Potassium triboron pentaoxide

Endpoint summary

Additional information

Boron is an essential plant micronutrient with an average total concentration of 10 mg/kg in the earth’s crust (Adriano, 2001). Dissolution of boron-bearing minerals (e.g. tourmaline, muscovite), irrigation waters, fertilizers, atmospheric deposition of emitted boron (e.g. coal fly ash) as well as the soil’s buffering capacity, affect the boron concentration in soil. The natural level of boron in soils largely depends upon the soil parent material. In general, soils derived from igneous rocks and those of tropical and semitropical regions of the world are considerably lower in boron content compared with soils derived from sedimentary rocks and those of arid and semiarid regions. The content of total boron in the latter group may range up to 200 mg/kg, particularly in alkaline, calcareous soils, while that for the former group is usually lower than 10 mg/kg (Swaine, 1955, cited in Adriano, 2001).

The oceans are the largest global reservoir for boron with a global average concentration of about 4.6 mg B/L (Argust, 1998, Park and Schlesinger, 2002). However, boron may range in concentration from 0.52 mg B/L in the Baltic Sea to 9.6 mg B/L in the Mediterranean Sea (Argust, 1998).

Natural events such as generation of seasalt aerosols over the ocean, biomass burning, rock weathering and volcanic activity are estimated to release 2 x 10^9 kg B/year (Park and Schlesinger, 2002) Formation of seasalt aerosols and their transfer to land represents the largest flux of boron from the sea to the terrestrial environment, estimated as 1.44 x 10^9 kg B/yr by Park and Schlesinger (2002). They estimate riverine transfer to the oceans to be about 0.58 Tg B/yr.

Most anthropogenic releases of boron to the environment are from global coal combustion, estimated as 2 x 10^8 kg B/yr (Park and Schlesinger, 2002). Boron produced from mining is estimated to be about 3.1 x 10^8 kg B/yr (Argust, 1998) with about half the processed boron being used in products that are unlikely to release boron to the environment (glass, fiberglass and ceramics) (Park and Schlesinger, 2002).

Most anthropogenic boron (excluding coal-related materials) in Europe originates from mines in Turkey and California. Ratios of the boron isotopes ¹¹B and ¹⁰B provide a tool to distinguish locally-derived boron from anthropogenic boron, although this has not been widely done (Vengosh et al., 1994; Chatelet and Gaillardet, 2005). ¹¹B separates preferentially into dissolved boron (i.e. boric acid), whereas ¹⁰B is preferentially incorporated into the solid phase (Vengosh et al., 1994). The boron-11 isotope enrichment value (identified as δ¹¹B) ranges from about 39‰ in seawater, to about 0‰ in average continental crust, to -0.9 to +10.2‰ in sodium borate minerals from Turkey and California (Vengosh et al., 1994). The ratio has been used to identify anthropogenic boron fractions in surface waters (Chatelet and Gaillardet, 2005) and groundwaters (Vengosh et al., 1994, Kloppmann et al, 2005).