GEOCHEMICAL CONTROLS ON ARSENIC IN URANIUM MINE TAILINGS

One of the most important environmental issues facing the uranium mining industry in Saskatchewan, Canada is the long-term stability of arsenic in mine wastes. Hydrometallurgical processing of uranium ores results in the dissolution of primary arsenic minerals. Effective removal and safe deposition of this unwanted impurity is a key issue facing several mining industries. Solids and associated porewaters containing arsenic within tailings impoundments present an analog to the study of the geochemical controls on arsenic in non-contaminated environments. In both mine wastes and natural environments, pH, Eh, temperature, as well as the arsenic mineralogy control the long-term stability of arsenic. A detailed geochemical investigation was initiated at the Rabbit Lake uranium mine site in northern Saskatchewan to determine the long-term stability of arsenic in mine tailings. Arsenic concentrations in the tailings solids and porewater ranged from 0.01 to 1.2 % wt/wt (n=14) and 0.24 to 140 mg/L (n=72). In all samples of tailings porewater, the arsenic concentrations exceeded the drinking water standards (0.050 mg/L). Using atomic absorption/hydride generation it was determined that greater than 90% of the arsenic in samples of tailings porewater was in the As5+ state. This was important because the toxicity and mobility of arsenic is dependent upon the valence state of arsenic present in aqueous environments with As3+ being up to 60 times more toxic than As5+. An investigation into the speciation and mineralogy of the arsenic bound species in the tailings solids was conducted using synchrotron light. The application of this technique showed that the secondary arsenic minerals in the tailings solids contained arsenic exclusively in the As5+ state. It was also determined that in areas of tailings rich in iron the arsenic was adsorbed to 2-line ferrihyrdite through bi-dentate linkages. The results of this study will improve our understanding of geochemical controls on arsenic in both contaminated and natural groundwaters.