Actinides in the environment

Actinides in the environment refer to the sources, environmental behaviour and effects of actinides in the environment. Environmental radioactivity is not limited solely to actinides; also, actinides such as uranium and radium specifically are of note.

Inhalation versus ingestion
In general for the insoluble actinide oxides such as high fired uranium dioxide and MOX fuel if it is swallowed then it will pass through the digestive system with very little actinide dissolving. As the actinide oxide can not dissolve, it can not be absorbed into the body of the person or animal. With such an oxide the dose a person is committed to after a given intake of activity is higher for inhalation than for ingestion as the insoluble compound will remain in the lungs, where it will then irradiate the lung tissue.

Low fired oxides and soluble salts such as the nitrates can be absorbed with greater ease through the digestive system. So they are able to enter the bloodstream after being swallowed. If they are inhaled then it is possible for the solid to dissolve and leave the lungs. Hence the dose to the lungs will be lower for the soluble form.

Radon and radium in the environment
Radon and radium are not actinides—they are both radioactive daughters from the decay of uranium. Aspects of their biology and environmental behaviour is discussed at radium in the environment.

Thorium in the environment
In India a large amount of Thorium ore can be found in the form of monazite in placer deposits of the Western and Eastern coastal dune sands, particularly in the Tamil Nadu coastal areas. The residents of this area are exposed to a naturally occurring radiation dose ten times higher than the worldwide average. .

Occurrence


Thorium is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium, and is about as common as lead. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals, the most common being the rare earth-thorium-phosphate mineral, monazite, which contains up to about 12% thorium oxide. There are substantial deposits in several countries. 232Th decays very slowly (its half-life is about three times the age of the earth) but other thorium isotopes occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than 232Th, though on a mass basis they are negligible.

Effects in humans
Thorium has been linked to liver cancer. In the past thoria (thorium dioxide) was used as a contrast agent for medical X-ray radiography but its use has been discontinued. It was sold under the name Thorotrast.

Uranium in the environment
Uranium is a natural metal which is widely found. It is present in almost all soils and it is more plentiful than antimony, beryllium, cadmium, gold, mercury, silver, or tungsten and is about as abundant as arsenic or molybdenum. 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).

Seawater contains about 3.3 parts per billion of uranium by weight as uranium(VI) forms soluble carbonate complexes. The extraction of uranium from seawater has been considered as a means of obtaining the element.

Due to the very low specific activity of uranium the chemical effects of it upon living things can often outweigh the effects of its radioactivity.

Additional uranium has been added to the environment in some locations as a result of the nuclear fuel cycle and the use of depleted uranium in munitions.

Neptunium in the environment
Like plutonium, neptunium has a high affinity for soil. 

Environmental chemistry
Plutonium like other actinides readily forms a dioxide plutonyl core (PuO2). In the environment, this plutonyl core readily complexes with carbonate as well as other oxygen moieties (OH-, NO2-, NO3-, and SO4-2) to form charged complexes which can be readily mobile with low affinities to soil.


 * PuO2(CO3)1-2
 * PuO2(CO3)2-4
 * PuO2(CO3)3-6

PuO2 formed from neutralizing highly acidic nitric acid solutions tends to form polymeric PuO2 which is resistant to complexation. Plutonium also readily shifts valences between the +3, +4, +5 and +6 states. It is common for some fraction of plutonium in solution to exist in all of these states in equilibrium.

Plutonium is known to bind to soil particles very strongly, see above for a X-ray spectrscopic study of plutonium in soil and concrete. While cesium has very different chemistry to the actinides, it is well known that both cesium and many of the actinides bind strongly to the minerals in soil. Hence it has been possible to use 134Cs labeled soil to study the migration of Pu and Cs is soils. It has been shown that colloidal transport processes control the migration of Cs (and will control the migration of Pu) in the soil at the Waste Isolation Pilot Plant according to R.D. Whicker and S.A. Ibrahim, Journal of Environmental Radioactivity, 2006, 88, 171–188.

Americium in the environment
Americium often enters landfills from discarded smoke detectors. The rules associated with the disposal of smoke detectors are very relaxed in most municipalities. For instance in the UK it is permissible to dispose of an americium containing smoke detector by placing it in the dustbin with normal household rubbish, but each dustbin worth of rubbish is limited to only containing one smoke detector.

In France a truck transporting 900 smoke detectors has been reported to have caught fire, it is claimed that this led to a release of americium into the environment. 

Humans have become contaminated with americium, the worst case was that of Harold McCluskey. It is interesting to note that Harold McCluskey did not die of cancer but of heart disease (which he had before the accident). It is likely that the medical care which he was given saved his life; it should be noted that due to the difference in the chemistry of americium (the +3 oxidation state is very stable) to plutonium (where the +4 state can form in the human body) the americium has very different biochemistry to plutonium.