Molybdenum is widely distributed in trace amounts in nature, occurring chie y as insoluble molybdenite (MoS2) and soluble molybdates (MoO42-). Molybdenum is relatively mobile in the environment because soluble compounds predominate at pH > 5. The solubility of molybdenum increases as redox potential is lowered. Below pH 5, adsorption and coprecipitation of the molybdate anion by hydrous oxides of iron and aluminum are effective at removing dissolved molybdenum. The weathering of igneous and sedimentary rocks (especially shales) is the main natural source of molybdenum to the aquatic environment.
Molybdenum metal is used in the manufacture of special steel alloys and electronic apparatus. Molybdenum salts are used in the manufacture of glass, ceramics, pigments, and fertilizers. The use of fertilizers containing molybdenum is the single most important anthropogenic input to the aquatic environment. Other contributions to the aquatic environment come from mining and milling of molybdenum, the use of molybdenum products, the mining and milling of some uranium and copper ores, and the burning of fossil fuels. Fresh water usually contains less than 1 mg/L molybdenum. Concentrations ranging between 0.03 and 10 |jg/L are typical of unpolluted waters. Levels as high as 1500 |j.g/L have been observed in rivers of industrial areas. The average concentration of molybdenum in nished drinking w ater is about 1 to 4 |J.g/L.
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