Science Inventory

Sequestration of U(VI) from Acidic, Alkaline, and High Ionic-Strength Aqueous Media by Functionalized Magnetic Mesoporous Silica Nanoparticles: Capacity and Binding Mechanisms

Citation:

Li, D., S. Egodawatte, D. Kaplan, S. Larsen, S. Serkiz, J. Seaman, K. Scheckel, J. Lin, AND Y. Pan. Sequestration of U(VI) from Acidic, Alkaline, and High Ionic-Strength Aqueous Media by Functionalized Magnetic Mesoporous Silica Nanoparticles: Capacity and Binding Mechanisms. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 51(24):14330-14341, (2017). https://doi.org/10.1021/acs.est.7b03778

Impact/Purpose:

In this work, we developed magnetic mesoporous silica nanoparticles (MMSNs) that were further grafted with dihydroimidazole (DIM), polyacryloamidoxime (AD), phosphonate (PP), phosphonate-amino (PPA), poly(propylenimine) dendrimer (PPI), and poly(amidoamine) dendrimer (PM). The new functionalized MMSNs are magnetic for the purpose of easy postuse retrieval, possess high surface areas and mesopore structures to provide more active binding sites, and greater accessibility, with organic binding ligands that can selectively retain U and improve adsorption capacity. The objectives of this work were as follows: (1) evaluate the adsorption capacities using batch adsorption experiments of these functionalized MMSNs for removing U from three model model systems: high salt water (HSW) simulant, low pH artificial groundwater (AGW) and high pH AGW; (2) identify the U speciation sorbed to the functionalized MMSNs using U L3-edge synchrotron X-ray absorption near structure (XANES), Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) spectroscopies; and (3) elucidate the molecular mechanisms responsible for U species binding to the functionalized MMSNs surfaces using U L3-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. One of the key novel findings in this work is that the dominant U species bound to the most effective sorbents were uranyl or uranyl hydroxide, rather than the expected uranyl carbonate species. It is further postulated that the surface functional groups of the MMSNs out-competed the carbonate ligands associated with the aqueous U species. This information is of interest to Regional and Program Office decision makers, States, and local affected communities.

Description:

Uranium (VI) exhibits little adsorption onto sediment minerals in acidic, alkaline or high ionic-strength aqueous media that often occur in U mining or contaminated sites, which makes U(VI) very mobile and difficult to sequester. In this work, magnetic mesoporous silica nanoparticles (MMSNs) were functionalized with several organic ligands. The functionalized MMSNs were highly effective and had large binding capacity for U sequestration from high salt water (HSW) simulant (54 mg U/g sorbent). The functionalized MMSNs, after U exposure in HSW simulant, pH 3.5 and 9.6 artificial groundwater (AGW), were characterized by a host of spectroscopic methods. Among the key novel findings in this work was that in the HSW simulant or high pH AGW, the dominant U species bound to the functionalized MMSNs were uranyl or uranyl hydroxide, rather than uranyl carbonates as expected. The surface functional groups appear to be out-competing the carbonate ligands associated with the aqueous U species. The uranyl-like species were bound with N ligand as η² bound motifs or phosphonate ligand as a monodentate, as well as on tetrahedral Si sites as an edge-sharing bidentate. The N and phosphonate ligand-functionalized MMSNs hold promise as effective sorbents for sequestering U from acidic, alkaline or high ionic-strength contaminated aqueous media.