Acidification and Radionuclide Leaching in Unsaturated Uranium Mill Tailings
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Abstract
The results of a 7 year leaching study on tailings that were approximately 20 years old, fresh tailings, and fresh tailings solidified by a proprietary process are presented. All experiments were conducted in duplicate under conditions which emulate the unsaturated zone of tailings. The 20 year old tailings received water at three rates which were approximately equal to 0.6, 1.3 and 7 times the natural rainfall rate. The fresh tailings and solidified fresh tailings received water at approximately 2 times the natural rainfall rate. The main dissolved ions in the leachates from the 20-year old tailings were hydrogen, iron and sulphate. The concentrations of these ions and the leaching patterns of conductivity and total dissolved solids (TDS) concentrations suggests that the acidification rate is independent of the water addition rate in these unsaturated tailings, as would be expected if pyrite oxidation is bacterially controlled and follows zero order kinetics. In the old tailings, 0.8% of the Ra-226 in the lysimeters with the higher water addition rates, and only 0.02% of the Ra-226 in the lysimeters with the lowest water addition rate , were leached during the study. Ra-226 levels in the low-rate leachate generally decreased over the 7 years from 3.7 Bq/L (100 pCi/Ll to less than 0.56 Bq/L (15 pCi/L). Ra-226 levels in the high-rate leachate remained relatively constant at about O. 74 Bq/L (20 pCi/L) for the first 3 years, then increased to peak at 19 Bq/L (514 pCi/L) after 6 years. This increase was hypothesized to result from the release of Ra-226 being retarded until sulphate concentrations had decreased sufficiently to allow dissolution of a barium-radium-sulphate coprecipitate formed on the surface of quartz solids in the leaching pachucas. Leaching of thorium isotopes appears pH dependent, suggesting that their dissolution is solubility and/or kinetically controlled as a function of pH. In the fresh tailings, the pH remained at about 7 during the first two years, then declined to reach 2.2 by the end of the study. The data suggest that gypsum (calcium sulphate) was dissolving during the entire leaching period and that about 2 to 3 years were required before sufficient base minerals were consumed to allow the colonization of iron-oxidizing Thiobacillus ferrooxidans and pyrite oxidation reactions to dominate leachate chemistry. The pH of leachate from the solidified fresh tailings lysimeters remained at about 11.7 during the first two years, then declined to reach 7.1 at the end of the study. The pH was higher than the pH of the fresh tailings leachate due to the alkaline nature of the chemical additives used in the solidification process, and because solidification minimized oxidation of the pyrite in the tailings and/or encapsulated the pyrite within a solidified matrix for a period of time. Leachate pH decreased and calcium and Ra- 226 concentrations increased dramatically after 2 years, however, suggesting that the encapsulation process was breaking down and allowing gypsum and Ra-226 dissolution to occur. The results suggest that the solidification process merely delays the commencement of pyrite oxidation and does not prevent it from eventually occurring.
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