Chemical toxicity of natural uranium

Uranium is a naturally occurring element, with an atomic number of 92 and density as high as 18 g/cc. Its radioactive nature was discovered in 1896 by A.H. Becqueral. Initially, it was used as a coloring agent in glass and as a homeopathic medicine. Subsequently, uranium is used as a fuel in nuclear power reactors. The uranium deposits are mined (mainly from Singhbhum area of Bihar) and the metal is produced by series of chemical and metallurgical operations.

Uranium is distributed widely in nature and is estimated to be present in the earth’s crust to the extent of about 4 parts per million. It is more abundant than many familiar elements such as gold, silver and mercury. Uranium content of sea water is 3 parts per billion. Traces of uranium are also found in both food and drinking water. The average daily intake of uranium through food and fluids is estimated to be 1.9 micro gram.

The chemical toxicity of uranium is more dominating when it is in water soluble compounds such as carbonates, nitrates, phosphates, fluorides, etc. One of the three oxides of uranium, i.e., UO3 is more soluble as compared to other insoluble oxides, viz., UO2 and U3O8. In soluble form, it can be transported by ground/surface water. Very small part of the ingested uranium is absorbed by the body which ultimately gets deposited in bone and kidney. While passing through kidney, uranium gets precipitated, thus increasing kidney burden. The assessed safe threshold for uranium in kidney is in the range of 1 to 3 microgram/g of kidney tissue. The quantity of uranium in blood that might produce a human fatality is 60 mg. The threshold limit value in blood for uranium induced proteinurea is 2.7 mg. Drinking water standards vary considerably and are reported in the range of 15 to 100 microgram uranium per liter of water.

To control the kidney burden, uranium should be kept in body fluids as a stable complex, such as bicarbonate complex, so that it is filterable in kidney and excreted through urine. That is the reason why sodium bicarbonate solution (1 to 1.5% in saline) is used for internal decontamination of uranium.