Investigation of the Environmental Fate of Tritium in the Atmosphere

3.2 Chemical Transformations of Tritium
INVESTIGATION OF THE ENVIRONMENTAL FATE OF TRITIUM IN THE ATMOSPHERE
Industrial tritium releases predominantly consist of HTO and HT, and under some conditions,
tritiated methane (CH3T) (ATSDR, 2002; McCubbin et al., 2001). The residence times of HT
and CH3T in the atmosphere are not well known, although mean values of 5 to 10 years have
been estimated (e.g., Burger, 1979). Of the possible anthropogenic forms of tritium, HTO is
of most interest, since it tends to be much more biologically-active than other forms (UN,
ILO and WHO, 1983).
When tritium is released as a gas (HT) or as HTO in its vapour form (such as from airborne
releases), it can be concentrated by ordinary chemical separation of hydrogen and water
vapour from air. In contrast, when present as HTO in aqueous solutions (such as in liquid
effluents), it is not amenable to separation and concentration by conventional waste-treatment
techniques (Blomeke, 1964; Jacobs, 1968). Instead, isotopic separations are required, which
may be difficult. The more satisfactory procedures of isotopic separation are usually
physical in nature because mass differences result in greater relative differences in physical
properties than in chemical properties (Moeller, 1954). As a result of the similarity of the
properties of HTO and H2O, tritium generally follows ordinary water in processing streams
and in the environment under such conditions (Haney, 1964; Moeller, 1954).
3.2.1 Tritium Transformation from HT to HTO
Upon release into the atmosphere, key removal processes for HT include photochemical
oxidation in air and bacterial oxidation in soil, resulting in transformation to HTO (Dunstall
et al., 1985a, 1985b; Hart, 2008). The atmospheric process is slow, with a half-life of > 5
years, while a much higher oxidation rate occurs at the air-soil interface with a half-life of
40 minutes to 5 hours (Mishima and Steelec, 2002; McFarlane et al., 1978, 1979). The
chemical reactions are:
H 2+ OH H + H O
2 photochemical Equation 3.3
H + O 0 + M HO 2 + M (M =other molecule)
2H 2 +O 2 H 2O bacterial
Some of the HTO that is formed at the air-soil interface is taken up by plants through their
roots with transpiration water, and some is emitted to the atmosphere (Neil, 1991, 1992).
This HTO is available for uptake by animals and humans through inhalation and ingestion,
as well as by plants through their leaves.
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