Recovery of Rare Earth Elements From Sulfate-rich Acid Mine Water: Looking Through the Keyhole the Exchange Reaction for Cationic Resin

The ion exchange process for the recovery of rare earth elements (REE) from acid waters is investigated through combined experimental and theoretical approaches. Loading and elution experiments using a strongly acidic cation resin were carried out batchwise and in columns by applying laboratory solutions as well as crude acid mine drainage (AMD). Loading experiments, which assessed the competition of different species during the ion exchange process, showed the preference for La in detriment to Y. The resin was eluted with different inorganic acids and salts to remove the adsorbed REE. The best result was attained with the use of CaCl2 solution, having approximately 80 % of the REE eluted in the batch experiments. It was also noticed that as the flow rate decreased there was an increase in the elution from 80 % to 100 % for light REE and 50-90 % for heavy REE. DFT calculations indicated that the chemical models [LnSO4(H2O)7]+ and [Ln(SO4)2(H2O)5]- (Ln--La, Y) are adequate to describe the chemical speciation. Furthermore, these species effectively interact with the resin sulfonic groups in a similar way as to the [Ln(H2O)9]3+ species, explaining why the loading results are high even in the presence of sulfate. For elution, the CaCl2 solution was modeled with the [Ca(H2O)6]2+ species, displacing the La and Y species from the resin. The estimated free energies involved in the mechanisms with and without SO42-are similar. Therefore, DFT calculations indicate that the functional group, which is present in the cation exchange resin, acts as a complexing agent.