Water redox

Lindberg, R. D. and Runnells, D. D. (1984). Ground-water redox reactions An analysis of equilibrium state applied to Eh measurements and geochemical modeling. Science 225,925-927. 

Fig. 5.6 (Left) Comparison of band energy levels for different II-VI compounds. Note the high-energy levels of ZnSe. Representation is made here for electrodes in contact with 1 M HQO4. The reference is a saturated mercury-mercurous sulfate electrode, denoted as esm (0 V/esm = +0.65 V vs. SHE). (Right) Anodic and cathodic decomposition reactions for ZnSe at their respective potentials (fidp, Fdn) and water redox levels in the electrolytic medium of pH 0. (Adapted from [121])...

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

Finally, because the Mo ocean budget should be strongly sensitive to bottom water redox conditions, particularly perturbations associated with expanded deep sea anoxia, determination of Mo concentrations or their variations in ancient seawater, via sedimentary proxies, has been proposed as an indicator of global ocean paleoredox change (e.g., Emerson and Huested 1991). 

The coordination chemistry of vanadium is strongly influenced by the oxidizing/reducing properties of the metallic centre, and the chemistry of vanadium ions in aqueous solution is limited to oxidation states +2, +3, +4 and +5, although V2+ can reduce water. Redox potentials are given in Table 1 and an E vs. pH diagram is shown in Figure 1. 

There are, of course, many other types of reaction that can profitably be studied in solvents other than water. Redox reactions will be discussed in Section 9.4, where we will look at analogies between reduction/ oxidation and acid/base processes, and the use of non-aqueous solvents as media suitable for the use of strongly oxidising or reducing agents. 

Walton-Day, K., D.L. Macalady, M.H. Brooks, and V.T. Tate. 1990. Field methods for measurement of ground water redox chemical parameters. Ground Water Monitor. Rev. 10, 81-89. 

Sufficient DO data were not obtained from basalt-synthetic Grande Ronde groundwater experiments to allow determination of a definitive rate law. A first order kinetic model with respect to DO concentration was assumed. Rate control by diffusion kinetics and by surface-reaction mechanisms result in solution composition cnanges with different surface area and time dependencies (32,39). Therefore, by varying reactant surface area, determination of the proper functional form of the integrated rate equation for basalt-water redox reactions is possible. 

Other examples of redox-sensitive elements include heavy elements such as uranium, plutonium, and neptunium, all of which can exist in multiple oxidation states in natural waters. Redox conditions in natural waters are also indirectly important for solute species associated with redox-sensitive elements. For example, dissolution of iron (hydr)oxides under reducing conditions may lead to the solubilization and hence mobilization of associated solid phase species, e.g. arsenate, phosphate (see Sections 3.3.2.1, 3.3.3.2, and 3.3.4.1). 

The main equilibria in the water redox system are the following ... 

Piper D. Z. and Isaacs C. M. (1995) Minor elements in Quaternary sediment from the Sea of Japan a record of surface-water productivity and intermediate-water redox conditions. Geol. Soc. Am. Bull. 107, 54-67. 

Under aerobic conditions the redox potential deviates widely from the potentials of soil redox couples. In anaerobic soils, redox potentials may be more quantitatively related to ion activities. The Fe24" and perhaps Mn2+ concentrations are high and tend to dominate the redox potential. The range of redox potentials that have been measured in soils is shown in Fig. 4.7. The envelope around those data was considered by the investigators to be the extreme limits of likely redox potentials and pH values in soils and natural waters. Redox potentials can closely approach the H+-H2 potential, because it is nearly reversible at the platinum electrode. 

When combining the RuOEP CO with a transition metal, such as ruthenium or platinum, the bandedge shift increases up to 0.48 electron volts (eV) in the positive direction. These combination treatments allowed overlap of the water redox potentials to occur in the dark. The effect of pH on the flat band potential of an untreated electrode and... 

Keywords Acid-volatile sulphide Bioavailability Pore water Redox Sediment Trace metals... 

The redox reactions of iron are involved in several important phenomena occurring in natural waters and water treatment systems. The oxidation of reduced iron minerals, such as pyrite (FeS2(s)), produces acidic waters and the problem of acid mine drainage. The oxidation/reduction of iron in soil and groundwaters determines the iron content of these waters. Redox reactions are intimately involved in the removal of iron from waters. As we have already seen in Section 7-6, the oxidation of metallic iron is an important corrosion reaction. 

FIGURE 6.2 Eh-pH phase diagram for oxygen-water redox couple (Bolin et al., 1979). 

Drastic change in ground water redox potential. Depending on the nature of change there are oxidation and reduction brirriers. Oxidation barrier is usually caused by penetration in the subsurface of surface water rich in O. Reduction barrier is usually associated with penetration of deep no-oxygen water to the surface. 

Table 2,1 - Various ways of writing the two water redox couples...

The terms used to denote both the water redox couples presented in section 2.1.23 correspond with the terms used to write these two same couples in acidic or basic media. 

As already emphasised, the water redox couples play a particularly important role in all electrochemical systems involving an aqueous electrolyte. The corresponding current-potential curves are rather complex, notably because water autoprotolysis occurs . However some qualitative characteristics can be retained, especially for very acidic or... 

Figure 3.12 shows the shape of the Pourbaix diagram as applied to the CI(+I)/CI(-I) system. In the same pH range, the two straight iines symboiizing the water redox coupies are aiso indicated using grey lines. 

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