Uranium separation from acid mine drainage using anionic resins - An experimental/theoretical investigation of its chemical speciation and the interaction mechanism

Uranium speciation and its complexes are an important environmental issue mainly when it is associated with acid rock drainage (ARD). Ion exchange resins are the most appropriate technology for the removal of low levels of uranium from highly concentrated sulfate solutions. The chemical speciation of uranium/sulfate system indicates that the predominant species at acidic conditions is the neutral [UO2SO4] (uranyl sulfate). Therefore, the uranium removal process cannot be described based solely as anionic exchange. The present work investigated the chemical speciation of the UO22+ (uranyl) in the presence of sulfate using density functional calculations (DFT) of the chemical species and their interaction with the resins to elucidate such counter-intuitive mechanism. It was demonstrated the great selectivity and loading capacity towards uranium from experiments with naturally occurring ARD solution and 3 different commercial anionic resins. The high concentration of sulfate along with the uranium affinity to sulfate play an important role in such process. We have demonstrated that the mechanism cannot be described as ion exchange but rather a complexing mechanism in which the uranium and its neutral [UO2SO4] (uranyl sulfate) species interacts with the sulfate adsorbed on the resins. This is a unique mechanism in which the uranium and its neutral sulfate complex is adsorbed through a complexing mechanism rather than ion exchange. This is a strategy that may be explored to remove other metal ions of environmental interest.