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The Elements in the Second Rank

4 Boron

4.1 Identity

Table 4.1
CAS No., EINECS No. and molecular weight for boron

4.2 Physico-chemical properties

Boron is not found in its elemental form in nature but it occurs commonly as boric acid (B(OH)₃), borate compounds such as B(OH)₄^- and Na₂O•2B₂O₃•10H₂O (sodium borate), boron halides (e.g. BCl₃) and boron hydrides as e.g. B₂H₆ [2, 13]. While boric acid and borates are water soluble, most boron halides and hydride hydrolyses with contact with water, forming boric acid. Boric acid in the aquatic environment is found as B(OH)₄^- . Metallic boron is not water soluble. Below selected physico-chemical data for metallic boron are shown (data from [1, 3]).

Table 4.2
Physico-chemical data for metallic boron

4.3 Uses and consumption

4.3.1 Uses

Boron is used in form of sodium borate as bleaching agent in detergents. On a weight basis, this is the most important use of boron [8, 14].

Enamel can be based on borates. These can also be the starting point in the formation of glass from supercooling of borate. The borates have also a capacity to dissolve oxides and can therefore be used as a flux [14].

Boron is used in low concentration in metal alloys (especially steel) acting as a hardener. In a wide range of products such as plastics, oil, fat and other materials where increased thermal conductivity are required, boron is used as additive [18]. It is especially the compounds triisopropyl borate (TIPB) and triphenyl boron (TPB), which are in focus as e.g. catalysts [19].

Boron trioxide (also named boron oxide) is used together with SiO₂ and Na₂CO₃ for preparation of heat resistant Pyrex glass and as flame retardant in paint.

Weak solutions of boric acid, B(OH)₃ are used for mouth and eye rinsing purposes [6].

Boron compounds are used as dietary supplement. It is believed to impede losses of calcium, phosphor and magnesium through the urine [16, 17].

The boron compounds are used in tanning of skin, in cosmetics, photographical materials, soaps and detergents. Certain pesticides and wood preservatives can contain boron compounds [15].

The layered structure of boric acid is utilised in lubricants. The compound boron nitride can form both graphite- and diamond like structures [14]. Boric acid and borax (Na₂B₄O₇) are used as combined flame retardant and biocide in some alternative building isolation materials (wool, paper and flax).

Metal alloys with boron have increased hardness and melting point and can therefore be used where such properties are required [14].

A new product based on boron is the so-called "Boron Nitride Nanotubes", which is used for surface treatment where a very hard surface is required such as the exterior of airplane windows [20]. Boron nitride can in many occasions replace fluoro-based polymers and is also used in paints [23, 24].

4.3.2 Consumption

The consumption of detergents in Denmark with and without bleaching agents is approximately 36,000 tons pr. year [8]. If assumably half of this amount contains 15% borate as bleaching agent, 2,700 tons of borate is consumed yearly. With a boron content of 18%, the boron consumption is approximately 500 tons.

4.4 Emissions to and occurrence in the environment

Boron exists naturally in the environment and is found in e.g. sea water as B(OH)₄^- in the mg/L-range.

The use of borate and boric acid in industry and households are the major emission sources, while only a small part of the boron emission comes from combustion of fossil fuels.

Table 4.3 shows the background concentration of boron in the aquatic and terrestrial environment.

Table 4.3
Typical background concentration of boron in the environment. Data from [2, 11]

A limited investigation of the levels of boron (and the other elements covered by this study) in the major emissions and waste streams in society was conducted in the autumn of 2001, see Table 4.4.

In Danish waste streams very high concentrations of boron are found in sludge and compost. Boron is also among the metals which are found in highest concentration in sediment from runoff retention basins, see Table 4.4. Boron concentration is low in treated waste water and leachate from landfills.

Table 4.4
Levels of boron in selected emissions and waste products from measurements conducted as part of this study in the autumn of 2001.

4.5 Danger classification

The boron compounds boron tribromide, boron trichloride and boron triflouride are listed on the Danish list of dangerous compounds and are classified as very toxic when inhaled and/or ingestion. Furthermore, these compounds are corrosive [10]. Inorganic and organic boron compounds are not regarded as carcinogenic. The boron compounds should be labelled with the risk sentence R14 (reacts violently with water).

Table 4.5
Classification of and risk phrases for boron compounds. Data from [10]

4.6 Toxicology

Chronic exposure to boron, boric acid and boron derivates in working and indoor environments from ingestion, uptake through skin or mucous membrane results in loss of appetite, loss of weight, vomiting, mild diarrhea, skin eruption and anaemia [25].

Investigations with mouse, rat and dog exposed to sodium borate and boric acid for prolonged periods show that males are more sensitive than females, and that the testicles are the target organ. Depending on the dose effects vary from minor impairment of spermatozoon formation to complete testicle atrophy. Congenital malformation has been observed in experimental animals [26].

No inorganic or organic boron compounds have been tested positive for carcinogenic effects.

4.7 Environmental properties

4.7.1 Environmental chemistry

Most boron compounds have relatively high water solubility and are regarded as mobile. In aquatic environments, boron is found primarily as B(OH)₄^- from which metal complexes are formed [2]. In soil, sorption of boron compound depends on soil pH, content of aluminum oxides and ion oxides, particle size, and content of organic material.

4.7.2 Environmental toxicology

Boron is an essential nutrient for plants, but can also be phytotoxic in high concentrations. The environmental toxicity is low, see Table 4.6. The data on acute aquatic toxicity of boron compounds stated as EC₅₀ shows, that adverse effects in aquatic organisms are not likely to occur under normal circumstances. In the terrestrial environment, the LC₅₀ is 10,000 ppm for quail and mallard.

Table 4.6
test results for environmental toxicity of boron. Data from [5, 12]

* LD₅₀ (mg/kg)

4.7.3 Bioaccumulation

Being an essential micronutrient, boron is assimilated by microorganisms and plants [2]. Boron is found in animals and plants and is accumulated in e.g. algae and plants, but food web magnification is not expected.

4.8 Conclusions

Boron is used in industry and households as e.g. bleaching agent in detergents. Boron halides are classified as toxic by ingestion and inhalation. Boron is found as B(OH)₄^- in aquatic environment. This compound has low acute toxicity on aquatic organisms.

4.9 References

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