Chemically, gallium (Ga) is similar to aluminium. It is not attacked by air and does not react with water. When cold, gallium reacts with chlorine and bromine, and when heated, with iodine, oxygen and sulphur. There are 12 known artificial radioactive isotopes, with atomic weights between 64 and 74 and half-lives between 2.6 minutes and 77.9 hours. When gallium is dissolved in inorganic acids, salts are formed, which change into insoluble hydroxide Ga(OH)3 with amphoteric properties (i.e., both acidic and basic) when the pH is higher than 3. The three oxides of gallium are GaO, Ga2O and Ga2O3.
Occurrence and Uses
The richest source of gallium is the mineral germanite, a copper sulphide ore which may contain 0.5 to 0.7% gallium and is found in southwest Africa. It is also widely distributed in small amounts together with zinc blendes, in aluminium clays, feldspars, coal and in the ores of iron, manganese and chromium. On a relatively small scale, the metal, alloys, oxides and salts are used in industries such as machine construction (coatings, lubricants), instrument making (solders, washers, fillers), electronics and electrical equipment production (diodes, transistors, lasers, conductor coverings), and in vacuum technology.
In the chemical industries gallium and its compounds are used as catalysts. Gallium arsenide has been widely used for semiconductor applications including transistors, solar cells, lasers and microwave generation. Gallium arsenide is used in the production of optoelectronic devices and integrated circuits. Other applications include the use of 72Ga for the study of gallium interactions in the organism and 67Ga as a tumour-scanning agent. Because of the high affinity of macrophages of the lymphoreticular tissues for 67Ga, it can be used in the diagnosis of Hodgkin’s disease, Boeck’s sarcoid and lymphatic tuberculosis. Gallium scintography is a pulmonary imaging technique which can be used in conjunction with an initial chest radiograph to evaluate workers at risk of developing occupational lung disease.
Workers in the electronics industry using gallium arsenide may be exposed to hazardous substances such as arsenic and arsine. Inhalation exposures of dusts are possible during the production of the oxides and powdered salts (Ga2(SO4)3, Ga3Cl) and in the production and processing of monocrystals of semiconductor compounds. The splashing or spilling of the solutions of the metal and its salts may act on the skin or mucous membranes of workers. Grinding of gallium phosphide in water gives rise to considerable quantities of phosphine, requiring preventive measures. Gallium compounds may be ingested via soiled hands and by eating, drinking and smoking in workplaces.
Occupational diseases from gallium have not been described, except for a case report of a petechial rash followed by a radial neuritis after a short exposure to a small amount of fumes containing gallium fluoride. The biological action of the metal and its compounds has been studied experimentally. The toxicity of gallium and compounds depends upon the mode of entry into the body. When administered orally in rabbits over a long period of time (4 to 5 months), its action was insignificant and included disturbances in protein reactions and reduced enzyme activity. The low toxicity in this case is explained by the relatively inactive absorption of gallium in the digestive tract. In the stomach and intestines, compounds are formed which are either insoluble or difficult to absorb, such as metal gallates and hydroxides. The dust of the oxide, nitride and arsenide of gallium was generally toxic when introduced into the respiratory system (intratracheal injections in white rats), causing dystrophy of the liver and kidneys. In the lungs it caused inflammatory and sclerotic changes. One study concludes that exposing rats to gallium oxide particles at concentrations near the threshold limit value induces progressive lung damage that is similar to that induced by quartz. Gallium nitrate has a powerful caustic effect on the conjunctivae, cornea and skin. The high toxicity of the acetate, citrate and chloride of gallium was demonstrated by intraperitoneal injection, leading to death of animals from paralysis of the respiratory centre.
Safety and Health Measures
In order to avoid contamination of the atmosphere of workplaces by the dusts of gallium dioxide, nitride and semiconductor compounds, precautionary measures should include enclosure of dust-producing equipment and effective local exhaust ventilation (LEV). Personal protective measures during the production of gallium should prevent ingestion and contact of gallium compounds with the skin. Consequently, good personal hygiene and the use of personal protective equipment (PPE) are important. The US National Institute for Occupational Safety and Health (NIOSH) recommends control of worker exposure to gallium-arsenide by observing the recommended exposure limit for inorganic arsenic, and advises that concentration of gallium arsenide in air should be estimated by determining arsenic. Workers should be educated in possible hazards, and proper engineering controls should be installed during production of microelectronic devices where exposure to gallium arsenide is likely. In view of the toxicity of gallium and its compounds, as shown by experiments, all persons involved in work with these substances should undergo periodic medical examinations, during which special attention should be paid to the condition of the liver, kidneys, respiratory organs and skin.