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Background on boron chemistry of environmental relevance

Boron is found almost exclusively 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 for practical purposes they can be regarded chemically inert.

 

Elemental Boron

A number of forms of elemental boron are described in the literature and various types are supplied as commercial products. These differ primarily in being either crystalline or amorphous and also having varying levels of impurities. Although highly pure crystalline boron is available, more common commercial forms, which are often amorphous, are typically somewhat impure, often containing 85-99% B, depending on grade. Indeed, highly pure elemental boron is difficult to prepare. The amount and identity of impurities in commercial boron products is dependent on manufacturing method. Boron products may contain significant amounts of magnesium or other metals, carbon and nitrogen, and generally substantial amounts of oxygen. The amount of water-soluble boron in these products is typically specified and may be 0.5% or more. This water-soluble boron represents for the most part water leachable boron oxide impurity.

Boron is a hard refractory and very water-insoluble material. Although reactions of with oxygen or water to form boron oxides are highly favoured thermodynamically, elemental boron is remarkably stable under normal environmental conditions. The reactivity of elemental boron depends on its form and purity. Crystalline boron is less reactive than amorphous boron (Laubengayer et al., 1943), but both forms are inert to air and water at room temperature. Detailed studies on the reaction of boron with oxygen and water show that boron does not react in bulk under normal atmospheric conditions at significant rates (Wang et al., 1993). Only at high temperatures do these reactions proceed to form boron oxide, which converts to boric acid in the presence of water (Wang et al., 1993; Foo et al., 1991). This indicates that the normal environmental conditions are not sufficiently energetic to surmount the activation energy barrier to conversion of elemental boron to the boric acid, making the rate of this conversion extremely slow. Discounting any leachable boron oxide impurities, elemental boron is far less soluble in water than common borate minerals, such as colemanite [CaB3O4(OH)3·H2O], which generally exhibit solubility on the order of 0.1%.

The inertness of elemental boron is attributed to the fact that boron is largely present in icosahedral B12-clusters, an arrangement that confers remarkable kinetic stability. The various types of amorphous and crystalline boron differ in the arrangement and overall order of these clusters, as well as the presence of interstitial boron or other elements that bond to and link B12-clusters into networks, but boron in these materials is predominantly tied up in these stable icosahedral structures. Although boron reacts only superficially with oxygen under normal conditions, particles of elemental boron processed under atmospheric conditions may possess a very thin passivating boron oxide surface layer that lends resistance to further oxidation of the bulk of the material (Greenwood and Earnshaw, 1997).

 

Ferro boron

The term ferro boron refers to alloys of iron with boron. As an alloy, ferro boron has a variable composition. Many forms of ferro boron are offered commercially. Common commercial products contain 17-25% boron and often contain various other non-ferrous constituents, including carbon, aluminium, silicon, phosphorus, and sulphur. These other constituents may be impurities resulting from the manufacturing process or intentionally added components to enhance performance in particular applications. A typical analysis is 18% B, 0.50% C max, 0.50% Si max, 0.2% Al max, 0.003% P max and 0.01% S max.

Ferro boron is produced by either a carbothermic or aluminothermic process. Carbothermic production involves the reaction of boric acid, carbon and iron in an electric arc furnace. Aluminothermic production involves the reaction of boric acid, iron ore, aluminium and sometimes magnesium, in a ladle.

Ferro boron is used in the production of steel, cast iron, neodymium-iron-boron magnets and in amorphous metals (9). Ferro boron is also used in some processes to “boronize” metal surfaces to add abrasion and corrosion resistance

Additions of 0.001-0.003% boron to unalloyed and low-alloyed steels raise hardenability. Ferro boron can also be added to carbon steels for automotive strip stock as a nitrogen scavenger. As boron has a high neutron absorption capability, 0.5-1.0% B is added in the form of ferro boron to some stainless steels used in the nuclear industry (Roskill Information Services, 2010).

Ferro boron dissolves in mineral acids, but is stable to air and water. Upon prolonged exposure to moist air commercial ferroboron changes in appearance from bright metallic to dull gray with rusty-like stains (Baudis and Fichte, 1995). This tarnishing is indicative of some degree of surface oxidations, but aside from superficial reactivity ferro boron displays similar stability to other ferro alloys.

 

Summary

Both ferro boron and elemental boron are inert materials under environmental conditions. Fundamental chemistry considerations indicate that these materials will ultimately decompose with formation of boric acid, the common form that boron takes in the natural world, but the rates at which this occurs is very slow (geological scale) and beyond relevance for REACH. Typical boron minerals, such as colemanite, are substantially more soluble and release boron, as boric acid, at higher rates than elemental boron or ferro boron.