The ability to successfully sequester and separate trivalent lanthanides and actinides has applications to the nuclear fuel cycle, processing of irradiated target materials, and nuclear forensics. The challenge in separating the trivalent lanthanides and actinides is their similar size and charge. As a result of this, standard size exclusion or ion exchange techniques become much more challenging. Instead of focusing on size or charge for separation, their differences in interactions with complexing ligands can be utilized. Another challenge in trivalent lanthanide and actinide separations is that these metals are often dissolved in highly acidic matrices. Thus, the stability of the extractant ligand used to complex the metal is at risk of degradation.

The most common sequestration and separation routes based on complex formation for lanthanides and actinides use liquid-liquid separations, which can generate large volumes of hazardous organic waste. By moving to solid-liquid separations, in which the organic phase is replaced by a solid-phase extractant (SPE), the production of large volumes of waste can be minimized. The most common method of making SPEs for lanthanide and actinide separations is coating an organic extractant ligand on either a silica or polymer support. As a result of only coating the ligand rather than bonding it to the solid support, these materials can suffer from ligand degradation and detachment. Chapter 5 focuses on trivalent lanthanide and actinide interactions with a commercially available SPE. While those materials were found to have use for certain applications, their limitations motivated the organically-modified mesoporous silica (OMMS) development presented in Chapter 7.

The work presented here is a study on solid-phase extractants for lanthanide and actinide separations, particularly focused on OMMS. The OMMS materials are made by covalently binding the extractant ligand to the mesoporous silica, to increase the stability of these SPEs in the presence of acidic media. Chapters 6 and 7 discuss the synthesis and characterization of two types of OMMS materials as well as macroscopic and molecular level studies of these materials with trivalent lanthanides and actinides. Additionally, the stability of the OMMS materials in the presence of acid was probed using nuclear magnetic resonance (NMR) spectroscopy.

The first OMMS material discussed is a caprolactam (CA)-modified mesoporous silica. The behavior of Eu(III), Sc(III), and Al(III) was studied with the CA-modified mesoporous silica. Al and Sc were studied as comparative trivalent metals that had smaller ionic radii than Eu. The results of batch sorption experiments and mainly solid-state NMR spectroscopy studies are presented. The acid stability studies showed that the CA-modified mesoporous silica degraded in two ways in the presence of acid: 1) the 7-membered ring of the ligand opened, and 2) a portion of the isolated ligands on the surface were cleaved at the silane anchor. It was determined that while the CA materials were not effective sorbents for Eu(III) from acidic matrices, they did bind Al(III) and Sc(III). Through the NMR studies, both Al and Sc were found to complex to the CA ligand and the surface silanols simultaneously. This work was the first time solid-state NMR had been used to study the surface, ligand, and metals directly of OMMS materials via their NMR active nuclei (29Si, 13C, 1H, 27Al, and 45Sc).

The second OMMS material studied was a diglycolamide (DGA)-modified mesoporous silica. The main focus of this work was the interactions of the DGA-modified mesoporous silica with Eu(III) and Am(III), but also touched on other trivalent species such as La(III), Lu(III), and Y(III). Again, the stability of this material in the presence of acid was investigated using NMR spectroscopy and while the DGA ligand was found to withstand acid treatment, the isolated ligand cleavage at the silane anchor still occurred. The DGA-modified mesoporous silica was found to efficiently bind the trivalent species at high acid concentrations, particularly below pH 1. As a result of the mesoporous substrate and subsequently high ligand loading for the associated mass, the sorption capacity for Eu(III) (379 umol/g) was found to be higher than other diglycolamide SPEs. Through infrared spectroscopy and NMR spectroscopy experiments, the trivalent metals were found to bind to the carbonyl and ether oxygens of the DGA ligand. Fluorescence spectroscopy of Eu(III) and X-ray absorption spectroscopy studies elucidated that the trivalent metals are complexing as three ligands per metal center. Proof-of-concept column experiments demonstrate the use of DGA-modified mesoporous silica as a stationary phase to which Eu was both sorbed and then eluted.

A major focus of this work was the interplay of macroscopic and molecular level studies to best characterize metal interactions with OMMS materials. While both the bulk and molecular level investigations can provide information about these systems, neither gives the full picture in itself. In designing materials for separations, knowledge of the complete picture is necessary in order to best optimize methodologies and predict separation behavior.