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Environmental fate & pathways

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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 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 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).

Justification for grouping of different borate compounds

This report covers sodium pentaborate (EC# 234-522-7).

For 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

Substance

Formula

Conversion factor for equivalent dose of B (multiply by)

Boric acid

H3BO3

0.1748

Boric Oxide

B2O3

0.311

Disodium tetraborate anhydrous

Na2B4O7

0.2149

Disodium tetraborate pentahydrate

Na2B4O7.5H2O

0.1484

Disodium tetraborate decahydrate

Na2B4O7.10H2O

0.1134

Disodium octaborate tetrahydrate

Na2B8O13·4H2O 

0.2096

Sodium metaborate (anhydrous)

NaBO2

0.1643

Sodium metaborate (dihydrate)

NaBO2·2H2O

0.1062

Sodium metaborate (tetrahydrate)

NaBO2·4H2O

0.0784

Sodium pentaborate (anhydrous)

NaB5O8

0.2636

Sodium pentaborate (pentahydrate)

NaB5O8∙5H2O

0.1832