Reductive dissolution level

The Electron Transfer Step. Inner-sphere and outer-sphere mechanisms of reductive dissolution are, in practice, difficult to distinguish. Rates of ligand substitution at tervalent and tetravalent metal oxide surface sites, which could be used to estimate upward limits on rates of inner-sphere reaction, are not known to any level of certainty. 

In order that the oxidative and the reductive dissolution reactions may proceed, the affinity for the reaction is required to be positive the reaction affinity is represented by the difference between the Fermi level, ef(sc), of the electrode and the equivalent Fermi level Er(dK) of the ion transfer reaction (Refer to Sec. 4.4.). 

Fig. 9-16. Polarization curves of anodic oxidative dissolution and cathodic reductive dissolution of semiconductor electrodes of an ionic compound MX iiixcp) (iMxh )== anodic oxidative (cathodic reductive) dissolution current solid curve = band edge level pinning at the electrode interface, dashed curve = Fermi level pinning.

Fig. 9-17. Thermodynamic stability of electrodes of compound semiconductors relative to oxidative and reductive dissolution in the state of band edge level pinning (a) oxidative dissolution is thermodynamically impossible (eFXp.< Cb) oxidative dissolution may occur (er(p.dK)> Ev)> (c) reductive dissolution is thermodynamically impossible (cnn.M> E ), (d) reductive dissolution may occur < Cc) pip. sk) (cpbi. d i) = equivalent Fermi...

Fig. 9-18. Band edge levels and equivalent Fermi levels of oxidative and reductive dissolution reactions of compound semiconductors in aqueous sohitions at pH 7 en ) - F(a

Figure 9-18 illustrates the band edge levels of compound semiconductor electrodes in aqueous solutions and the equivalent Fermi levels of the following oxidative and reductive dissolution reactions ... 

Bennett, B. and Dudas, M.J. (2003) Release of arsenic and molybdenum by reductive dissolution of iron oxides in a soil with enriched levels of native arsenic. Journal of Environmental Engineering and Science, 2(4), 265-72. 

Figure 4. Multiscale aspects of oxide surfaces in aqueous environments, (b) represents a single trace across a hematite (001) surface undergoing biotically mediated reductive dissolution (a). Relatively clean steps 2.6 nm in size on the left of the figure give way to a complex surface morphology on the right side of the image where biotic dissolution was pervasive. The wavelet transform, to /2 levels, is shown in (c). (b) provided by Kevin Rosso, Pacific Northwest National Laboratory.

Desorption on Metal Reduction Many bacteria and archaea can respire on Mn(lll/tV) and Fe(lll) oxides, leading to their dissolution, with the potential for concomitant displacement of arsenic into the aqueous phase (Cummings et al., 1999). In fact, within most soils and sediments, total As levels correlate with Fe content rather than Al or clay content (Smedley and Kinniburgh, 2002), and thus reductive dissolution—transformation of Fe(lll) phases should have a major impact on arsenic. Respiratory reduction of Fe in sediments generally occurs in zones where O2, NOs , and Mn(lV) [all being oxidants of Fe(ll) and alternative electron acceptors] are diminished (Lovley, 2000). 

Fluorides. Most woddwide reductions in dental decay can be ascribed to fluoride incorporation into drinking water, dentifrices, and mouth rinses. Numerous mechanisms have been described by which fluoride exerts a beneficial effect. Fluoride either reacts with tooth enamel to reduce its susceptibihty to dissolution in bacterial acids or interferes with the production of acid by bacterial within dental plaque. The multiple modes of action with fluoride may account for its remarkable effectiveness at concentrations far below those necessary with most therapeutic materials. Fluoride release from restorative dental materials foUow the same basic pattern. Fluoride is released in an initial short burst after placement of the material, and decreases rapidly to a low level of constant release. The constant low level release has been postulated to provide tooth protection by incorporation into tooth mineral. 

One of the most ingenious ways in which corrosion is inhibited is to strap a power pack to each leg (just above the level of the sea) and apply a continuous reductive current. An electrode couple would form when a small portion of the iron oxidizes. The couple would itself set up a small voltage, itself promoting further dissolution. The reductive current coming from the power pack reduces any ferric ions back to iron metal, which significantly decreases the rate at which the rig leg corrodes. 

In an oversimplified way one can say that acids of the volcanoes have reacted with the bases of the rocks the composition of the ocean (which is at the first endpoint (pH = 8) of the titration of a strong acid with a carbonate) and the atmosphere (which with its pco2 = 10 3 5 atm is nearly in equilibrium with the ocean) reflect the proton balance of reaction (5.25). Oxidation and reduction are accompanied by proton release and proton consumption, respectively. (In order to maintain charge balance, the production of e will eventually be balanced by the production of H+.) Furthermore, the dissolution of rocks and the precipitation of minerals are accompanied by H+-consumption and H+-release, respectively. Thus, as shown by Broecker (1971), the pe and pH of the surface of our global environment reflect the levels where the oxidation states and the H+ ion reservoirs of the weathering sources equal those of the sedimentary products. 

XANES spectra provide information about contaminants present at too low a level to produce EXAFS spectra. It has been used to investigate the oxidation of As on Mn-goethite (Sun et al. 1999). This technique has also provided information about the valence of Fe in Fe oxide films during cathodic reduction in a borate buffer (Schmicki et al. 1996), about the dissolution of Fe oxide films in acidic solutions (Vir-tanen et al. 1997) and about the orientation of styrene molecules adsorbed on FeO (111) and Fe304 (111) (Wuehn et al. 2000). 

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