Controlling lanthanide separations using functionalized ordered mesoporous carbon solid supports

Abstract

The lanthanides are some of the most challenging elements to separate from one another, yet the need for high-purity lanthanides is crucial for several different disciplines. Extraction chromatography (EXC) is a separation technique that offers multiple separation stages in a single column to achieve high-purity separations. Common EXC supports for lanthanide separations are made of porous polymeric materials. This dissertation focuses on ordered mesoporous carbon (OMC) nanoparticles as an alternative support material for lanthanides separations. The initial effort centers on the applicability of OMC as an extraction chromatographic support. OMC physisorbed with bis-(2 ethylhexyl) phosphoric acid (HDEHP) was compared to an analogous, commercially available EXC resin. The distribution coefficient for Eu3+ was greater using HDEHP-OMC than the EXC resin, even after adjusting for differences in extractant concentration. The maximum Eu3+ adsorption capacity for the HDEHP-OMC was at least twice that of the EXC resin. The results indicate that the high surface area of the OMC plays a significant role in the adsorption capacity, and therefore distribution coefficient, of the lanthanide ion. An improved chromatographic separation of Nd3+ and Eu3+ was achieved with the HDEHP-OMC material. Maximum Eu3+ adsorption studies using OMC physisorbed with N,N,N',N'-tetra-n-octyl diglycolamide (TODGA) and an analogous EXC resin showed hyperstoichiometric uptake of Eu3+ by both supports. Electroanalytical techniques were used to probe the redox properties of Eu adsorbed on TODGA-OMC. The results show that the trivalent oxidation state is stabilized by the complexation with TODGA and that through controlled polarization, desorption of a reduced Eu2+ species is possible. Differential pulse voltammograms from matrix confined Eu in the TODGA-OMC electrodes indicated that under high Eu3+ loading, Eu species—resembling an aquated Eu cation—were present. Small-angle X ray scattering (SAXS) and extended X ray absorption fine structure (EXAFS) techniques were used to examine the TODGA-OMC and analogous EXC resin under varying Eu3+ loading conditions. These results show that under high Eu3+ loading conditions, microcrystalline-like domains exist for the TODGA-OMC material but are absent for the EXC resin.