December 27, 2009 by
Filed under mobile nuclear medicine
Boron is a non metallic element and the only non-metal of the group 13 of the periodic table of the elements. It is electron-deficient, possessing a vacant p-orbital. It has several forms, the most common of which is amorphous boron, a dark powder, unreactive to oxygen, water, acids and alkalis. It reacts with metals to form borides. At standard temperatures boron is a poor electrical conductor but becomes a good conductor at high temperatures. The most economically important compound of boron is sodium tetraborate decahydrate Na2B4O7 · 10H2O, or borax, used for insulating fiberglass and sodium perborate bleach. Boric acid is an important compound used in textile products. Compounds of boron are used in organic synthesis, in the manufacture of a particular type of glasses, and as wood preservatives. Boron filaments are used for advanced aerospace structures, due to their high-strength and light weight. An early use of borax was to make perborate, the beaching agent once widely used in household detergents. Boron compound also came into the average home in the guise of food preservatives, especially for margarine and fish.
Boron is not present in nature in elemental form. It is found combined in borax, boric acid, kernite, ulexite, colemanite and borates. Volcanic spring waters sometime contain boric acids. Boron is an element that occurs in the environment mainly through natural processes. Seawater contains approximately 4-5 ppm of this element. River water generally contains only 10 ppb. In seaweed 8-15 ppm and in mussels 4-5 ppm (dry mass) of boron was found. Boron dissolved in water occurs as B(OH)3 (aq) or B(OH)4- (aq). Under normal circumstances boron does not react with water. However, some boron compounds may do so. For example, the boron trifluoride ethyl ether complex reacts with water, forming diethyl ether- BF3, and releasing some highly flammable gases. A number of boron compounds, such as boron tri iodide, are hydrolysed in water. Boron salts are generally well water soluble. Boric acid has a water solubility of 57 g/L, borax of 25.2 g/L, and boron trioxide of 22 g/L. Boron trifluoride is the least water soluble boron compound, with a water solubility of 2.4 g/L. Some boron compounds, such as boron nitrite are completely water insoluble.
The most abundant minerals containing boron are kernite, borax, ulexite and colemanite. It can also be found in slate and in loam rich rock formations. Air-tight soil contains boron concentrations of between 5 and 80 ppm. Boron rich places, such as fumaroles, contain boric acid, borates and boron minerals. The degree of binding to clay minerals is mainly pH-dependent. Boron is released from rocks and soils through weathering, and subsequently ends up in water. In industry the pure element is rarely used, except for metal boride production, or to improve aluminum conductivity. Metal borides are processed to for instance turbines, rocket power, containers for high-temperature reactions, and electrodes. The hardness of steel is enhanced by adding boron.
Sodium perborate is applied as bleach in detergents. This eventually forms borate, which directly damages water plants. Borates are applied as water softeners. Other boron compounds are applied in glass, glass fibre, ceramic and email production. Glass is more solid and heat resistant when boron is added, and glass fibres are applied as insulation. Boron compounds may also be found in cleansing agents, batteries, illegal preservatives, and eye drops. The most important compounds in this respect are borax, boron oxide and boric acid. Boric acid and borax are added to fertilizers and pesticides in large amounts. The element is also present in impregnation and wood preservatives. It is applied as an abrasive as boron carbide and boron nitrite. Boron has a special function in a polymer matrix. It is applied to regulate nuclear reactors as a cooling agent in dangerous situations. It also absorbs neutrons in the reactor core. Boron often ends up in soil and groundwater through domestic landfills, when these are inadequately sealed. It serves as a typical indicator compound that indicates the presence of other hazardous substances.
It occurs naturally in the environment due to the release into air, soil and water through weathering. It may also occur in groundwater in very small amounts. Humans add boron by manufacturing glass, combusting coal, melting copper and through the addition of agricultural fertilizers. The concentrations of boron that are added by humans are smaller that the naturally added concentrations through natural weathering. Boron exposure through air and drinking water is not very likely to occur, but the risk of exposure to borate dust in the workplace does exist. Boron exposure may also occur from consumer products such as cosmetics and laundry products. Plants absorb boron from the ground and through plant-consuming animals it can end up in food chains. Boron has been found in animal tissue, but it is not likely to accumulate. When animals absorb large amounts of boron over a relatively long period of time through food or water the male reproductive organs will be affected. When animals are exposed to boron during pregnancy their offspring may suffer from birth defects or delayed development. Furthermore, animals are likely to suffer from nose irritation when they breathe in boron.
Boron is a dietary requirement for a number of organisms, and it plays an important role in mitosis. This applies to green algae, and some higher plant species. Boron deficiencies cause growth problems and difficulties in sugar mobilization. The boron compound that is absorbed most is boric acid. Plants contain 30-75 ppm of boron (dry mass). The toxic mechanism starts at concentrations exceeding 100 ppm. This may decrease crop yield. Grass species tolerate relatively high boron concentrations, but pine species are particularly susceptible. However, trees do require large amounts of boron compared to other plant species. A tolerable boron concentration in soils is approximately 25 ppm. High boron concentrations in water may be toxic to fish species, regarding concentrations of 10-300 mg/L. For water plants mainly borate is hazardous. Boron is not a dietary requirement for vertebrates. Boric acid is mildly water hazardous, but boron halogens are strongly water hazardous. Boron is averagely mobile and is transformed slowly. It may therefore spread rapidly through water. Boron consists of two stable and fourteen instable isotopes.
Humans can be exposed to boron through fruit and vegetables, water, air and consumer products. We have a regular daily intake of about 2 mg and about 18 mg in out body in total. When humans consume large amounts of boron-containing food, the boron concentrations in their bodies may rise to levels that can cause health problems. Boron can infect the stomach, liver, kidneys and brains and can eventually lead to death. When exposure to small amounts of boron takes place irritation of the nose, throat or eyes may occur. It takes 5 g of boric acid to make a person ill and 20 grams or more to put its life in danger. Eating fish or meat will not increase the boron concentrations in our bodies, as boron does not accumulate within the tissues of animals. The human body contains approximately 0.7 ppm of boron, an element that is not considered a dietary requirement. Still, we absorb this element from food stuffs, because it is a dietary requirement for plants. Daily intake is approximately 2 mg. The amount of boron present in fruits and vegetables is below the toxicity boundary.
At a daily intake of over 5 g of boric acid the human body is clearly negatively influenced, causing nausea, vomiting, diarrhoea and blood clotting. Amounts over 20 g are life threatening. Boric acid irritates the skin and eyes. Skin contact with boron trifluoride may cause corrosion. A possible correlation exists between the amount of boron in soils and drinking water, and the occurrence of arthritis among people. Both boric acid and borax are applied in medicine in certain amounts. Neutron absorbing characteristics of boron are applied in brain tumour treatment (boron neutron capture therapy).
Dr.Badruddin Khan teaches Chemistry in the University of Kashmir, Srinagar, India.His E.mail is:khanbudr@yahoo.co.in