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The Elements in the Second Rank

5 Gallium

5.1 Identity

Table 5.1
CAS No., EINECS No and molecular weight for gallium

5.2 Physico-chemical properties

Gallium is in the same group in the periodic table as boron and aluminum and can have the valences +2 (Ga(II)) and +3 (Ga(III)), though +3 is the normal valence. It is a heavy metal among the transition elements cadmium and lead. Gallium is one of few metals with a melting point close to room temperature.

Table 5.2
Physico-chemical data for metallic gallium. Data from [1, 3]

5.3 Uses and consumption

5.3.1 Uses

Gallium is used mostly in the electronics industry. It is used together with indium and arsenic in e.g. photo-voltaic cells for automation. These are grouped according to their composition, e.g.: InGaAs and HgCdTe [10]. The consumption of gallium is expected to increase as the used of the classical heavy metals is phased out. This applies especially for mercury, which in many cases can be substituted with gallium [13, 18].

Gallium (together with arsenic or as gallium phosphide) is used in solar cells, transistors, semi conductors, laser equipment (especially violet and blue laser), light emitting transistors, photoelectric cells, DVD's, CD's, and for electronic cooling [6, 12-15, 17-20]. An expansion of the electronics industry is assumed to account for the major part of any increment in consumption in the coming years.

Gallium is used in metal alloys for e.g. dental purposes [13, 18].

5.3.2 Consumption

The yearly consumption in Denmark is assumed to be small. Based on the per capita consumption in USA, the consumption is estimated to be about 0.15-0.24 tons/year.

Table 5.3
The relative distribution of the gallium consumption in USA and the consumption in Denmark in 1997, based on [6]

5.4 Emissions to and occurrence in the environment

The background concentration of gallium in the aquatic environment is low. In sediment and soil, the concentration of gallium is up to 100 mg/kg. Coal and oil contain gallium in concentrations of 1-35 mg/kg and 0.01-1.2 mg/kg, respectively, and combustion of fossil fuels therefore contributes to the emission of gallium to the environment. A Swedish study showed that the gallium concentration in ash from incineration of coal was between 4 and 7 mg/kg [6]. Despite the increment in the use of gallium the last 10 to 20 years, it is estimated that natural weathering of gallium still accounts for a considerable part of the total gallium emission [8].

Table 5.4
Typical background concentration of gallium in the environment. Data from [8]

A limited investigation of the levels of gallium (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 5.5.

In Danish waste streams, gallium was found mainly in compost and sewage sludge and sediment from road runoff retention basins. A low gallium concentration was found in stack gas, cleaned waste water, and landfill leachate, se Table 5.5.

Table 5.5
Levels of gallium in selected emissions and waste products from measurements conducted as part of this study in the autumn of 2001.

5.5 Danger classification

Gallium or inorganic gallium compounds are not on the list of dangerous compounds [7].

5.6 Toxicology

Low gallium assimilation can be observed from the gastrointestinal tract and the toxicity from ingestion is considered to be low. In animal experiments gallium caused kidney injury and had toxic effects on muscle nerves. Blindness and paralysis has been reported in rats and aplastic changes have been reported in dogs. The assimilation pattern was not stated but is assumed to be intravenous. No additional data regarding toxicology of gallium or effects related to exposure from working environment was found [22]. Data regarding the use of different gallium compounds for therapeutic purposes are available but not directly applicable to describe gallium toxicity.

5.7 Environmental properties

5.7.1 Environmental chemistry

Gallium is biologically reactive and forms stable complexes with nitrogen and sulphur groups in living organic material. In the aquatic environment organisms such as algae can assimilate gallium. As for nutrients, gallium is assimilated and released again with the decomposition of dead organic material. However, a fraction of gallium will bind to other particulate matter.

Gallium is found as Ga(OH)₄^- in fresh and sea water. The solubility is strongly dependent on the pH-value. Ga(OH)₄^- is highly soluble at low pH-values while the solubility is low a neutral and high pH-values.

Compared to aluminum, which also forms low soluble hydroxides, gallium will to a lesser degree be present in acidic environments [6].

5.7.2 Environmental toxicology

The toxicity of GaCl₃ on rainbow trout is the only test result found regarding the environmental toxicity of gallium. With a LC₅₀-value of 3.5 mg/L for a 28 days test, it is regarded as toxic to aquatic organisms [5]. According to [6], gallium is somewhat more toxic than zinc.

Table 5.6
Test results for environmental toxicity. Data from [5]

5.7.3 Bioaccumulation

It is not known if gallium is assimilated in different organisms and whether it can accumulate in the food web.

5.8 Conclusions

The use of gallium has increased in the last decades, mainly due to applications in the electronics industry. Weathering accounts compared to anthropogenic emission for a considerable part of the total gallium emission to the environment. The increased use of gallium has resulted in an increase of gallium in the waste streams. A certain amount of the total emission comes from fossil fuel combustion. The available data regarding fate and effect of gallium in the environment is not sufficient to allow a thorough evaluation. It is assumed that gallium is less toxic than the heavy metals cadmium and lead, but more toxic than zinc.

5.9 References

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