Tritium fractionation in anomalous water bound to environmental samples

Abstract The quantification of the hydrogen isotope effect involved in the physi- and chemisorption of water on to environmental samples provides the possibility for the characterization of their bound water. This is realized by distilling suspensions of samples in tritiated water ( HTO H 2 O ) under near-equilibrium conditions at 5 °C and 5 mbar, and measuring the specific tritium activity in fractions of the condensed gas phase in the course of water removal. The tritium fractionation is caused by isotope effects during the evaporation of water as well as during the sorption of water on to the sample matrix. The two isotope effects are separated by comparing the fractionation in the sample—water mixtures with that in pure water. Three typical soils and maize plant seeds are chosen for investigation. The experimental procedure is tested for model substances with well known amounts of bound water, e.g. Na-tartratedihydrate and molecular sieve. The investigated examples show a different water sorption capacity from 4 to 60% related to the dry substance. Dehydration kinetics further demonstrate the existence of at least four types of sorbed water with distinct fractionation factors. They are characterized by the specific-activity ratio of sorbed water to free water, which ranges from 0.97 to 1.46.