The geochemical distribution of radium-226 in Cluff Lake uranium mill tailings
Goulden, Wanda Denice
The choice of an input 'source term' for radium concentration into numerical transport models has been a challenge when attempting to predict long term impacts of uranium mill tailings storage. Physical and chemical separation methods were used to establish the geochemical distribution of radium in a fresh Cluff Lake uranium mill tailings sample. Analysis of the experimental data suggested that radium in Cluff Lake uranium mill tailings is present as a barium radium sulfate [(Ba,Ra)S0₄] coprecipitate. Even though radium is known to be strongly adsorptive, only a minor amount of radium absorption was calculated in the presence of competing cations in the Cluff Lake tailings pore fluids. While approximately 80% of the total radium present in the tailings was associated with the < 75μm size fraction (dominated by gypsum, illite, chlorite and iron hydroxide), the data suggests that the common trace mineral barite present in this fraction controls the radium pore fluid concentration. Of the total radium mass present in the tailings, 0.35% was released during deionized water leaching, which removed all of the readily water soluble fraction (11% of the solids mass). Coprecipitation theory using barite as a host mineral for radium adequately predicts equilibrium radium pore fluid concentration. The calculated equilibrium value agrees well with in situ empirical data for Cluff Lake pore fluids, column lysimeter leachates and the empirical experimental data from this study. The results of this study have several environmental implications for stability and transport of ²²⁶Ra associated with sulfuric acid leached and lime neutralized uranium mill tailings. If the barium and radium solids concentrations are known, the equilibrium radium pore fluid concentration can be predicted using coprecipitation theory. The majority of the radium mass (>99%) will be highly stable in the solid phase when in contact with deionized water (or by extrapolation, very low ionic strength waters). The radium concentration in solution is sensitive to sample disturbance; therefore, sampling should be performed with care not to agitate the tailings solids during liquid recovery.