Reactive iron

Scheme 10.5 Tentative mechanism for cytochrome P450-cata-lyzed epoxidation of a double bond. The reactive iron-oxo species VII (see Scheme 10.4) reacts with the olefin to give a charge transfer (CT) complex. This complex then resolves into the epoxide either through a radical or through a cationic intermediate.

Mass spectrometry is a useful tool to detect the existence of reactive iron-imido intermediates. In intramolecular aromatic aminations, Que and coworkers used electrospray ionization mass spectrometry to show the presence of a molecular ion at m/z 590.3 and 621.2, which could be attributed to the formation of [(6-(o-TsN-C6H4)-TPA)Fe ]+ and [(6-(o-TsN-C6H4)-TPA)Fe° OMe)]+. With the isoto-... 

In a study directed to the analysis of the role of Fe and the generation of H2O2 in Escherichia coli (McCormick et al. 1998), hydroxyl radicals were specihcally trapped by reaction with ethanol to give the a-hydroxyethyl radical. This formed a stable adduct with a-(4-pyridyl-l-oxide)-iV-t-butyl nitroxide that was not formed either by superoxide or hydroxyl radicals. The role of redox-reactive iron is to use EPR to analyze the EPR-detectable ascorbyl radicals. 

Once a moderately reactive iron cluster activates the first hydrogen molecule, it rapidly reacts to adsorb more hydrogen, eventually reaching a saturation level. The saturation studies by... 

Finally, it would appear that these highly reactive iron powders will be of value as a general reducing agent. Reaction of nitrobenzene with four equivalents of iron and one equivalent of n-butanol in THF at room temperature was very exothermic. After one hour at room temperature followed by reflux for one hour, the reaction mixture gave 88% aniline upon workup. 

Aniline has also been obtained (93%) from a homogenous reduction with preformed tetraethylammonium hydridotri-iron undecacarbonyl [10], In this reduction, the yield is comparable, or superior, to that obtained when the more reactive iron pentacarbonyl or di-iron nonacarbonyl complex is used in the absence of a phase-transfer catalyst. 

Note that plots, such as Figure 5.1, provide information only on the net outcome of chemical reactions. In the case of iron, a small addition does take place in estuaries as a result of desorption of Fe from the surfaces of riverine particles. As these solids move through the estuarine salinity gradient, the major cation concentrations increase and effectively displace the iron ions from the particle surfeces. Since this release of iron is much smaller than the removal processes, the net effect is a chemical removal of iron. Sedimentation of these iron-enriched particles serves to trap within estuaries most of the riverine transport of reactive iron, thereby preventing its entry into the oceans. 

Canfield, D.E. (1989) Reactive iron in marine sediments. Geochim. Cosmochim. Acta 53 619-632... 

The reactive iron required for the process costs approximately 375 to 450 per ton (D12778Q, p. 88 D213354, p. 5). Because the EnviroMetal technology is a patented process, a licensing fee (approximately 15% of capital costs) may also be required (D20317Y, p. 29). Capital cost information for several in situ EnviroMetal Process installations is summarized in Table 1. 

Availability of reactive iron in sediments also has been postulated to control S retention. Reactive iron may limit fluxes of S recycled from sediments by rendering sulfide immobile and less amenable to oxidation by bacteria or chemical agents. Availability of iron strongly influences total S content and isotopic signature of marine sediments (198). Canfield (94) ob-... 

Three other lines of evidence may support the hypothesis that availability of reactive iron limits S retention. 

The degree of pyritization of iron (DOP) or the fraction of reactive iron that is bound with sulfur appears to increase rapidly with increasing S Fe ratios and may approach an asymptotic value of about 75% at S Fe ratios of 3 (Figure 6). 

Figure 6. The degree of pyritization, defined as the fraction of reactive iron present as pyrite, is a measure of the extent to which available iron has reacted with sulfur (226). In lake sediments, iron monosulfides frequently are as abundant as pyrite and hence were included with pyrite in the values calculated for surface sediments from 13 lakes and presented here. Even this correction neglects Fe(II) that may have been reduced by sulfide but may be present as siderite. Availability of iron appears to be more important than bottom-water oxygenation in determining the degree of pyritization. In the right-hand graph, darkened squares represent sediments known to experience seasonal anoxia only the uppermost point experiences permanent anoxia. (Data are from references 30, 34, 56, and 61.)...

Recently we presented (23) the results of an experimental study on the kinetics and mechanisms of the reaction of lepidocrocite (y-FeOOH) with H2S. With respect to the interaction between iron and sulfur, lepidocrocite merits special attention. It forms by reoxidation of ferrous iron under cir-cumneutral pH conditions (24), and it can therefore be classified as a reactive iron oxide (19). The concept of reactive iron was established by Canfield (19), who differentiated between a residual iron fraction and a reactive iron fraction (operationally defined as soluble in ammonium oxalate). The reactive iron fraction is rapidly reduced by sulfide or by microorganisms. 

In summary, the reaction of H2S with y-FeOOH is a fast surface-controlled process. Equations 8 and 9 can be used to estimate an upper limit of sulfide oxidation rates in sediments with reactive iron (assuming reactive iron to be represented by lepidocrocite). The surface-area concentration A of reactive iron can be calculated according to... 

Reactions 12a and 12b consume dissolved sulfide. This fact fits nicely with the data of White et al. (35), who could not detect free sulfide in their study. Dissolved sulfide is frequently absent in freshwater sediments (e.g., 39 see Urban, Chapter 10, for a discussion). This lack of sulfide is explained by an excess of reactive iron over the total sulfide concentration (19, 40). 

FeS can still be found in such environments because of an insufficient supply of an oxidant to react with sulfide. This insufficiency may happen when either no sulfide-reactive iron exists or the rate of reoxidation of (mi-... 

The CRS AVS ratio reflects the trophic state of a lake (cf. Urban (28), Table V). This observation may be explained by the preceding kinetic considerations. The high organic matter supply in eutrophic lakes leads to an intensive mineralization rate by both iron- and sulfate-reducing bacteria. However, reoxidation of Fe2+ to ferric oxide or of sulfide to sulfate does not take place because an anoxic hypolimnion prevents penetration of oxygen. Therefore FeS can build up, but the sediment becomes depleted with respect to reactive iron. 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

On the other hand, a permanent supply of ferric oxides to the sediments is provided by sedimentation of allochthonous material. It is unknown to what extent these oxides are reactive with respect to sulfide or whether a predigestion of ferric oxides by bacteria is needed. Various studies indicate that 50% of freshwater sediment iron exists as iron oxide and 20% of the iron is reactive (72). Future studies should be directed to a better understanding of the existence of reactive iron. 

In addition to a better understanding of the reaction of sulfide with ferric oxides and its role in pyrite formation, a more exact definition of the term reactive iron is critical. Does reactive iron mean a different iron oxide fraction for bacterial dissolution (e.g., weathering products such as goethite or hematite) than for reaction with sulfide (e.g., reoxidized lepidocrocite) In other words, is there a predigestion of ferric oxides by bacteria that allows a subsequent rapid interaction of sulfide with ferric oxides ... 

The sterols that were chosen as substrates contained two double bonds, one at various positions in the side chain and A5 in the steroid nucleus. Whereas the latter double bond was never touched in reactions with the Fe(III) porphyrin vesicle system 183 in the presence of PhIO, the side chain double bonds of desmosterol 186 and fucosterol 187 were epoxidized to 188 and 189 in 32% and 22% yield, respectively (Fig. 31). In contrast, stigmasterol 190 was not reactive, since the double bond cannot approach the reactive iron-oxo intermediate. 

The kinetics of oxidation of aldehydes by the Fenton reagent [Fe(II)-H202-0H-] have been studied.89 It has been suggested that different reactivities of PhIO in iron(III)-porphyrin-catalysed alkene epoxidation may be due to the formation of a more reactive iron(IV)-0-IPh complex.90 The iron(m) complex of tetrakis(3,5-disulfonato-mesityl)porphyrin catalyses the oxidative degradation of 2,4,6-trichlorophenol to 2,6-dichloro-l,4-benzoquinone with KHSO5 as the oxygen atom donor a peroxidase-type oxidation is thought to be involved.91... 

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