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Uranium in the environment refers to the science of the sources, environmental behaviour and effects of actinides in Earth's environment and in animals (including humans). This page is a subpage of actinides in the environment.

Contents

Occurrence

From the land

Uranium ore

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−1 to 11.7 mg kg−1. (United Nations Scientific Committee on the Effects of Atomic Radiation, 1993, Report to the General Assembly, with scientific annexes, New York)

It 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)[1] 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 236U is only formed in nuclear reactors fueled with 235U, but the 238U/235U ratio should be used with some caution. In Africa a set of natural nuclear fission reactors operated in one uranium rich area, the Oklo region in Gabon.

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 [1] "A total of just over 290 metric tons of DU projectiles were fired by the US during the Gulf War (comapred [sic] to 9 tons in Kosovo and 3 tons in Bosnia and Herzegovina)."[2] The potential long-term effects 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.[3] Studies by the University of Denver conclude that there is no risk of radiological effects on humans from DU munitions. However, there exists evidence that the density of uranium particles may lead to a variety of health effects. Uranium is a pyrophoric metal, which causes an incendiary affect in DU weapons, but also scatters the inert uranium 238 isotope into the air. DU exists normally in our environment. We eat it and breathe it in trace amounts on a daily basis. However, in areas where DU weapons have been used, uranium saturation can be dangerous. Because uranium is so heavy, the human body is poor at removing it. If inhaled in large amounts, it can cause respiratory arrest and is difficult to remove. If ingested, the kidneys filter uranium from the blood, and expel it in urine, but because of the difficulty of removing the large atom, it tends to accumulate more quickly than it is removed in areas of high concentrations of DU. Because of the weight of the atom, some believe this puts dangerous strain on kidney functions, and can lead to kidney failure.

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

"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 [6] and thus could not affect populations more than a few kilometers from target areas. [7]

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. [4]

In pure water, schoepite {(UO2)8O2(OH)12.12(H 2O)} is formed [5] 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. [6][7]

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 temperatures 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 irradiation has little effect on the corrosion rate.[8].

Oxygen gas inhibits the corrosion of uranium by water. [9].

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 because of 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 handled by vitrification, in the west it is converted 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,[8] 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 published 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) [9].

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) [10] 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.[11] 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 engineering research center) at FZK (Karlsruhe) has been published.[12] 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.

2,6-dihydroxybenzoic acid

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.

Benzene-1,2,4,5-tetracarboxylic acid

A PhD thesis on the interactions of uranium with Boom Clay has been published.[13]

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. [14]

The antidote for uranium in humans is 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.[15]). An article on the design of new actinide antidotes can be read at Chemical Reviews, 2003, 103, 4207-4282.

Tiron which is a phenoloic aromatic disulfonic acid which is an alternative to bicarbonate which has already been tested in animals

Humans

Studies have shown that the use of DU ammunition has no measurable detrimental health effects, either in the short or long term. The International Atomic Energy Agency reported in 2003 that, "based on credible scientific evidence, there is no proven link between DU exposure and increases in human cancers or other significant health or environmental impacts."[16]

Gulf War syndrome

A two year study headed by Al Marshall of Sandia National Laboratories analyzed some health effects associated with accidental exposure to depleted uranium during the 1991 Gulf War, but did not consider any nonradiological reproductive toxicity, developmental toxicity, or immunological effects. Marshall’s study concluded that the reports of serious health risks from DU exposure are not supported by veteran medical statistics and were consistent with earlier studies form Los Alamos and the New England Journal of Medicine.[17]

In the Balkans war zone, an absence of problems is seen by some as evidence of DU muntions' safety: "Independent investigations by the World Health Organization, European Commission, European Parliament, United Nations Environment Programme, United Kingdom Royal Society, and the Health Council of the Netherlands all discounted any association between depleted uranium and leukemia or other medical problems."[18]

However in 2004, the UK Pensions Appeal Tribunal Service granted a single disability claim to a Gulf War soldier who attributed his aching joints and children's health problems to depleted uranium. This claim was vigorously disputed by the MoD.[19] [20]

Some studies have indicated that DU passes into humans more easily than previously thought after battlefield use.[21] [22]

Birth defects

Most scientific studies have found no link between uranium and birth defects, but some claim statistical correlations between soldiers exposed to DU, and those who were not, concerning reproductive abnormalities.

One study concluded that epidemiological evidence is consistent with an increased risk of birth defects in the offspring of persons exposed to DU.[23] Environmental groups and others have expressed concern about the health effects of depleted uranium,[24] and there is some debate over the matter. Some people have raised concerns about the use of this material, particularly in munitions, because of its mutagenicity,[25] teratogenicity in mice,[26][27] and neurotoxicity [28] and its suspected carcinogenic potential. Additional concerns address unexploded DU munitions leeching into groundwater over time.[29].

Several sources have attributed the increase in the rate of birth defects in the children of Gulf War veterans and in Iraqis to depleted uranium inhalation exposure,[30][31] A 2001 study of 15,000 February 1991 U.S. Gulf War combat veterans and 15,000 control veterans found that the Gulf War veterans were 1.8 (fathers) to 2.8 (mothers) times more likely to have children with birth defects.[32] In a study of UK troops, "Overall, the risk of any malformation among pregnancies reported by men was 50% higher in Gulf War Veterans (GWV) compared with Non-GWVs". The conclusion of the study stated " We found no evidence for a link between paternal deployment to the Gulf war and increased risk of stillbirth, chromosomal malformations, or congenital syndromes. Associations were found between fathers' service in the Gulf war and increased risk of miscarriage and less well-defined malformations, but these findings need to be interpreted with caution as such outcomes are susceptible to recall bias. The finding of a possible relationship with renal anomalies requires further investigation. There was no evidence of an association between risk of miscarriage and mothers' service in the gulf."[33]

However, as yet, all evidence surrounding DU and birth defects by credible scientific sources has been deemed circumstantial. There is as yet no concrete evidence to suggest that direct exposure to DU is unsafe unless inhaled or ingested.

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.[34] These bacterium change soluble U(VI) into the highly insoluble complex forming U(IV) ion, hence stopping chemical leaching.

See also

References

  1. ^ Gulf veteran babies 'risk deformities' | Politics | The Observer
  2. ^ p68 UNEP study, 2003
  3. ^ Depleted Uranium
  4. ^ http://www.the-conference.com/2003/Gold2003/abstracts/A493.pdf
  5. ^ http://webmineral.com/data/Schoepite.shtml
  6. ^ http://www.royalsoc.ac.uk/downloaddoc.asp?id=1183
  7. ^ http://www.royalsoc.ac.uk/downloaddoc.asp?id=1182
  8. ^ M. McD. Baker, L. N. Less, S. Orman, Trans. Faraday Soc., 1966, 2513-2524 DOI: 10.1039/TF9666202513
  9. ^ 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.
  • Radioactivity, Ionizing Radiation and Nuclear Energy, by J. Hala and J.D. Navratil.ç
  • Martín-Gil J., Martín-Gil F.J, José-Yacamán M., Carapia-Morales L. and Falcón-Bárcenas T. Microwave-assisted Synthesis of Hydrated Sodium Uranyl Oxonium Silicate. Polish Journal of Chemistry. 2005. 79, 1399-1403







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