Boron in drinking-water
Background document for development of WHO Guidelines for Drinking-water Quality
General description
Identity
Boron (CAS no. 7440-42-8) is never found in the elemental form in nature. It exists as a mixture of the 10B (19.78%) and 11B (80.22%) isotopes (Budavari et al., 1989). Boron's chemistry is complex and resembles that of silicon (Cotton & Wilkinson, 1988).
Physicochemical properties
Elemental boron exists as a solid at room temperature, either as black monoclinic crystals or as a yellow or brown amorphous powder when impure. The amorphous and crystalline forms of boron have specific gravities of 2.37 and 2.34, respectively. Boron is a relatively inert metalloid except when in contact with strong oxidizing agents.
Sodium perborates are persalts, which are hydrolytically unstable because they contain characteristic boron–oxygen–oxygen bonds that react with water to form hydrogen peroxide and stable sodium metaborate (NaBO2·nH2O) . Boric acid is a very weak acid, with a pKa of 9.15, and therefore boric acid and the sodium borates exist predominantly as undissociated boric acid [B(OH)3] in dilute aqueous solution at pH <7; at pH >10, the metaborate anion B(OH)4- becomes the main species in solution. Between these two pH values, from about 6 to 11, and at high concentration (>0.025 mol/litre), highly water soluble polyborate ions such as B3O3(OH)4-, B4O5(OH)4-, and B5O6(OH)4- are formed.
The chemical and toxicological properties of borax pentahydrate Na2B4O7·5H2O, borax Na2B4O7·10H2O, boric acid, and other borates are expected to be similar on a molar boron equivalent basis when dissolved in water or biological fluids at the same pH and low concentration.
Major uses
Boric acid and borates are used in glass manufacture (fibreglass, borosilicate glass, enamel, frit, and glaze), soaps and detergents, flame retardants, and neutron absorbers for nuclear installations. Boric acid, borates, and perborates have been used in mild antiseptics, cosmetics, pharmaceuticals (as pH buffers), boron neutron capture therapy (for cancer treatment), pesticides, and agricultural fertilizers.
Environmental fate
Waterborne boron may be adsorbed by soils and sediments. Adsorption–desorption reactions are expected to be the only significant mechanism influencing the fate of boron in water (Rai et al., 1986). The extent of boron adsorption depends on the pH of the water and the concentration of boron in solution. The greatest adsorption is generally observed at pH 7.5–9.0 (Waggott, 1969; Keren & Mezuman, 1981; Keren et al., 1981).
In natural waters, boron exists primarily as undissociated boric acid with some borate ions. As a group, the boron–oxygen compounds are sufficiently soluble in water to achieve the levels that have been observed (Sprague, 1972). Mance et al. (1988) described boron as a significant constituent of seawater, with an average boron concentration of 4.5 mg/kg.