1. Occurrence and Significance
Boron (B) is the first element in Group IIIA of the periodic table. It has an atomic number of 5, an atomic weight of 10.81, and a valence of 3. The average abundance of B in the earth’s crust is 9 ppm. In soils it is 18 to 63 ppm, in streams it is 10 μg/L, and in groundwaters it is 0.01 to 10 mg/L. The most important mineral is borax, which is used in the preparation of heat-resistant glasses, detergents, porcelain enamels, fertilizers, and fiberglass.
The most common form of boron in natural waters is H3BO3. Although boron is an element essential for plant growth, in excess of 2.0 mg/L in irrigation water, it is deleterious to certain plants and some plants may be affected adversely by concentrations as low as 1.0 mg/L (or even less in commercial greenhouses). Drinking waters rarely contain more than 1 mg/L B and generally less than 0.1 mg/L, concentrations are considered innocuous for human consumption. Seawater contains approximately 5 mg/L B, and this element is found in saline estuaries in association with other seawater salts.
The ingestion of large amounts of boron can affect the central nervous system. Protracted ingestion may result in a clinical syndrome known as borism.