Radioactive waste contains a wide range of isotopes, but many are either short-lived radionuclides or are stable (i. e. non-radioactive) isotopes. The radionuclides that are problematic environmentally are those that are long-lived, have a high activity, are present in relatively large quantities and/or are bioavailable. Details of some key radionuclide contaminants are given in Table 4. All are present in significant quantities in the environment from nuclear weapons testing or the nuclear fuel cycle, except for 60Co; uranium and radon also occur naturally. Cobalt-60 is widely used in medical and industrial appli­cations requiring a radiation source, and is created by neutron activation of 59Co.

Several of these radionuclides are bioavailable. Radon exists as a gas and inhalation of radon can cause cancer; it is the second leading cause of lung cancer in the USA.39 Strontium(n), an analogue for Ca21 can be accumulated in bone, whilst Cs1 is analogous to K1 and so can be transported into cells via the K1 transport mechanisms; both 99Tc and 129I can be accumulated in the thyroid gland.40,41

The environmental fate of radionuclides is controlled by a number of factors; these will be discussed in detail in sections 4 and 5. However, the oxidation state

of the radionuclide will have a significant impact on its chemical behaviour, transport and bioavailability, particularly for redox-sensitive radionuclides. As can be seen from Table 4, the actinide elements can exist in a range of oxidation states, leading to fairly complicated chemical behaviour. The most stable oxidation state of uranium in the environment is U(vi), as UO221, but it is also stable as U(iv) under reducing conditions.42 The most stable and dominant oxidation states of neptunium and plutonium in the environment are +v (as NpO2+) and +iv, respectively. However, in the environment, neptunium can also exist in the +iv and +vi oxidation states, whilst plutonium can also be present in + iii and +v oxidation states.43 The higher, environmentally stable oxidation states (v, vi) of the actinide elements tend to be more soluble and therefore more mobile, whilst An(iv) species (An = actinide), with a high charge/radius ratio, are prone to hydrolysis and polymerisation, forming colloids and precipitates, and readily sorb to mineral surfaces.1,44

Of the fission products listed, technetium and iodine are redox active, but caesium and strontium have only one stable oxidation state each: Cs1 and Sr21. Therefore changes in the redox environment do not directly affect the chemistry of caesium and strontium, but the environmental behaviour and bioavailability of Cs1 and Sr21 do relate to their oxidation state. The low charge density on Cs1 means that it is only weakly complexed by ligands and tends to bond via electrostatic interactions rather than covalent bonding. it is also highly soluble and so is mobile in the environment, with interactions with mineral phases being the dominant mechanism of retardation.44 Like Cs1, Sr21 does not complex strongly to ligands and tends to be soluble in the environment, but it can co-precipitate with calcium sulfate or carbonate.44,45 The mobility of technetium is primarily controlled by the oxidation state, with two stable states found in the environment: +vii and +iv. Under aerobic conditions, technetium will exist in the+vii oxidation state, as TcO4, and in this form it is highly soluble and mobile. Under more reducing conditions, Tc(iv) is stable and will tend to exist as insoluble TcO2.2 For iodine, the most important oxidation states in the environment are: 41, 0 and +v. In aqueous environments, +v (as IO3_) and 41 (as i4) are the dominant forms, but in soils, iodine can be mostly present as organic species.1,2 The redox chemistry of cobalt is relatively simple, with just two stable oxidations states: +ii and+111. Co(ii) is the dominant oxidation state in solution, as it tends to be more soluble than Co(iii), but Co(iii) can be stabilised and mobilised by certain ligands (see section 5.3).46,47