Actinides in Biological Systems
Actinides are radioactive heavy elements with atomic numbers between 89 and 103. Based on their occurrence, they can be separated into two groups: naturally occurring and synthetic elements. However, the line between them is not a sharp one. This is due to human activities in using and testing nuclear power and nuclear weapons as well as to a smaller degree due to natural nuclear reactors in the Proterozoic era. Thorium, protactinium, uranium and, in much smaller amounts, plutonium are the naturally occurring elements. Neptunium and all elements with atomic numbers (AN) larger than 94 are synthetic elements. The isotopes of elements beginning with einsteinium (AN=99) have relatively short lifetimes of radioactive decay (<472 d for 252Es) and their availability is very limited. Therefore, no information about their behaviour in biological systems has been published until recently. Especially uranium, neptunium, plutonium and to some extent americium can exist in different oxidation states. Therefore, for these elements redox reactions in biological systems are of great significance. Actinides can accumulate in the human body. However, the target accumulating organs differ. While protactinium mainly accumulates in the kidneys and bone, plutonium is incorporated in lung, liver and bone. All actinides have long residence times in the human body. Therefore, it is sometimes not straightforward to distinguish between chemical toxicity and radio-toxicity. Most of the scientific literature deals with monitoring of radioactivity and calculation of doses in living systems; however, this aspect will not be the focus of this chapter. Several bodies of data are available, dealing with questions of decontamination of actinides inhaled or ingested into the human body. Lanthanides (elements of the 4f series) show similar chemical behaviour to actinides in the same oxidation state. The behaviour of actinides in biosystems that are of common interest includes pathways of uptake and excretion, plus strategies of protection and defence employed by organisms. The long-living elements thorium and uranium mainly show heavy metal effects; this behaviour changes within the actinide series, starting with neptunium where radiological effects also have to be taken into account.