Anion structural effects on interaction of rare earth element ions with Dowex 50W X8 cation exchange resin

The effect of the anion in the original rare earth element (REE) solution on the reversible ion exchange of Ce nitrate and Ce sulfate with the Dowex 50W X8 was investigated using attenuated total reflection infrared (ATR-IR) spectroscopy, continuous flow reactor studies coupled with inductively coupled plasma mass spectroscopy (ICP-MS), and density functional theory (DFT). The simulated IR spectrum at the DFT B3LYP/6-31G(d) level was compared to the experimental results to characterize the IR spectrum, molecular interactions, and bonding of the ion exchanged species. The continuous liquid flow reactor studies show a capacity of 0.72 mmol/g sorbent for the Ce nitrateand 0.96 mmol/g sorbent for the Ce sulfate with the Dowex 50W X8. The flow reactor studies reveal the type of solute anion (SO(2-) or NO3(-)) associated with the REE during cation exchange significantly affects the sorption capacity of the Dowex 50W X8 ion exchange resin. The calculated REE binding energy (BE) and the DFT optimized structures suggest that the differences in sorption capacity is the result of the formation of different types of partially ion-exchanged Ce(2)((3+)())2SO(4)((2-)) and Ce(3+) 3NO3((-)). These results suggest that the solute anion affects the equilibrium constants of the Dowex resin by the formation of a charged layer capable of retaining the counterion. Modifying the sulfonic acid site (H+) in the Dowex 50W X8 with the NH4+ counterion does not affect the sorption capacity and retention times of the Ce nitrate and Ce sulfate species. These results suggest that the counterions and co-ions having a finite size, may limit access to the Dowex sulfonate active site where the type of REE cation as a nitrate or sulfate in solution may significantly modify the sorption capacity of the ion exchange resin. Similar results are obtained during sorption with nitrates and sulfates of Sm and Yb. (C) 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.