a)) is much greater than the amount of O2 remaining after closure, then with time, all O2 will be reduced to H2O by these minerals, producing iron hydroxide in the process. This would ensure that the reducing intensity would return to values at least as low as the redox potential of the Fe(0H)3(s)/FeC03(s) couple (near —0.05 V). This is below the threshold for corrosion of either copper or uranium oxide by O2. It is also shghtly above the threshold for sulphide production by sulphate reduction (—0.2 V). The presence of ferrous minerals thus buffers the redox intensity of the repository to conditions that are favourable for repository performance. 

Although RDC provides a rigorous definition of the buffer capacity required to maintain the redox intensity below that associated with copper corrosion, engineering application to real... 

The redox potential for reduction of chromate therefore represents a critical threshold, below which chromium is contained, and above which chromium is mobilized. Sufficient reducing capacity to maintain the redox intensity below this threshold could be provided, for example, by including zero-valent iron or organic compost in reactive engineered barriers. This is not necessarily suggested as an engineering solution for Cr containment in particular each contaminant and each site need full and detailed consideration. It is only used here to illustrate the point that redox transformations have a potentially important role in the design and performance of reactive barriers for containment of wastes other than radionuclides. 

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