Mn oxidation rates

It is interesting to compare the expected rates of Mn oxidation via abiotic mechanisms with the rates expected from the biological kinetic rate law described above. Abiotic Mn oxidation rates at pH 8.03 were measured in seawater by von Langen et al. (1997) who reported a first-order rate constant of l.lxlO-6 (normalized for Po2 = 1 atm and T = 25°C). At this pH and for similar conditions, the cell concentration of L. discophora required to obtain the same rate would be only 0.30 pg/1 (Zhang et al., 2002) (i.e., approximately 3x10s cells/1). It is reasonable to assume that cell populations of Mn-oxidizing bacteria far greater than this would be possible in natural environments. Even smaller population sizes would be required to match abiotic rates (if they could be measured) at lower pH values. 

Figure 4. Calculated Mn oxidation rates in the layer of bottom water at depths between 81 and 86 m (see eq 2). The slightly negative values during the first quarter of the year are due to an overestimation of the resuspension flux.

In April 1991 a flux of 0.07 mmol of Mn(II) per m2 per day from the deeper sediment was obtained from a well-resolved gradient. A very similar profile was observed 1 year later. On the other hand, the diffusive flux of 0.46 mmol/m2 per day (June 1990) was clearly too low to sustain Mn oxidation rates of up to 3 mmol/m2 per day during summer. The 15-mm resolution of the dialysis samplers probably was too coarse to determine such gradients reliably. 

Summer and fall profiles at NWC are fit quite well by the model. The winter profile is not this may reflect an increase in Mn oxidation rate... 

Mn flux different from that controlled by production alone. For example, during winter the surface sediment is more highly oxygenated than at other times of year because of lowered microbial activity. As a result, Mn oxidation rates near the interface should be relatively high at that time compared to production rates. This change in the relative rates of consumption-production reactions would result in a greater decrease of the flux from the sediment than could be explained by the temperature dependence of production. 

The rate of this reaction in acidic condition is higher than inorganic reaction rate but not different in neutral-alkaline conditions (Fig. 6.13). Inorganic and biological oxidation rate is different for Fe and Mn. For instance, if Fe " and Mn " oxidation rates in HCOs solution are compared, it is found that Mn " does not oxidize less than pH 9, while Fe " oxidizes rapidly less than pH 6.6 (Stumm and Morgan 1970). 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

The oxidation of other monocarboxylic acids by both Cr(VI) and Mn(VII) is slow. Mare and RoCek examined the effect of COjH groups on the oxidation rates of methine and methylene groups. With a series of dicarboxylic acids... 

Arsenite can be oxidized by manganese dioxides in soils. The rate constants for the depletion of As(III) by bimessite and cryptomelane are much higher than those by pyrolusite due to the difference in the crystallinity and specific surfaces of the Mn oxides (Oscarson et al., 1983). The ability of the Mn dioxides to sorb As(III) and As(V) is related to the specific surface and the point-of-zero charge of the oxides. The one-to-one relationship between the amount of As(III) depleted and the amount of As(V) appearing in solution was reported by Oscarson and colleagues (1983). 

Still higher oxidation rates are achieved when hydrocarbons are oxidized in the presence of the catalytic system including the Co, Mn, and Br- ions. The Mn-Br binary system is less... 

When variable-valence metals are used as catalysts in the oxidation of hydrocarbons, the chain termination via such reactions manifests itself later in the process. This case has specially been studied in relation to the oxidation of paraffins to fatty acids in the presence of the K Mn catalyst [57], which ensures a high oxidation rate and a high selectivity of formation of the target product (carboxylic acids). As the reaction occurs, alcohols are accumulated in the reaction mixture, and their oxidation is accompanied by the formation of hydroxyperoxyl radicals. The more extensively the oxidation occurs, the higher the concentration of alcohols in the oxidized paraffin, and, hence, the higher is the kinetic... 

Mn(II) oxidation is enhanced in the presence of lepidocrocite (y-FeOOH). The oxidation of Mn(II) on y-FeOOH can be understood in terms of the coupling of surface coordination processes and redox reactions on the surface. Ca2+, Mg2+, Cl, S042-, phosphate, silicate, salicylate, and phthalate affect Mn(II) oxidation in the presence of y-FeOOH. These effects can be explained in terms of the influence these ions have on the binding of Mn(II) species to the surface. Extrapolation of the laboratory results to the conditions prevailing in natural waters predicts that the factors which most influence Mn(II) oxidation rates are pH, temperature, the amount of surface, ionic strength, and Mg2+ and Cl" concentrations. 

If, where the other ions are present, the Mn(II) oxidation rate is still described by equation 7 (or 8) then the psuedo-first order... 

In freshwater, Mn(II) oxidation is slightly slower than in 0.1M NaClO. The difference between the Mn(II) oxidation rate in freshwater and 0.1M NaCIO, is greatest at pH 8.5, at this pH the rate of Mn(II) oxidation is only 40% lower in the freshwater than in 0.1M NaClO. In the estuarine-water at pH 8.5 the rate of Mn(II) oxidation is 20 times slower than in 0.1M NaCIO,. The speciation calculations indicate why the model predicts the oxidation is slower than in natural waters (see, for example Table VII). 

The rates of Mn(II) removal in some natural waters are similar to the Mn(II) oxidation rates predicted on the basis of these laboratory studies. However, in other cases the rate of manganese removal in natural waters is much faster than that expected on the basis of this work. In these systems significant manganese removal may occur as the result of adsorption, bacterially mediated oxidation, or biological uptake. 

The oxidation rates of XOH were measured for the PVP complexes of the transition metal ions of the 4th series, i.e., Cr, Mn, Fe, Co, Ni, Cu and Zn ion. As can be seen in Fig. 2 (a), the Cu complexes exhibit the highest activity and the activity of the PVP-Cu catalyst is higher than that of the monomeric pyridine-Cu catalyst. To this Cu complex, equivalent amount of the second metal component was added i.e., the PVP-Cu, secondary metal ion mixed complexes were prepared. The activities of these mixed complexes are summarized in Fig. 2 (b). One notices that Mn ion increases the catalytic activity of the Cu ion although Cr and Fe ion inhibit the catalytic activity. Another important result in Fig. 2 (b) is that the effect of secondary metal ion is more clearly observed in the PVP system, comparing to the monomeric pyridine catalysts. 

The slowest growth rates are found in the Fe-Mn oxides that have formed predominantly by precipitation of solutes from seawater, being on the order of 1 to a few millimeters per million years. Because of slow formation rates, these hydrogenous precipitates tend to form only in areas where sedimentation rates are slow, such as the abyssal plains of the mid-Pacific Ocean, or where bottom currents are strong enough to prevent sediment accumulation, such as on submarine seamounts and plateaus. 

The Fe-Mn oxides that form from the diagenetic remobilization of sedimentary metals accrete at fester rates, on the order of himdreds of millimeters per million years. 

The observation that nodules grow at widely varying rates provides further support for the existence of multiple formation mechanisms. The nodules that accrete most slowly (1 mm per million years) appear to have formed primarily by the process of hydrogenous precipitation. This accretion rate is equivalent to the annual deposition of a layer that is only one atom deep. These slow rates cause a significant amount of metal-rich seawater to become occluded between the Fe-Mn oxide layers. 

The cycles of reduction and oxidation of Fe and Mn oxides in intermittently submerged soils provide opportunities for co-precipitation with trace metals. In most natural systems it is the rate of dissolution of the sohd phase that limits solid solntion formation rather than thermodynamics, so conditions in snbmerged soils are highly conducive to formation of solid solntions. 

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