Tritium accumulation in clay minerals (kaolinite, montmorillonite, palygorskite) was studied experimentally over a one year contact with tritiated water (initial activity 1.676x109 Bq/m3) under room conditions, and was compared to theoretical calculations. The amounts of tritium ions in bound water and structural hydroxyls were measured by the activity of water fractions released at characteristic temperatures determined on the basis of the mineral DTA patterns. Experiments showed that after a 400 day contact with tritiated water, tritium exchange rates (θ) were highest in palygorskite (θa = 0.42 in bound water and θs = 0.51 in the structure) compared to montmorillonite (θa = 0.15 and θs = 0.11, respectively) and kaolinite (θa = 0.34 and θs = 0.02, respectively). This could be due to different tritium concentrations in the solution near the bound-water layers, and also to certain features of the mineral structures, most of the kaolinite structural hydroxyls not interacting directly with bound HTO molecules.
Tritium accumulation in these minerals, under room conditions, can be described by a relatively simple two-stage isotope exchange model in which tritium ions first migrate from the solution to the bound layers and then substitute the protons of the structural hydroxyls. Rate constants for both stages of this isotope exchange were calculated on the basis of the experimental data.