1. Occurrence and Significance
Manganese (Mn) is the first element in Group VIIB in the periodic table. It has an atomic number of 25, an atomic weight of 54.94, and common valences of 2, 4, and 7 (and more rarely, valences of 1, 3, 5, and 6). The average abundance of Mn in the earth’s crust is 1060 ppm; in soils it is 61 to 1010 ppm; in streams it is 7 μg/L, and in groundwaters it is <0.1 mg/L. Manganese is associated with iron minerals, and occurs in nodules in ocean, fresh waters, and soils. The common ores are pyrolusite (MnO2) and psilomelane. Manganese is used in steel alloys, batteries, and food additives.
The common aqueous species are reduced Mn2+ and oxidized Mn4+. The aqueous chemistry of manganese is similar to that of iron. Since groundwater is often anoxic, any soluble manganese in groundwater is usually in the reduced state (Mn2+). Upon exposure to air or other oxidants, groundwater containing manganese usually will precipitate black MnO2. Elevated manganese levels therefore can cause stains in plumbing, laundry, and cooking utensils. It is considered an essential trace element for plants and animals. The United Nations Food and Agriculture Organization recommended maximum level for manganese in irrigation waters is 0.2 mg/L. The U.S. EPA secondary drinking water standard MCL is 50 μg/L.