Iron formation, rate

In the M. trichosporium OB3b system, a third intermediate, T, with kmax at 325 nm (e = 6000 M-1cm 1) was observed in the presence of the substrate nitrobenzene (70). This species was assigned as the product, 4-nitrophenol, bound to the dinuclear iron site, and its absorption was attributed primarily to the 4-nitrophenol moiety. No analogous intermediate was found with the M. capsulatus (Bath) system in the presence of nitrobenzene. For both systems, addition of methane accelerated the rate of disappearance of the optical spectrum of Q (k > 0.065 s-1) without appreciatively affecting its formation rate constant (51, 70). In the absence of substrate, Q decayed slowly (k 0.065 s-1). This decay may be accompanied by oxidation of a protein side chain. 

Temperature-Programmed EXAFS/ XANES Characterization of the Impact of Cu and Alkali Promoters to Iron-Based Catalysts on the Carbide Formation Rate... 

Fig. 8.39. Chemo-spectrophotometric evolution of the solar neighbourhood (left) and the whole Milky Way (right) as a function of time. Panels aA show the oxygen and iron abundances, bB the mass of stars and gas and the star formation rate, cC the extinction in B, V and K bands along a line of sight normal to the plane, dD the luminosity in solar units (taking extinction into account), eE the colour indices and fF the supernova rates. Note that panels aA are in linear units (see Fig. 8.16), while the others are all logarithmic. After Boissier and Prantzos (1999).

Although under chemolithoautotrophic growth conditions, cell densities of only 3-5 x 10 cells per milliliter were observed, the specific rate of Fe(III) reduced per cell unit was about 10 times faster than what had been published for any other Fe(lll) reducer. This strengthens the hypothesis that microbially mediated Fe(III) reduction by obligately anaerobic thermophiles could have been an important process on early Earth, when elevated temperatures were predominant (Baross 1998 Kashefi and Lovley 2000), which includes the involvement in the formation of specific Banded Iron Formations. In light of the properties of the above Fe(lll) reducers, the theories on the origin and biogeochemistry of Banded Iron Formations should be revisited. 

An equilibrium and kinetic study of the iron(II) phthalocyanine/nitric oxide system in DMSO, at 293 K, showed that formation of [Fe(pc)(NO)] obeys a simple second-order rate law, like [Fe(pc)] plus CO but unlike [Fe(pc)] plus dioxygen. A rate constant for dissociation of [Fe(pc)(NO)] was derived from its formation rate and equilibrium constants. " ... 

This parameter is nevertheless constrained by the relationship between the age and iron content of stars, the observed rate of supernova events (of the order of three per century), which is related to the current star formation rate, and the present gaseous fraction. After 10 billion years of evolution, the region of the Galaxy accessible to us, that is, the solar neighbourhood, still retains about 10% gas. 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Finally, all reactions may occur, and no simplification of Eq. (7.114) is possible. Both the slope of the plot and the intercept will be potential dependent and the latter will be greater than 1/A. These possibilities are all portrayed in Fig. 7.53. From such results the conclusions drawn for oxygen reduction on a pure Fe surface were that on the bare iron, the rate-determining step involves the formation of Oj, while on the passive layer it is oxygen chemisorption under Temkin conditions. 

The formation of pyrite in sediments depends on the availability of three parameters iron, sulfate, and organic matter (48). Although organic matter content controls the formation rate under marine sulfate-rich conditions, sulfate concentration is usually regarded as the limiting factor under fresh-... 

Iron solubility is an obvious factor in Fenton oxidation because the rate of hydroxyl radical formation is directly proportional to [Fe2+]. At elevated pH values, iron hydroxides and oxides form and precipitate, causing a dramatic decrease in hydroxyl radical formation rate. Iron chelators can be used to offset this factor. A related issue is the rate of Fe3 + reduction to Fe2+, which, if insufficient, can result in Fe2+ concentrations that are too low to... 

As previously discussed, the concentration of Fe2+ is an important factor in the rate of hydroxyl radical formation from hydrogen peroxide. Consequently, any process that can speed the reduction of Fe3+ to Fe2+ will increase the formation rate of hydroxyl radical. UV or visible radiation can play this role by photoreducing iron. However, the photo-Fenton process involves three additional mechanisms that can contribute to pollutant degradation (a) direct photolysis of H202 to yield two hydroxyl radicals (Eq. (22)) (b) photolysis of Fe(OH)2+ to form hydroxyl radical (Eq. (23)) and (c) degradation of pollutants by direct photolysis (i.e., absorption of a photon by the pollutant molecule followed by decomposition of the photoexcited pollutant molecule). 

Complexes of Fe(II), Cu(II) and Zn(II) with 1,10-phenanthroline (phen) and its hydrophobic derivatives exhibited remarkable interfacial adsorptivities, although the ligands themselves could be hardly adsorbed at the interface [40-42]. The extraction rate profiles of Fe(II) with phen and its dimethyl (DMP) and diphenyl (DPP) derivatives into chloroform were investigated using the HSS method [43]. Both the formation rate of the phen-derivative complex and the interfacial adsorptivity of the complex were remarkably dependent on the hydrophobicity of the ligand and the hydration tendency of anions. The initial extraction rate of the iron(II) complex is described by the following equation ... 

Opportunities for such secondary reactions certainly existed in the history of meteorites. Temperatures in the nebula (360-400 K, Table 1) may alone have been high enough for secondary reactions in the time available, 10 -10 yr. Kinetic studies of a similar reaction (formation of benzene from alcohols, amines, or fatty acids on Fe Oj or iron-rich peat catalysts Galwey, 1972) indicate a benzene formation rate of 5 x 10 molecules g yr at 360 K. At this rate it would take only 5000 years to transform all the meteoritic carbon to benzene. Further opportunities were provided by brief thermal pulses during chondrule formation, impact, or transient shocks. Of couree, any high-temperature episodes must have happened early or on a local scale, to permit survival of other, more temperature-sensitive compounds. 

The half-hfe of Fe in the solutions from which the Griquatown iron formation was deposited was obviously many orders of magnitude longer than this, and it is useful to inquire into the cause for the difference. Stumm and Lee (1961) have shown that the rate of oxidation of Fe in aqueous solutions is governed by the equation... 

The time, t, required for the passage of water across the Campbellrand platform is uncertain. The modern Bahamas are probably a reasonable analogue for the Campbellrand platform. On the Bahama Banks tidal currents of 25 cm s are common, and velocities of 1 m s have been recorded in channels (SeUwood, 1986). At a rate of 25 cm s it would have taken seawater —1 month (4 X 10" min) to traverse the —800 km diameter of the Campbellrand platform. This period is much longer than the time required to precipitate Fe oxyhydroxide after the oxidation of Fe to Fe (Grundl and Delwiche, 1993). The best estimate of the residence time of seawater on the Grand BahamaBankis —1 yr(Morseefa/., 1984 Millero, personal communication). The pH of the solutions from which the iron formations were deposited was probably less than that of seawater today, but probably not lower than 7.0. 

Stone and Tilley have shown that the spectral changes which occur on spinel formation can be related to the extent of reaction of the component oxides, although the kinetics were also followed by chemical analysis. Keyser et have studied the reaction of zirconium silicate with calcium oxide by a radio tracer method. Wagenblast and Damask have used internal friction measurements to study the rate of precipitation of carbon in iron. The rate of decrease of the Snoek peak can be related to the growth kinetics of the iron carbon precipi-... 

Figure 12. Comparison of carbon formation rates for iron foils from proposed model and from experiments.

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