Engineered, superparamagnetic, iron oxide nanoparticles (IONPs) have significant potential as platform materials for environmental sensing, imaging and remediation due to their unique size, physicochemical and magnetic properties. To this end, controlling the size and surface chemistry of the materials is crucial for such applications in the aqueous phase, and in particular, for porous matrixes with particle-surface interaction considerations. In this study, superparamagnetic, highly monodispersed 8 nm IONPs were synthesized and transferred into water as stable suspensions (remaining monodispersed) by way of an interfacial oleic acid bilayer surface. Once stabilized and characterized, particle-particle and model surface interactions (deposition and release) were quantitatively investigated and described systematically as a function of ionic strength (IS) and type with time-resolved dynamic light scattering (DLS), zeta potential, and real-time quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. The critical coagulation concentration (CCC) for oleic acid bilayer coated iron oxide nanoparticles (OA-IONPs) were determined to be 710 mM for NaCl (matching DLVO predictions) and 10.6 mM for CaCl2, respectively. For all conditions tested, surface deposition kinetics showed stronger, more favorable interactions between OA-IONPs and polystyrene surfaces compared to silica, which is hypothesized to be due to increased particle-surface hydrophobic interactions (when compared to silica surfaces).

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For the first time, this work develops and demonstrates precisely engineered manganese oxide nanoscale particles for the sorption of uranium, as uranyl, in water. Size controlled monodisperse nanocrystalline manganese oxides (12 to 28 nm) were systematically synthesized via thermal decomposition of manganese oleate and phase-transferred into water by ligand exchange and bilayer stabilization methods. Resulting monodisperse suspensions demonstrate significantly enhanced uranyl adsorption as a function of size, surface coating chemistries, and solution pH. In particular, 12 nm particles coated with the unsaturated-unsaturated carbon chains linked bilayers, (e.g. oleic acid-oleyl phosphate linked bilayer coatings) have binding capacities well over 600 mg U per g of Mn, which is the highest reported uranium sorption capacity for any manganese based sorbent to date. Further, we spectrally identify significant uranyl reduction as part of the adsorption mechanismIJs) for high capacity materials. Last, oleyl-based (phosphate and carboxylic) functionalized bilayered nanocrystals were extremely stable in the presence of high ionic strength/type (>800 mM); calcium (>19 mM), including the presence of uranyl cations (from 0.1 to 60 ppm). Taken together, these data demonstrate the potential for engineered monodisperse manganese oxide nanocrystals as ultra-high capacity platform sorbent materials for uranium separation at environmentally relevant ionic strengths and pH.Environ. Sci.: Nano This journal is † Electronic supplementary information (ESI) available: Supporting information includes detailed descriptions for control over the diameter of manganese oxide nanocrystals, the histograms of the diameter distribution of manganese oxide nanocrystals, HR-TEM image of manganese oxide, uranium sorption isotherm on commercial samples IJFe 3 O 4 and MnO), size and surface coating thickness dependent uranium sorption isotherm on engineered PEGylated manganese oxide nanocrystals, size dependent uranium sorption isotherm on oleic acid bilayered manganese oxide nanocrystals, FT-IR measurement of engineered nanocrystals before and after uranium sorption, attachment efficiencies of both bilayered-and pegylated-manganese oxide nanocrystals, two tables of the maximum uranium sorption capacity and the sorption constant for the materials used in the research, two tables of XPS binding energies of individual peaks of the U4f and Mn2P spectra, and a table of hydrodynamic diameters of the nanocrystal samples designed in the research. See Environmental radionuclide detection, separation, and treatment remain widespread, global challenges. Advances in nanoscale engineering now allows for precisely tuned sorbent materials with extreme surface to volume ratios and specific bulk and surface chemistries for advanced environmental applications. In this work, for the first time, we have systematically developed and demonstrated manganese oxide nanocrystals as a platform material for ultra-high capacity uranium sorption and separation under environmentally ...

Herein, we describe engineered superparamagnetic iron oxide nanoparticles (IONPs) as platform materials for enhanced uranyl (UO) sorption and separation processes under environmentally relevant conditions. Specifically, monodispersed 8-25 nm iron oxide (magnetite, FeO) nanoparticles with tailored organic acid bilayered coatings have been systematically evaluated and optimized to bind, and thus remove, uranium from water. The combined nonhydrolytic synthesis and bilayer phase transfer material preparation methods yield highly uniform and surface tailorable IONPs, which allow for direct evaluation of the size-dependent and coating-dependent sorption capacities of IONPs. Optimized materials demonstrate ultrahigh sorption capacities (>50% by wt/wt) at pH 5.6 for 8 nm oleic acid (OA) bilayer and sodium monododecyl phosphate (SDP) surface-stabilized IONPs. Synchrotron-based X-ray absorption spectroscopy shows that iron oxide core particle size and stabilizing surface functional group(s) substantially affect U(VI)-removal mechanisms, specifically the ratio of uptake via adsorption versus reduction to U(IV). Taken together, tunable size and surface functionality, high colloidal stability, and favorable affinity toward uranium provide distinct synergistic advantage(s) for the application of bilayered IONPs as part of the next-generation material-based uranium recovery, remediation, and sensing technologies.

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