Boron

Abstract Boron is the fifth element of the periodic table and the only electron‐deficient nonmetallic element. Consequently, boron has a high affinity for electronegative atoms such as oxygen and forms strong covalent boron–oxygen bonds in compounds known as borates . The boron atoms can bond to oxygen to form either planar trigonal BO 3 or negatively charged tetrahedral BO 4 − units. Essentially all boron‐containing minerals are inorganic borate salts containing a mixture of trigonal and tetrahedral boron atoms. These naturally occurring borates are converted to a large variety of commercial boron‐containing products, 98% of which are also borates. Boron in elemental form is an expensive refractory material that is produced by the costly reduction of borates, and used in relatively small quantities. In this chapter this material is referred to as elemental boron . The term boron alone is used in a generic sense here to mean boron as it is included in any chemical combination in boron‐containing materials. The inorganic boron‐containing materials share many chemical and biological characteristics thought to be due to unique properties of the boron element. For this reason, this chapter deals rather extensively with these shared characteristics first. Following this, individual inorganic boron‐containing compounds that have a significant use in industry are considered and to the extent that they have unique attributes, are discussed here. Borates are ubiquitous in nature in the form of a variety of borate minerals. Yet, in relatively few locations have they been found in quantities sufficiently concentrated to mine commercially. Borate minerals are typically found in nature in low concentrations as alkali‐metal and alkaline‐earth borate and borosilicate minerals, and, very rarely, as boric acid. The element boron has been known to be an essential nutrient in vascular plants since the mid‐1920s and is commonly used in fertilizer applications. It is also generally accepted to be essential for diatoms and marine algal flagellates. More recently, several reports have been published on the essentiality of boron in animals. It has been reported to be essential in the frog, Xenopus laevis , for reproduction and development. Embryos from frogs cultured in low boron environments showed increased necrosis and poor viability. The ovaries and testes of adult frogs maintained on low boron for 120 days were distinctly atrophied. In fish (trout and zebra fish) development and retinal health was reported to be adversely affected. Embryonic trout growth is stimulated in a dose‐dependent manner. Most zebra fish embryos did not survive the early blastula development stage in a low boron environment. Those that did survive were reported to be photophobic and were shown to have retinal abnormalities. Studies are currently under way in mice and rats. Similar to the frog and fish studies, mice embryos from the low boron exposure were affected at a very early development stage as shown by reduced blastula cell counts. Boron stimulates growth in vitamin D–deficient chicks, including modulation of cartilage calcification and positive affects on the growth plate of one‐day‐old chicks. Numerous reports from the USDA Grand Forks Human Nutrition Research Center in North Dakota have indicated a nutritional role for boron in humans and animals. Positive benefits from nutritional boron have been reported for bone health, cell membrane function, psychomotor skills and cognitive processes of attention and memory, response to estrogen therapy, control of inflammatory disease, enzyme regulation, energy metabolism, and macroscale mineral metabolism. A World Health Organization (WHO) expert committee has concluded that boron is “probably essential”. Since boron is a required nutrient in plants, it occurs naturally in all foods. Fruits, vegetables, and nuts contain the highest levels. The mean dietary boron consumption in the United States is reported to be 1.17 mg/day for adult males and 0.96 mg/day for adult females. Boron occurs naturally as a mixture of the stable isotopes, boron‐10 and boron‐11, in a ratio of approximately 4:1 and an atomic weight of approximately 10.81. Slight variations in this ratio are observed depending on the source of the original boron mineral, but these variations are too small to affect normal analytical determinations. Some of the more important physical and chemical properties of boron compounds of commercial significance and widespread exposure are listed. These compounds are listed in decreasing order of exposure, but the order is approximate, since in some cases the available information on production and usage is incomplete. Although inorganic borates represent by far the most important class of commercial compounds, the total list includes a number of examples with diverse chemical, physical, and toxicological properties.