Manganese is the 12th most abundant element in the Earth's crust and constitutes approximately 0.1% of it. Chemical forms of manganese in their natural deposits include oxides, sulfides, carbonates, and silicates. Anthropogenic sources of manganese are predominantly from the manufacturing of steel, alloys, and iron products. Manganese is widely used as an oxidizing agent, as a component of fertilizers and fungicides, and in dry cell batteries. Methylcyclopentadienyl manganese tricarbonyl (MMT) improves combustion in boilers and motors and can substitute for lead in gasoline as an antiknock agent. Concentrations of manganese in groundwater normally range between 1 and 100 mgl-1, with most values being below 10 mgl—1 Typical airborne levels of manganese (in the absence of excessive pollution) range from 10 to 70ngm—3.
Manganese is a transition element located in group VIIA of the periodic table. It occurs in 11 oxidation states ranging from —3 to +7, with the physiologically most important valences being +2 and +3. The +2 valence is the predominant form in biological systems and is the form that is thought to be maximally absorbed. The +3 valence is the form in which manganese is primarily transported in biological systems.
The solution chemistry of manganese is relatively simple. The aquo-ion is resistant to oxidation in acidic or neutral solutions. It does not begin to hydrolyze until pH 10, and therefore free Mn2+ can be present in neutral solutions at relatively high concentrations. Divalent manganese is a 3d5 ion and typically forms high-spin complexes lacking crystal field stabilization energies. The previous properties, as well as a large ionic radius and small charge-to-radius ratio, result in manganese tending to form weak complexes compared with other first-row divalent ions, such as Ni2+ and Cu2+. Free Mn2+ has a strong isotropic electron paramagnetic resonance (EPR) signal that can be used to determine its concentration in the low micromolar range. Mn3+ is also critical in biological systems. For example, Mn3+ is the oxidative state of manganese in superoxide dismutase, is the form in which trans-ferrin binds manganese, and is probably the form of manganese that interacts with Fe3+. Given its smaller ionic radius, the chelation of Mn3+ in biological systems would be predicted to be more avid than that of Mn2+. Cycling between Mn3+ and Mn2+ has been suggested to be deleterious to biological systems because it can generate free radicals. However, at low concentrations Mn2+ can provide protection against free radicals, and it appears to be associated with their clearance rather than their production.
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