Boron (B) and water
Boron and water: reaction mechanisms, environmental impact and health effects
|Seawater contains approximately 4-5 ppm boron. 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, for boron compounds may be the case. 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.
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.
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.
Boron naturally occurs mainly as boric acid and as boric acid salts. Boric acid can be removed by ion exchangers, but very slowly, because of its resemblance to silicate.