Reduction by Fe

The reductions by Fe(ll) of chloro(ethylenediaminetriacetatoacetate)cobaltate (III), Co(Y)CP , and its conjugate acid, Co(HY)CP, have been investigated by Pidcock and Higginson. At hydrogen-ion concentrations > 5x10 M... 

The stoichiometry of the reduction by Fe(ll) of cumene hydroperoxide is 1 1 (in contrast to reduction of H2O2) but the ratio A[Fe(II)]/A[ROOH] increases greatly in the presence of oxygen. The Arrhenius parameters for reduction of this and related hydroperoxides are quite similar to those of the Fenton reaction (Table 21). The production of acetophenone and ethane in high yield and the simple, second-order kinetics are consistent with the scheme... 

At 22 °C the overall second-order coefficient for reductions by Fe(n) and Cu(I) are, respectively, 4x10 and 5xl0 l.mole .sec Products in the presence of olefins demonstrate unequivocally the intermediacy of carbenoid transients, and a complex mechanism is put forward, viz. 

Chapelle, F. H. and D. R. Lovley, 1992, Competitive exclusion of sulfate reduction by Fe(III)-reducing bacteria a mechanism for producing discrete zones of high-iron ground water. Ground Water 30, 29-36. 

The midpoint reduction potential of cytochrome c and the kinetics of its reduction by Fe(EDTA) are also significantly influenced by substitutions at Phe-82. As measured by direct electrochemistry at pH 6 to eliminate any... 

Johnson TM, Bullen TD (2003) Selenium isotope fractionation during reduction by Fe(ll)-Fe(lll) hydroxide-sulfate (green rust). Geochim Cosmochim Acta 67 413-419 Johnson TM, Bullen TD, Zawislanski PT (2000) Selenium stable isotope ratios as indicators of sources and cycling of selenium Results from the northern reach of San Francisco Bay. Environ Sci Tech 34 ... 

Reductions by Cr are usually inner sphere and have a much higher rate constant with oxidants which can present potential chloride bridge. On this basis reductions by Fe are also inner sphere, whereas those of Fe (CN) and Ti(III) are outer sphere. 

Reduction by Fe(ll) results in an increase in the amount of iron oxides, which favor further reaction. Such autocatalytic behavior characterizes the oxidation of Fe(II) by and explains C Cl NO reduction by Fe(ll) in the absence of an iron mineral phase. Generalizing this behavior, it can be assumed that Fe(III) colloids derived from Fe(ll) oxidation in subsurface anoxic systems, together with other colloids, affect the environmental persistence of nitroaromatic contaminants. Colon et al. (2006), for example, elucidate factors controlling the transformation of nitrosobenzenes and N-hydroxylanilines, which are the two intermediate... 

Ru Ru step and a self-exchange rate of 2xlO" M s for the c -[Ru 0)2(L)] " /cw [Ru (0)2(L)]+ couple has been estimated a mechanism involving a pre-equilibrium protonation of ci5-[Ru (0)2(L)]+ followed by outer-sphere electron transfer is proposed for the Ru Ru step. For reduction by [Fe(H20)6] +, an outer-sphere mechanism is proposed for the first step and an inner-sphere mechanism is proposed for the second step. ... 

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). 

Experiments with sodium nitrate showed rapid reduction by Fe°, with nitrite as an intermediate and ammonia as final product. Iron acts as an electron donor and the reduction is coupled with metal corrosion (Equation 13.9). The reduction reaction in the model system was found to proceed in two sequential steps (Equation 13.30 and Equation 13.31), and the overall... 

Reduction by Fe° is a surface reaction, so reduction rates are most conveniently estimated from Equation 23 using surface-area normalized values of kobs (kSA). Representative values have been tabulated for a wide range of chlorinated solvents (161,162). The corresponding value for TCE is kSA = 3.9 x 10-4 L m 2 h 1 Thus, we can calculate a half-life of... 

Recently, research on nitro reduction by Fe° has been extended to environmental contaminants with multiple nitro groups, such as TNT and RDX [104-107], As expected, batch experiments show that TNT and RDX are rapidly reduced by Fe° to a complex mixture of products (Fig. 5). In contrast, column experiments with TNT have shown a very high capacity to... 

Like nitro and azo groups, the nitrosamine moiety is subject to reduction by Fe° and is present in some important environmental contaminants. One such contaminant is A-nitrosodimethylamine (NDMA), which is reduced by Fe° via a complex mechanism that gives the following overall reaction [114,115] ... 

Figure 8 Correction factors for the effect of temperature on the rate of reduction by Fe°. Arrows indicate the reference temperature around which most laboratory data are obtained (23 °C), and a more representative temperature for groundwater (15°C). Assuming EB 45 kJ mol-1 for TCE [160], the corresponding correction factor shown is 0.6 (i.e., rates will be slower in the field by 60%).

Figure 9 Comparison of previously reported values of kSA for reduction by Fe° with external mass transport coefficients estimated for batch, column, and rotating disk electrode reactors. References for the overall rate coefficients are given in Fig. 1 of Ref. 101. Mass transport coefficients were estimated for the batch and column reactors based on empirical correlations discussed in Refs. 125 and 101. Mass transport coefficients for the RDE were calculated using the Levich equation [178].

Because rates of reduction by Fe° vary considerably over the range of treatable contaminants, it is possible that there is a continuum of kinetic regimes from purely reaction controlled, to intermediate, to purely mass transport controlled. Fig. 9 illustrates the overlap of estimated mass transport coefficients (kmt) and measured rate coefficients (kSA). The values of kSA are, in most cases, similar to or slower than the kmi values estimated for batch and column reactors. The slower kSA values suggest that krxu < kml, and therefore removal of most contaminants by Fe° should be reaction limited or only slightly influenced by mass transport effects (i.e., an intermediate kinetic regime). 

However, information is still rare regarding the effects of ultraviolet fight on the zero-valent iron system. In the case of nitrate reduction by Fe(0), a detrimental effect of 254-nm irradiation on ferrous ion dissolution and ni-... 

There was no similar correlation between reactivity toward Fe(ll) and solvent exposure. FetSp and hCp exhibited similar rates of type 1 Cu(ll) reduction by Fe(ll) kohs > 1200s" ) while the rate with Co. cinereus Lac was >23s . In other words, laccases can use Fe(ll) as substrate but have no better than 1-2% of this activity in comparison to FetSp and hCp. In addition, they are at least 100-fold better than the ferroxidases in the turnover of bulky organic reductants. Combining the structure and reactivity features of these proteins indicates that the type 1 sites in the ferroxidases are less accessible to these large reductants and at the same time possess specificity elements that support the recognition and binding of Fe(II) as substrate. As outlined above, some of these elements may have been identified in hCp they remain uncharacterized in FetSp. 

The reaction of /r-superoxo complexes with reducing metal ions generally follows an outer sphere mechanism, and kinetic data have been reported for reduction by Fe and cobalt(II) chelates =>, Mo(V) ), [Ru(NH3)6] and... 

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