Probing the interaction of U() with phosphonate-functionalized mesoporous silica using solid-state NMR spectroscopy

The fundamental interaction of U( vi ) with diethylphosphatoethyl triethoxysilane functionalized SBA-15 mesoporous silica is studied by macroscopic batch experiments and solid-state NMR spectroscopy. DPTS-functionalized silica has been shown to extract U( vi ) from nitric acid solutions at or above pH 3. Extraction is dependent on pH and ionic strength. Single-pulse 31 P NMR on U( vi ) contacted samples revealed that U( vi ) only interacts with a fraction of the ligands present on the surface. At pH 4 the U( vi ) extraction capacity of the material is limited to 27-37% of the theoretical capacity, based on ligand loading. We combined single pulse 31 P NMR on U( vi )-contacted samples with batch studies to measure a ligand-to-metal ratio of approximately 2 : 1 at pH 3 and 4. Batch studies and cross-polarization NMR measurements reveal that U( vi ) binds to deprotonated phosphonate and/or silanol sites. We use 31 P- 31 P DQ-DRENAR NMR studies to compare the average dipolar coupling between phosphorus spins for both U( vi )-complexed and non-complexed ligand environments. These measurements reveal that U( vi ) extraction is not limited by inadequate surface distribution of ligands, but rather by low stability of the surface phosphonate complex. Solid-state NMR techniques combined with batch contact experiments elucidate how U( vi ) binds to phosphonate-functionalized mesoporous silica.