CONTROLS OF TOXIC ELEMENTS IN ABIOTIC REDUCTIVE DISSOLUTION OF URANIUM MILL RAFFINATES AND TAILINGS UNDER ALKALINE CONDITIONS

U mill tailings in northern Saskatchewan, Canada are alkaline (pH 8 to 10) and often contain elevated concentrations of the elements of concern (EOC) As, Se, Mo and Ni. These EOCs are immobilized within the tailings solids by secondary ferrihydrite (FH). Recent analysis of tailings solids (i.e., neutralized mill raffinates and tailings) also showed the presence of a significant reservoir of a secondary Mg-Al hydrotalcite nano-phase. Although the tailings are oxic and have remained so for more than 20 years, concern exists as to impact of the development of anaerobic conditions in the tailings and thus the long-term stability of the EOCs. Research suggests ferrihydrite is unstable under moderately reducing conditions (E_h ~ +100 mV) and may undergo phase transformation resulting in redox active species (e.g., Fe, As, and Se) being released into solution. A series of batch abiotic tests were conducted (7 day and 6 months) to investigate the impact of abiotic (via Fe(II)_(aq)) reduction on the sequestered EOCs in neutralized U-mill raffinates and tailings (pH 8 and 10). Under all abiotic reduction reactions tested, none of the neutralized raffinates, nor tailings samples (up to 8500 pmm As) released any significant (≤ 0.1 w.t. %) EOCs into the porewater. The optimum condition for lowest EOC release for all samples tested was observed when green rust formed. The lack of EOC release from the process samples, neutralized raffinates, and tailings samples occurred in spite of clear phase transformation (e.g. green rust) and even ferrihydrite/green rust crystallization to magnetite. The mechanisms involved in the EOC retention and phase transformation are discussed. In summary, these finding suggest that EOCs in the U mill tailings should remain sequestered in the solid phases, even if the tailings become anoxic at some time in the future.