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Environmental fate & pathways

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Boron is almost exclusively found in the environment in the form of boron-oxygen compounds, which are often referred to as borates. The high strength of the B-O bond relative to those between boron and other elements makes boron oxide compounds stable compared to nearly all non-oxide boron materials. Indeed, the B-O bond is among the strongest found in the chemistry of naturally occurring inorganic substances. A trivial exception occurs with some rare boron fluoride minerals, as the B-F bond is even stronger.

As a result of the high relative stability of boron oxides compared to other boron compounds they are the thermodynamicallyfavoureddecomposition products. This is an inescapable outcome of the laws of thermodynamics. Although virtually all boron compounds ultimately decompose under environmental conditions to the thermodynamically most stable state represented by boric acid, many boron compounds exhibit high kinetic stability and decompose extremely slowly under environmental conditions - in some cases so slowly that they can be regarded chemically inert for practical purposes.

In aqueous solution, the equilibrium distribution of B(OH)3and B(OH)4- is known to be strongly pH dependent, such that at values higher than 8.6 borate ion dominates while at lower pH boric acid is the dominant species (Hershey JP et al., 1986).

In seawater (pH=8.2) borate ion comprises +/- 28.5% of boron species (assuming the dissociation constant of boric acid pKB=8.597 (at 25 °C)), representing ca. 6% of seawater alkalinity (Dickson AG, 1990).

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 sea salt 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 sea salt 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 asδ 11B) 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 boric oxide (CAS#1303-86-2).

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 Table xx below.

Table xx: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