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Uranium in the environment, this page is devoted to the science of uranium in the environment and in animals (including humans). The legal and social issues associated with uranium are discussed elsewhere. This page is a subpage of actinides in the environment.
Occurrence
From the land
Uranium is a naturally occurring element found in low levels within all rock, soil, and water. This is the highest-numbered element to be found naturally in significant quantities on earth. According to the United Nations the normal concentration of uranium in soil is 300 μg kg
Main article: Uranium miningIt is considered to be more plentiful than antimony, beryllium, cadmium, gold, mercury, silver, or tungsten and is about as abundant as arsenic or molybdenum. It is found in many minerals including uraninite (most common uranium ore), autunite, uranophane, torbernite, and coffinite. Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores (it is recovered commercially from these sources).
From the sea
Seawater contains about 3.3 parts per billion of uranium by weight (3.3 µg/kg)http://www.webelements.com/webelements/elements/text/U/geol.html] as uranium(VI) forms soluble carbonate complexes. The extraction of uranium from seawater has been considered as a means of obtaining the element.
Sources
Note that uranium is present in most soils at a low concentration, so the mere fact that a soil contains uranium does not mean that it has been artificially contaminated by uranium. While it is possible to use the isotope signature to identify the origin of uranium in a sample, for instance
Metal
Munitions
See also Depleted uranium#Safety and environmental issues and Depleted uranium#Health Concerns
The potential danger of exposure to depleted uranium has received widespread publicity because of the use of DU munitions in the 1991 Gulf War and 1999 Kosovo War, as well as current conflicts . "A total of just over 290 metric tons of DU projectiles were fired by the US during the Gulf War (comapred to 9 tons in Kosovo and 3 tons in Bosnia and Herzegovina)." p68 UNEP study, 2003, The potential long-term effect on people living in areas where DU munitions were used, has also caused some concern.
Fragments from projectile DU munitions could cause substantial inhalation exposure risks in certain circumstances if the exposure was high. [http://www.globalsecurity.org/military/systems/munitions/du.htm associated with a number of health concerns, many of which are controversial.
Air crashes
Uranium metal, as depleted uranium, has been used in aircraft for trim weights in the past (although the practice has been discontinued), so after an air crash a release of uranium or its combustion products is possible.
Dispersion of uranium metal
- Uranium metal can disperse into the air and water, United Nations Environment Programme (UNEP) study says in part:
- "The most important concern is the potential for future groundwater contamination by corroding penetrators (ammunition tips made out of DU). The munition tips recovered by the UNEP team had already decreased in mass by 10-15% in this way. This rapid corrosion speed underlines the importance of monitoring the water quality at the DU sites on an annual basis."
Combustion
Studies of depleted uranium aerosol exposure suggest that uranium combustion product particles would quickly settle out of the air and thus could not affect populations more than a few kilometers from target areas.
The U.S. has admitted that there have been over 100 "friendly fire" incidents in which members of the U.S. military have been struck by DU munitions, and that an unknown number have been exposed to DU via inhalation of combustion products from burning DU munitions.
Corrosion
It has been reported that the corrosion of uranium in a silica rich aqueous solution forms both uranium dioxide and uranium trioxide.
In pure water, schoepite {(UO2)8O2(OH)12.12(H2O)} is formed in the first week and then after four months studtite {(UO2)O2·4(H2O)} was formed. A report on the corrosion of uranium metal has been published by the Royal Society.
Uranium metal reacts with water to form hydrogen gas, this reaction forms uranium dioxide and 2 to 9% uranium hydride. It is important to note that the rate of corrosion due to water is far greater than that caused by oxygen at tempertures around 100 °C. At pH values below 2 the corrosion rate at 100 °C goes down greatly, while as pH values go from 7 upwards the corrosion rate declines. Gamma irradation has little effect on the corrosion rate. M. McD. Baker, L. N. Less, S. Orman, Trans. Faraday Soc., 1966, 2513-2524 DOI: 10.1039/TF9666202513.
Oxygen gas inhibits the corrosion of uranium by water. M. McD. Baker, L. N. Less and S. Orman, Transactions of the Faraday Society, 1966, 62, 2525 - 2530 DOI: 10.1039/TF9666202525.
- Further reading
- Uranium + water reaction. Part 1.—Kinetics, products and mechanism M. McD. Baker, L. N. Less, S. Orman, Trans. Faraday Soc., 1966, 2513-2524 DOI: 10.1039/TF9666202513
- Uranium + water reaction. Part 2.—Effect of oxygen and other gases, M. McD. Baker, L. N. Less and S. Orman, Transactions of the Faraday Society, 1966, 62, 2525 - 2530 DOI: 10.1039/TF9666202525.
Compounds
From uranium mining
During the extraction of uranium ore and its processing, some releases of uranium occur. The releases of radium and other decay products of uranium are normally more important than the uranium in tailings ponds at the mines and ore processing centers.
See Uranium mining for further details.
From highly active waste in the form of glass
Note that while the vast majority of the uranium is removed by PUREX nuclear reprocessing, a small amount of uranium is left in the raffinate from the first cycle of the PUREX process. In addition due to the decay of the transplutonium minor actinides and the residual plutonium in the waste the concentration of uranium will increase on the waste. This will occur on a time scale of hundreds and thousands of years.
The waste from PUREX processing of used nuclear fuel is converted in the west into a borosilicate glass while in the former soviet bloc it is converted into a phosphate glass. The glass formed when placed in water will dissolve very slowly, according to the ITU it will require about 1 million years for 10% of the glass to dissolve in water.
See nuclear waste for more details.
From spent fuel
Spent uranium dioxide fuel is very insoluble in water, it is likely to release uranium (and fission products) even more slowly than borosilicate glass when in contact with water.
Behaviour in soil
A study has been done in the USA on the chemical form of uranium in soil, this was publsihed by Benjamin C. Bostick, Scott Fendorf, Mark O. Barnett, Phillip M. Jardine and Scott C. Brooks in Soil Science Society of America Journal 66:99-108 (2002) .
It has been suggested that it is possible to form a reactive barrier by adding something to the soil which will cause the uranium to become fixed. One method of doing this is to use a mineral (apatite) while a second method is to add a food substance such as acetate to the soil. This will enable bacteria to reduce the uranium (VI) to uranium (IV) which is much less soluble.
In peat like soils the uranium will tend to bind to the humic acids, this tends to fix the uranium in the soil. A report on the binding of uranium, other radioactive metals and non radioactive metal to humic acid has been published by the INE (German nuclear enginerring research center) at FZK (Karlsruhe) has been published. also see the paper by S. Pompe, K. Schmeide, M. Bubner, G. Geipel, K.-H. Heise, G. Bernhard and H. Nitsche in Radiochimica Acta, 2000, 88, 553-558 in which the effect of the phenol groups in the humic acid upon the binding of the uranium are studied. A series of papers have been written on coordination polymers or uranium(VI) with polycarboxylates, these have been used as models for the uranyl complexes of the humic acids.
For instance see G. Micera et. al., Inorganica Chimica Acta, 1985, 109, 135-139 which is a paper about the coordination of uranium to 2,6-dihydroxybenzoate which is a carboxylic acid which has phenolic groups close to the carboxylic acid group.
Some other work on the binding of actinides with aromatic carboxylates has been reported. A paper on the binding of neptunium(V) {neptunyl} with benzene-1,2,4,5-tetracarboxylic acid has been reported by F. Nectoux et. al., Journal of the Less-Common Metals, 1984, 97, 1-10.
A PhD thesis on the interactions of uranium with Boom Clay has been published.
It is interesting to note that A. Rossberg, L. Baraniak, T. Reich, C. Hennig, G. Bernhard and H. Nitsche, Radiochimica Acta, 2000, 88, 593-597 describes an EXAFS study of the interactions of uranium with the degradation products of wood such as protocatechuic acid (3,4-dihydroxy-benzoic acid), catechol (2-hydroxyphenol), pyrogallol (1,2,3-trihydroxybenzol), and vanillic acid (4-hydroxy-3-methoxybenzoic acid).
Health effects
Soluble uranium salts are toxic, though less so than those of other heavy metals such as lead or mercury. The organ which is most affected is the kidney. Soluble uranium salts are readily excreted in the urine, although some accumulation in the kidneys does occur in the case of chronic exposure. The World Health Organization has established a daily "tolerated intake" of soluble uranium salts for the general public of 0.5 μg/kg body weight (or 35 μg for a 70 kg adult): exposure at this level is not thought to lead to any significant kidney damage.
The antidote for uranium in humans in bicarbonate, this is used because uranium(VI) forms complexes with carbonate. An alternative is to use Tiron (O. Braun, C. Contino, M.-H. Hengè, E. Ansoborlo and B. Pucci, Analusis, 1999, 27, 65-68.). An article on the design of new actinide antidotes can be read at Chemical Reviews, 2003, 103, 4207-4282.
Animals
It has been reported that uranium has caused reproductive effects, and other health problems in rodents, frogs and other animals.
Uranium was shown to have cytotoxic, genotoxic and carcinogenic effects in animal studies (PMID 7694141, PMID 16283518). It has been shown in rodents and frogs that water soluble forms of uranium are teratogenic (PMID 16124873, PMID 11738513, PMID 12539863)
Bacterial biochemistry
It has been shown in some recent work at Manchester that bacteria can reduce and fix uranium in soils.
Further reading
Radioactivity, Ionizing Radiation and Nuclear Energy, by J. Hala and J.D. Navratil
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