Outer hydrous layer

At gold oxide, an insulating oxide is formed up to 17 = 2.0 V. Then, the valence band reaches the Fermi level, and oxygen evolution and corrosion starts. A conducting hydrous layer is formed at a constant rate [108]. The electrode capacity remains constant, since the potential drop across the inner oxide is electronically limited, and the outer hydrous layer is ionically conducting. ETR can take place at the inner AU2O3/Au(OH)3-interface. 

Outer Hydrous Layer on the Passive Oxide Film... 

Ferrous hydroxide [Fe(OH)2] and hydrated ferrous oxide (Fe0.nH20) is first diffusion barrier layer formed on the surface. As the pH of saturated Fe(OH)2 is about 9.5, the surface of steel corroding in aerated pure water is always alkaline. Due to incipient oxidation green coloured Fe(OH)2 is formed. Ferrous oxide is converted to hydrous ferric oxide or ferric hydroxide at the outer rust layer as dissolved oxygen is available by the following reaction. 

Several types of mineralizations, massive-type stockwork mineralization and a sulfide-bearing sediment layer were described (Halbach et al., 1989). The outer portion of the massive sulfide sample (late-stage) is composed of barite, realgar, orpiment, amorphous silica and hydrous Fe-Mn oxides, small amounts of sphalerite, galena and pyrite. The central portion consists mainly of sphalerite, pyrite and galena with small... 

Surface complex formation of an ion (e.g., cation) on the hydrous oxide surface. The ion may form an inner-sphere complex ("chemical bond"), an outer-sphere complex (ion pair) or be in the diffuse swarm of the electric double layer. (From Sposito, 1989)... 

Surface Complex Formation. Metal ions form both outer and inner sphere complexes with solid surfaces, e.g. hydrous oxides of iron, manganese, and aluminium. In addition, metal ions, attracted to charged surfaces, may be held in a diffuse layer, which, depending upon ionic strength, extends several nanometres from the surface into solution. 

Figure 9.10. (a) Surface complex formation of an ion (e.g., cation) on the hydrous oxide surface. The ion may form an inner-sphere complex ( chemical bond ), an outer-sphere complex (ion pair), or be in the diffuse swarm of the electric double layer. (The inner-sphere complex may still retain some aquo groups toward the solution side.) (From Sposito, 1989.) (b) A schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups ( s ), inner-sphere complexes ( a ), outer-sphere complexes ( /3 ), and the diffuse ion swarm ( d ). (Adapted from Sposito, 1984.)... 

The uranium loading of 3600 ppm amounts to less than 3 % of the total uranium capacity. A high attrition resistance of the resin beads, expected in contrast to hydrous titanium oxide, could be demonstrated in long-term experiments. Granules exposed to natural sea water in a fluidized bed for more than 6 months showed almost no attack of the outer surface however, the initial white color of the beads changed to brown, probably due to the complexation of transition metals. Uranium is mainly accumulated in a narrow surface layer of the beads. The high uranium selectivity of the resin can be deduced from Table 7. 

VIBRATIONAL SPECTROSCOPY Infrared and Raman spectroscopies have proven to be useful techniques for studying the interactions of ions with surfaces. Direct evidence for inner-sphere surface complex formation of metal and metalloid anions has come from vibrational spectroscopic characterization. Both Raman and Fourier transform infrared (FTIR) spectroscopies are capable of examining ion adsorption in wet systems. Chromate (Hsia et al., 1993) and arsenate (Hsia et al., 1994) were found to adsorb specifically on hydrous iron oxide using FTIR spectroscopy. Raman and FTIR spectroscopic studies of arsenic adsorption indicated inner-sphere surface complexes for arsenate and arsenite on amorphous iron oxide, inner-sphere and outer-sphere surface complexes for arsenite on amorphous iron oxide, and outer-sphere surface complexes for arsenite on amorphous aluminum oxide (Goldberg and Johnston, 2001). These surface configurations were used to constrain the surface complexes in application of the constant capacitance and triple layer models (Goldberg and Johnston, 2001). 

For this purpose it is possible to extend to a multiple oxide the one-site model of Johnson (1984), which provides a thermodynamic description of the double layer surrounding simple hydrous oxides. Briefly, in this model the double layer charge is divided into the charge inside the slip plane, slip plane a[d]. While occupied sites, a[tl] is obtained from the Poisson-Boltzmann equation. Note that unlike the triple-layer model (Davis et al., 1978) which allows ions to form surface complexes at two different planes (0 or / ) instead of al the slip plane only, this model does not distinguish between inner- and outer-sphere complexes. Expression of the... 

Hydrous ferrous oxide (FeO reH20) or ferrous hydroxide [Fe(0H)2l composes the diffusion-barrier layer next to the iron surface through which O2 must diffuse. The pH of a saturated Fe(0H)2 solution is about 9.5, so that the surface of iron corroding in aerated pure water is always alkaline. The color of Fe(0H)2, although white when the substance is pure, is normally green to greenish black because of incipient oxidation by air. At the outer surface of the oxide film, access to dissolved oxygen converts ferrous oxide to hydrous ferric oxide or ferric hydroxide, in accordance with... 

---