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

Abstract

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.

Organic layer formation and sorption of U(vi) on acetamide diethylphosphonate-functionalized mesoporous silica

“…Similar 1-3 nm decreases in average pore diameter have been observed in other silica materials after modification with an organic monolayer. 21,43,44 While the nitrogen adsorption isotherms of AcPhos-SBA 1and 2are both type IV with H 1 hysteresis loops, the shapes of the isotherms for AcPhos-SBA (3) and (4) look different. The isotherm and hysteresis loop for AcPhos-SBA (3) appear to be type IV-H 1 .…”

“…The U(VI) sorption isotherm for AcPhos-SBA (2) was not described particularly well by either the Langmuir or Freundlich models, which is evidence that a more complex sorption environment exists between uranyl cations and AcPhos-SBA (2). In our previous work, 21 we observed that the sorption of U(VI) on diethylphosphonate (DPTS)-functionalized SBA-15 from pH 4 nitric acid solutions was described slightly better by the Freundlich model than the Langmuir model. Combined with evidence from NMR spectroscopy, observation of the Freundlich sorption isotherm led us to believe that the uranyl ions were complexed to both ligands and surface silanols.…”

“…Previous work 21 showed that increasing the ionic strength resulted in an increase in extraction of U(VI) from nitric acid solutions by phosphonate-functionalized SBA-15. Therefore, sorption of U(VI) from pH 2 and pH 3 solutions containing no additional salt and 3 M NaNO 3 was compared for AcPhos-SBA (2), (3), and (4).…”

“…A similar chemical shift has been observed for phosphonate species bound to uranyl cations. 21 Therefore, we attribute the chemical shift at 29 ppm to AcPhos-ligands interacting with uranyl cations. These AcPhosligands could contain two P−OEt groups, two P−OH groups, or one of each.…”

“…12 There have been several investigations of U(VI) extraction using diethylphosphonate-modified mesoporous silica. [13][14][15][16][17][18][19][20][21] Additionally, Dudarko et al used phosphonic acid tethered to mesoporous silica for the extraction of trace U(VI) from aqueous solution under mildly basic conditions. 22 Fryxell et al report the anchoring of the AcPhos functional group to silica supports as an extension of the sol-gel process.…”