Micronutrients and Other Trace Elements

Abstract

The solubilities of most minerals from which trace metals in natural waters originate are favored by low pH. The concentration of the free ion of a dissolved trace element usually is much lower than is the total concentration of the trace element. This results from ion pair associations between the free trace ion and major ions, complex ion formation, hydrolysis of metal ions, and chelation of metal ions. Several trace elements---zinc, copper, iron, manganese, boron, fluorine, iodine, selenium, cadmium, cobalt, and molybdenum---are essential to plants, animals or both. A few other trace elements are suspected but not unequivocally proven to be essential. There are some reports of low micronutrient concentrations limiting the productivity of water bodies; but primary productivity in most water bodies apparently is not limited by a shortage of micronutrients. Trace elements---including the ones that are nutrients---may be toxic at high concentration to aquatic organisms. Excessive concentrations of several trace metals in drinking water also can be harmful to human health. Instances of trace element toxicity in aquatic animals and humans usually have resulted from anthropogenic pollution. Nevertheless, excessive concentrations of trace metals in drinking water sometimes occur naturally---an example is the presence of chronically-toxic concentrations of arsenic in groundwater that serves as the water supply for several million people in a few provinces of Bangladesh and adjoining India.