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Environmental fate & pathways

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Background on boron chemistry of environmental relevance

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 substances. As a result of the high relative stability of boron oxides compared to other boron compounds they are the thermodynamically favoured decomposition 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.

Inorganic borates (e.g., boric acid, sodium tetraborates) are soluble in water. The chemical species present in solution depend on concentration and pH.

Only two soluble boron species in ordinary soils can be expected (Adriano, 2001). The nonionized species, [B(OH)3], is the predominant species expected in soil solution. Boric acid, [B(OH)3], is a very weak, monobasic acid that acts as a Lewis acid by accepting a hydroxyl ion to form the borate anion, [B(OH)4]-. At pH greater than 9.2, [B(OH)4]- becomes predominant.

B(OH)3 + 2H2O ↔ [B(OH)4]- + H3O+              pKa = 9.2

In the pH range of 7 to 11, both species can be found. With higher boron concentrations (B > 0.025 M) and increasing pH also polymeric boron forms can also precipitate, which are usually very rare and unstable in soils. Therefore, boric acid and borate ions are the predominant B-forms in the natural soil system (Power and Woods, 1997; de Vette et al., 2001).

Boron as a natural element is not degradable. However, boron and its inorganic compounds are subject to chemical transformation processes (adsorption, complexation, precipitation, fixation) once released to the environment. One consequence of the transformation is that the mobility/bioavailability and the potential for toxicity, induced by the borate species, is changed and in many cases reduced or even removed over time. Thus, these natural processes achieve a similar result as is sought in the demonstration of biotic and abiotic degradation of synthetic organic chemicals.

Please also refer to the read-across statement attached to section 13.