Mn2+, oxidation

Morgan (11) derived a rate law that adequately describes the observed kinetics and was able to extract rate constants for both homogeneous and particle-catalyzed reactions. In laboratory experiments with sterile, filtered synthetic solutions, Mn2+ oxidation proceeds much more slowly than in natural waters. It may not occur at all at neutral or acidic pH, especially in the absence of catalytically active surfaces such as preformed oxidation products (12, 13). 

In spite of the high similarity between MnP and VP oxidation sites, crystal structures of wild and recombinant VP, and kinetics of its mutated variants, revealed some significant differences. These result in efficient Mn2+ oxidation in the presence of only two of the three residues forming the binding site [9, 61]. Some MnP residues, which are not conserved in VP, could be responsible for these... 

A few years ago, a different LiP-type peroxidase was reported in the white-rot basidiomycete Trametes cervina. This enzyme has an exposed tyrosine residue that seems to be involved in catalysis [73]. Curiously, this catalytic tyrosine occupies the equivalent position to that of an aspartate residue forming the Mn2+ oxidation site in MnP and VP. Additional work is necessary to confirm the catalytic role of this amino acid residue. 

Quantifying Reactions 7.64 and 7.65 requires fixing the pH and partial pressure of 02 (p02) at some predetermined value and providing OH" upon demand. This is accomplished with a pH-stat technique. The technique utilizes a pH electrode as a sensor so that as OH" is consumed, (during Mn2+ oxidation), the instrument measures the rate of OH consumption and activates the autoburete to replace the consumed OH . It is assumed that for each OH" consumed, an equivalent amount of Mn2+ is oxidized. 

The data in Figure 7.15 demonstrate kinetics of Mn2+ oxidation using the pH-stat technique. The data show at least two major slopes. The first slope (near the origin) represents Reaction 7.64, whereas the second slope represents the autocatalytic part of the reaction (Reaction 7.65). The data demonstrate that the reaction is pH-dependent. As pH increases, the autocatalytic part of the reaction represents the mechanism by which most Mn2+ oxidizes. Similar reactions for Fe2+ are shown in Figure 7.16. Note that Fe2+ oxidizes at a much lower pH than Mn2+. 

Figure 7.15. Pseudo first-order Mn2+ oxidation under various pH values at pC>2 of 0.2 in duplicate using a pH-stat technique (from Evangelou, unpublished data).

Adams, L.F. and Ghiorse, W.C., Characterization of extracellular Mn2+-oxidizing activity and isolation of an Mn2+-oxidizing protein from Leptothrix discophora SS-1, J. Bacteriol., 169, 1279, 1987. 

Brouwers, G.J. et al., Stimulation of Mn2+ oxidation in Leptothrix discophora SS-1 by Cu2+ and sequence analysis of the region flanking the gene encoding putative multicopper oxidase mofA, Geomicrobiol. J., 17, 25, 2000a. 

Figure 1. Electron free energy levels calculated for the approximate pH of the oxic-anoxic interface of the Black Sea (pH 7.75). Dissolved species other than H are assumed to have unit activity. The strongest oxidants are at the top, and the strongest reductants are at the bottom. Such diagrams are a simple way to evaluate the feasibility of redox reactions. For example, ammonia and Mn2+ oxidation by nitrate may be feasible, but the actual free energy available will depend on the in situ concentrations at the site of reaction. All such reactions are, most likely, mediated by bacteria. The vertical separation of the different oxidants from organic matter (CH20) is proportional to the energy available from the different respiration reactions (1).

Mn oxidation and scavenging in plumes appear to be dominated by microbial activity (see review in Winn etal., 1995). Plume bacteria are often characterised by capsules of abundant extracellular polymer matrices that are believed to scavenge Mn from solution (e.g. Cowen etal., 1986, 1999). Radiotracer experiments (54Mn uptake) and elevated microbial biomass at plume depths suggest that microbial metabolic activity enhances Mn scavenging (Cowen etal., 1986). However, it is not clear if this activity includes active Mn2+ oxidation or is simply due to non-enzymatic processes. 

Mariganese-containing catalases have been isolated from three species of bacteria Lactobacillus plantarum [27], Thermus ihemtophUus [28], and Thermoleophilum album [18]. X-ray crystallographic structure analysis [29] has shown that these catalases contain a dinudear manganese core. During catalysis, the dinudear manganese active site cydes between the Mn"- and Mn2"oxidation states [30]. 

It is evident that the abrupt change of the potential in the neighbourhood of the equivalence point is dependent upon the standard potentials of the two oxidation-reduction systems that are involved, and therefore upon the equilibrium constant of the reaction it is independent of the concentrations unless these are extremely small. The change in redox potential for a number of typical oxidation-reduction systems is exhibited graphically in Fig. 10.15. For the MnO, Mn2+ system and others which are dependent upon the pH of the... 

The nitrogen source in the medium is the amino add glutamate. There are several cations K Mn2, Cn2, Zn2, Mg2, Co2, Fe2, Ca2 Mo6. Phosphate (POi") is the major anionic component. Fumaric add is a TCA cycle intermediate and may improve metabolic balance through the catabolic pathways and oxidation through the TCA cyde. Peptone may improve growth through the provision of growth factors (amino acids, vitamins, nudeotides). 

The composition of lithium-manga-nese-oxide spinel electrodes that are of interest for lithium battery applications fall within the Li[Mn2]04 - Li4Mn5Ot2 -Li2[Mn4]0() tie-triangle of the Li-Mn-0... 

If Li[Mn2]04 is heated above -780 °C, oxygen-deficient spinels LiMn204 (S <0.14) are produced [132, 144, 145]. The loss of oxygen lowers the manganese oxidation state below 3.5 and triggers a mild Jahn-Teller distortion the da ratio in the tetragonal LiMn204 (7 phase varies between 1.02 and 1.07. 

K.12 Which do you expect to be the stronger oxidizing agent Explain your reasoning, (a) KBrO or KBr03 (b) Mn04 or Mn2+. 

Use data from Appendix 2B to determine whether chlorine gas will oxidize Mn2+ to form the permanganate ion in an acidic solution. 

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