Manganese complexes, oxidation-reduction

In well-aerated soil, it is expected that all species will be in their highest oxidation states. However, this does not happen for reasons elucidated in previous chapters. In well-aerated soil, both ferrous and ferric iron can exist along with elemental iron.3 Zinc, copper, and especially manganese can apparently exist in a mixture of oxidation states simultaneously in soil. Add to this a multitude of organic species that are also capable of oxidation-reduction reactions and the result is truly a complex voltammetric system [12,13],... 

The oxidation-reduction potentials and reaction constants of oxidation of iron and manganese differ and these reactions can occur in different amounts of oxygen. That is why the level of appearance of particulate manganese is situated higher than that of particulate iron [63]. Bacteria have been shown to oxidize manganese [64], whereas iron oxidation is possible without bacteria but can be carried out with bacteria [50]. Reduced iron can be oxidized by particulate manganese, forming complex compounds [65]. 

The present paper summarizes the oxidation-reduction chemistry of the gluconate complexes of Mn(II), Mn(III), and Mn(IV) in alkaline media. Electrochemical, spectrophotometric, and magnetic susceptibility measurements have been used to establish the formulas and chemical characteristics of the complexes. The oxidation-reduction chemistry for the manganese complexes formed by other ligands with polyhydroxyl functions also has been determined. 

The oxidation-reduction chemistry of the manganese-gluconate complexes can be expressed by Reactions 6-8. 

Table II. Oxidation—Reduction Chemistry of Manganese (II) Complexes...

Kinetic studies of the Fe(SOD) indicate a fairly simple oxidation-reduction cycle in which 02 is bound to the Fe(III) form of the protein and oxidized to 02, followed by binding of a second 02 to the resulting Fe(II) form and reduction to H202. The kinetics of Mn(SOD) are more complicated and the turnover number (1300 s-1 at 25 °C) is much lower than for Fe(SOD) (26,000 s-1) however, a similar catalytic cycle is believed to occur. The manganese enzyme kinetics are complicated by a side reaction to form a dead end complex, possibly a Mn(III) peroxide complex (61, 62). 

Since about 1983 there has been an increasing deluge of publications on manganese compounds that might provide models for—or at least provide some insight into— the role of Mn in photosynthesis and several enzyme systems. While we cannot attempt to discuss these complicated systems,44 for photosynthesis it appears that tetranuclear Mn complexes are involved that undergo oxidation/reduction cycles in photosystem II where the overall reduction involved is... 

Overall, this refined use of hydrogen concentrations supported the results of the bioassays and the complex system of redox zones inferred from the distribution of dissolved redox-sensitive species. The Grindsted Landfill (DK) plume is host to all of the proposed redox reactions, but also to secondary oxidation-reduction reactions involving ammonium, methane, manganese oxides, ferrous iron, and sulfides. 

---