Iron reduction surfaces

An XPS Investigation of iron Fischer-Tropsch catalysts before and after exposure to realistic reaction conditions is reported. The iron catalyst used in the study was a moderate surface area (15M /g) iron powder with and without 0.6 wt.% K2CO3. Upon reduction, surface oxide on the fresh catalyst is converted to metallic iron and the K2CO3 promoter decomposes into a potassium-oxygen surface complex. Under reaction conditions, the iron catalyst is converted to iron carbide and surface carbon deposition occurs. The nature of this carbon deposit is highly dependent on reaction conditions and the presence of surface alkali. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

Hence, these Qc values are a quantitative measure for the relative affinities of the various NACs to the reactive sites. Figs. 14.10e and/show plots of log Qc versus h(AtN02)/0.059 V of the 10 monosubstituted benzenes. A virtually identical picture was obtained for the log Qc values derived from an aquifer solid column and from a column containing FeOOH-coated sand and a culture of the iron-reducing bacterium, Geobacter metallireducens (GS15). Furthermore, a similar pattern (Fig. 14.10c) was found when correlating relative initial pseudo-first-order rate constants determined for NAC reduction by Fe(II) species adsorbed to iron oxide surfaces (Fig. 14.12) or pseudo-first-order reaction constants for reaction with an iron porphyrin (data not shown see Schwarzenbach et al., 1990). Fig. 14.12 shows that Fe(II) species adsorbed to iron oxide surfaces are very potent reductants, at least for NACs tv2 of a few minutes in the experimental system considered). 

Redox Reactions. Aquatic organisms may alter the particular oxidation state of some elements in natural waters during activity. One of the most significant reactions of this type is sulfate reduction to sulfide in anoxic waters. The sulfide formed from this reaction can initiate several chemical reactions that can radically change the types and amounts of elements in solution. The classical example of this reaction is the reduction of ferric iron by sulfide. The resultant ferrous iron and other transition metals may precipitate with additional sulfide formed from further biochemically reduced sulfate. Iron reduction is often accompanied by a release of precipitated or sorbed phosphate. Gardner and Lee (21) and Lee (36) have shown that Lake Mendota surface sediments contain up to 20,000 p.p.m. of ferrous iron and a few thousand p.p.m. of sulfide. The biochemical formation of sulfide is undoubtedly important in determining the oxidation state and amounts of several elements in natural waters. 

If the geology of the formation of green silicate minerals is relatively well defined, especially at near surface or surface conditions, the question remains how much of the iron is in a reduced oxidation state and how In the case of reduction of iron in surface environments if most of the iron goes to Fe " ", one mineral is formed if only part of it is reduced, another is formed. This is the fundamental geochemical aspect of the genesis of green clay minerals they contain iron in both oxidation states. 

Fe " then gets oxidized to Fe + in a further reaction. Any oxidation must be accompanied by a reduction taking place at the same time, but not necessarily at the same location. In fact, the electrons produced by the oxidation are consumed by the reduction of oxygen at a cathodic site elsewhere on the iron s surface. The reduction half-reaction is... 

Reduction plays a major role in the behavior of iron and manganese as well as in the behavior of pollutant chemicals in the environment. The solids Fe(OH)3 and Mn02 are strong adsorbents of many chemicals, especially metals. When particles of these oxides form in, or are transported into, surface waters, they can sorb metals the suspended particles may then be removed from the water column by settling, as previously discussed. Such a process may lead to either temporary or long-term deposition of the metals into the sediment. Retention in the sediment is only short term if iron reduction and manganese reduction subsequently lead to dissolution of the oxides and release of the metals back into the water column. 

Fig. 7.4 Measured iron concentrations in incubations (dots) and model (solid line) results of dissolved iron within surface ocean water. The artificial light intensity (dashed line) drives the photochemical reduction. The gradual decrease of dissolved iron is caused by the uptake by phytoplankton (adopted from Johnson et al. 1994).

Once a framework for the availability of iron oxides is established, the kinetics of individual reactions provides insight into reaction rates and rate limiting steps for the overall reactivity of iron. Here, the kinetics of microbial iron oxide reduction is explored and in section 7.4.4.1 analog information are provided for the reduction by sulfide and ligands. Building on previous experimental results demonstrating the control of mineral surface area for the degree of iron reduction (Roden and Zachara 1996 Fig. 7.13), it was shown, that also the rate of microbial iron reduction in natural sediments is of first-order and controlled by the mineral surface area (Roden and Wetzel... 

Bioturbation is an effective transport mechanism to replenish freshly precipitated, highly reactive iron oxyhydroxide from the sediment surface to the zone of iron reduction. Vice versa, reduced iron phases, e.g. FeS, is transported to the oxic / suboxic zone, where it becomes oxidized. 

Flux and composition of reactive iron input to surface sediments bioturbation competitive iron reduction pathways, e.g. by sulfide. 

Other microorganisms promote corrosion of iron and its alloys through dissimilatory iron reduction reactions that lead to the dissolution of protective iron oxide/hy dr oxide films on the metal surface. Passive layers are either lost or replaced by less stable films that allow further corrosion. Obuekwe and coworkers [60] evaluated corrosion of mild steel under conditions of simultaneous production of ferrous and sulfide ions by an iron-reducing bacterium. They reported extensive pitting when both processes were active. When only sulfide was produced, initial corrosion... 

In the absence of an oxide layer, it appears that reduction of RX involves primarily ET from Fe (i.e., equations 1 and 4). An oxide free iron metal surface can be achieved in electrochemically-controlled laboratory systems (9), and may apply where localized corrosion occurs at defects in the oxide surface layer (see section on The Oxide as a Physical Barrier). Under environmental conditions, however, the layer of iron oxides that covers the Fe surface will contain Therefore, the... 

---