Redox condition

The reducing and oxidizing conditions in a sediment determine the chemical stability of the solid compounds and the direction of the spontaneous reactions. The redox state can be recognized as a voltage potential measured with a platinum electrode. This voltage potential is usually referred to as E or Eh defined by the Nernst equation, as described in Chapter 5  

The Nernst equation is applicable only if the redox reaction is reversible. Not all reactions are completely reversible in natural systems activities of reacting components may be too low or equilibrium may be reached very slowly. In a sediment, the biotic microenvironment may create a redox potential that is different from the surrounding macroenvironment. For this reason, measurements of Eh in natural systems must be cautiously evaluated and not used strictly for calculations of chemical equilibria. Calculations of redox equilibria are in some cases valuable, in the sense that they will give information about the direction of chemical reactions. 

If a system is not at equilibrium, which is common for natural systems, each reaction has its own Eh value and the observed electrode potential is a mixed potential depending on the kinetics of several reactions. A redox pair with relatively high ion activity and whose electron exchange process is fast tends to dominate the registered Eh. Thus, measurements in a natural environment may not reveal information about all redox reactions but only from those reactions that are active enough to create a measurable potential difference on the electrode surface. 

The reader is referred to Berner (1963), Sillen (1965a,b), Breck (1972), Liss et al. (1973), Whitfield (1969, 1974), and Parsons (1975) for further information. 

In marine sediments, usually only the uppermost layer of the sediment exhibits oxidizing conditions 

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In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

It is also often taken for granted that many of the Earth s subsystems are exposed to free oxygen (O2), leading to a range of one-way reactions of reduced materials (such as organic carbon or metal sulfides) to an oxidized form. As pointed out many times in earlier chapters, the oxidation-reduction status of the planet is the consequence of the dynamic interactions of biogeochemical cycles. As is the case with the acid-base balances, there is considerable sensitivity to perturbations of "redox" conditions, sometimes dramatically as in the case of bodies of water that suddenly become anaerobic because of eutrophication. Another extreme... 

The effect of varying redox conditions since by altering the gas-phase experiments can readily be carried out under aerobic, microaerophilic, or anaerobic conditions. 

Bachmann A, P Walet, P, Wijnen, W de Bruin, JEM Huntjens, W Roelofsen, AJB Zehnder (1988) Biodegradation of alpha- and beta-hexachlorocyclohexane in a soil slurry under different redox conditions. Appl Environ Microbiol 54 143-149. 

Mormille MR, S Liu, JM Suflita (1994) Anaerobic biodegradation of gasoline oxygenates extrapolation of information to multiple sites and redox conditions. Environ Sci Technol 28 1727-1732. 

The redox condition of the deep fluids may be buffered by the chlorite-pyrite equilibrium. 

Liu, Y.-G., Miah, M.R.V. and Schmitt, R.A. (1988) Cerium a chemical tracer for paleooceanic redox conditions. Geochim. Cosmochim. Acta, 52, 1361-1371. 

Uranium and thorium partitioning into amphibole were also studied experimentally by Tiepolo et al. (2000a). Under the redox conditions of their experiments (FMQ-2 log units) U was dominantly tetravalent. They find no correlation between Aj, Djh and crystal composition, and, but again find a linear correlation with silica content ... 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

Related Polymer Systems and Synthetic Methods. Figure 12A shows a hypothetical synthesis of poly (p-phenylene methide) (PPM) from polybenzyl by redox-induced elimination. In principle, it should be possible to accomplish this experimentally under similar chemical and electrochemical redox conditions as those used here for the related polythiophenes. The electronic properties of PPM have recently been theoretically calculated by Boudreaux et al (16), including bandgap (1.17 eV) bandwidth (0.44 eV) ionization potential (4.2 eV) electron affinity (3.03 eV) oxidation potential (-0.20 vs SCE) reduction potential (-1.37 eV vs SCE). PPM has recently been synthesized and doped to a semiconductor (24). 

Typical Benzene Biodegradation Reactions under Various Electron Acceptor and Redox Conditions... 

A change in redox conditions (lower Eh) can partly or completely dissolve Fe and Mn oxides and liberate other coprecipitated metals. 

Patterson CG, Runnells DD. 1992. Dissolved gases in groundwater as indicators of redox conditions. Water-Rock Interaction, Proc Int Symp, 7th 1 517-520. 

The pros and cons of oxidative dehydrogenation for alkene synthesis using doped cerianites as solid oxygen carriers are studied. The hydrogen oxidation properties of a set of ten doped cerianite catalysts (Ce0.9X0.1Oy, where X = Bi, In, La, Mo, Pb, Sn, V, W, Y, and Zr) are examined under cyclic redox conditions. X-ray diffraction, X-ray photoelectron spectroscopy, adsorption measurements, and temperature programmed reduction are used to try and clarify structure-activity relationships and the different dopant effects. 

Fetzner, S., Bacterial Degradation of Pyridine, Indole, Quinoline, and Their Derivatives Under Different Redox Conditions.. Appl. Microbiol. Biotechnol., 1998. 49 pp. 237-250. 

Potential chemical interactions across dissimilar solvent systems and the sensitivity of oxide semiconductors to redox conditions and surface adsorption combine to make integration of solution-processed dielectrics and semiconductors a nontrivial undertaking. By maintaining appropriate aqueous... 

The removal rate of pharmaceuticals by WWTPs depends on factors such as reactor configuration, redox conditions, temperature, hydraulic retention time (HRT) and solid retention time (SRT). SRT is the average retention time of sludge... 

The sewer is dominated by heterotrophic microorganisms that degrade and transform wastewater components. These processes proceed under redox conditions determined by the availability of the electron acceptor. The importance of the processes for the sewer and the surroundings is not just caused by the removal and transformation of organic substrates — the electron donor—but is also a result of transformation of the electron acceptors exemplified by the formation of hydrogen sulfide from sulfate. 

In addition to the redox conditions associated with the characteristics of the sewer network as outlined in Table 1.1, a number of other sewer characteristics influence the process conditions. The following examples illustrate the close relations between design and operation characteristics and process conditions ... 

The microorganisms determined as active biomass constitute a central platform for description of the processes in a sewer. Classification of the microorganisms will, however, not be dealt with as a major subject in this context. But, it is especially important to determine under which redox conditions the microorganisms are active. 

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