Manganese-catalyzed water oxidation

Calcium, chloride and manganese are inorganic cofactors that are required for photosynthetic water oxidation. While manganese serves as the redox agent which catalyzes water oxidation, the biologcal roles of calcium and chloride have not been determined. 

An obvious approach to metal-catalyzed water oxidation is to mimic the stmcture and function of the OEC in PSII (Fig. 5). This species is based on a cubane stmcture with three manganese centers, four bridging oxo hgands, and one calcium cation on its vertices. An extra manganese center is oxo-bridged to one Mn-O edge of the cubane and aU cationic metal... 

Aqueous potassium permanganate solutions are not perfectiy thermodynamically stable at 25°C, because Mn02, not MnO is the thermodynamically stable form of manganese in water. Thus permanganate tends to oxidize water with the evolution of oxygen and the deposition of manganese dioxide, which acts to further catalyze the reaction. 

Some microorganisms can catalyze certain oxido-reduction reactions like the oxidation of iron and manganese in water, the oxidation of sulfur compounds, and oxidation-reduction of nitrogen compounds. Aerobic autotrophic bacteria of the type Thiobacillus can release soluble iron, copper, and sulfuric acid as sulphates into water. These organisms can be found everywhere in nature wherever an acidic environment is maintained in the presence of sulfide-containing minerals. 

Membranes can also be oxidized in the presence of iron, manganese, and other metals. These metals catalyze the oxidation of RO membranes. This type of oxidation tends to involve the entire RO skid rather than focus on the lead membranes. Again, when this type of degradation occurs, feed water passes into the permeate, resulting in an increase in permeate flow and a decrease in product quality. 

It is difficult to estimate what the rate of the metal ion catalyzed oxidative deamination reaction of amino acids would be in natural waters. Hamilton and Revesz (30) found that the rate of oxidation of alanine in the presence of pyridoxal and manganese ions was inhibited by EDTA. Since metal ions in natural waters can be complexed by a variety of organic and inorganic compounds, their effectiveness in catalyzing the oxidative deamination of amino acids may be reduced. Also, the fraction of dissolved amino acids which would be complexed by metal ions at the pH and metal ion and amino acid concentrations found in natural waters must be considered. At neutral pH, where the amino group of the amino acid is protonated, the fraction of the amino acid that would be in the form of the metal ion complex depends upon the equilibrium constant for the formation of the complex and the pK of the amino proton of the amino acid. The reactions for the formation of the Cu2+-alanine complexes can be written as... 

The system Ru(III)-EDTA catalyzes the oxidation of cyclohexane by molecular oxygen [26]. In the presence of manganese(II) acetate and molecular oxygen, alkenes and active methylene compounds react to yield cyclic peroxides [27]. a-Substituted cycloalkanones are oxidized to oxo acids by the copper(II) nitrate-dioxygen-acetic acid-water system [28a] ... 

The oxides of manganese also catalyze the oxidation of Mn to Mn Zapffe (256) found that the oxidation of manganese in a water treatment plant was speeded up, following aeration, by the catalytic infiuence of pyrolusite ()8-Mn02) and also by in situ precipitated manganese oxide. Hem (94) noted a catalytic effect of feldspar on the Mn to Mn reaction rate. It seems likely that this was caused by minute amounts of manganese oxide impurities on the feldspar surfaces. 

Figure 2. Manganese activity in solution as a function of pH for (1) equilibrium between Mn Oj ic) and yMnOOH(c) (2) disproportionation of Mn Oj where Mn O and 8Mn02 are present at unit activity (3) sur-face-catalyzed direct oxidation of Mn to Mnfi by aqueous in aerated water (4) condition of equal affinity for Reactions 2 and 3 to proceed...

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