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EC number: 234-522-7
CAS number: 12007-92-0
Boron represents an essential plant micronutrient with an average total
concentration of 10 mg/kg in the earth’s crust (Adriano, 2001).
Dissolution of B-bearing minerals (e.g. tourmaline, muscovite),
irrigation waters, fertilizers, atmospheric deposition of emitted B
(e.g. coal fly ash) as well as the soils’ buffer capacity, affect the B
concentration in soil. The natural level of B 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 B content compared with soils derived from
sedimentary rocks and those of arid and semiarid regions. The content of
total B 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
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 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
for grouping of different borate compounds
covers sodium pentaborate (EC# 234-522-7).
comparative purposes, exposures to borates are often expressed in terms
of boron (B) equivalents based on the fraction of boron in the source
substance on a molecular weight basis. As noted previously, only boric
acid and the borate anion are present at environmentally and
physiologically relevant concentrations. Read- across between the
different boron compounds can be done on the basis of boron (B)
equivalents. (See section on dissociation constants). Conversion factors
are given in the table below:
Conversion factors to boron equivalents
Conversion factor for equivalent dose of B (multiply by)
Disodium tetraborate anhydrous
Disodium tetraborate pentahydrate
Disodium tetraborate decahydrate
Disodium octaborate tetrahydrate
Sodium metaborate (anhydrous)
Sodium metaborate (dihydrate)
Sodium metaborate (tetrahydrate)
Sodium pentaborate (anhydrous)
Sodium pentaborate (pentahydrate)
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