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Surface-functionalized Mn–Fe oxide nanocrystals (NCs) were evaluated for single- and multi-sorbate scenarios considering As(V), Cr(VI), and U(VI) in varied water chemistries (deionized (DI), ground, and sea water) at pH 7.0. Multi-sorbate scenarios were further examined for competitive and/or cooperative effects. Precisely synthesized manganese ferrite (MnFe2O4) NCs were compared with iron oxide (Fe3O4) and manganese oxide (MnxOy) nanocrystal cores in terms of sorption capacities and colloidal stabilities. Positively charged cetyltrimethylammonium bromide (CTAB) and negatively charged oleyl phosphate (OP) were evaluated and compared as organic coatings. MnFe2O4 NCs exhibited both enhanced sorption performance and colloidal stability compared with Fe3O4 and MnxOy NC cores when functionalized with the same surfactant coating. For MnFe2O4 NCs, maximum sorption of As(V), Cr(VI), and U(VI) were observed to be 2.62, 3.43, and 4.27 mmol g−1, respectively (in DI water). Relative sorption capacity enhancement (compared with Fe3O4 and MnxOy) is due to increased surface grafting densities of MnFe2O4 NCs, providing a larger number of sorption sites (functional group) for target sorbates and higher repulsive energy (osmotic and elastic–steric interaction) for increased stability, and thus maintaining available surface area. For As(V) and Cr(VI) multi-sorbate systems, all materials evaluated preferentially adsorbed As(V) over Cr(VI). This preference was further investigated and observed using a novel quartz crystal microbalance (QCM) technique.

The application of positively charged polymers is a common treatment strategy for the sorption and separation of dissolved hexavalent chromium Cr(VI). In particular, polyethylenimine (PEI) has been demonstrated as an effective polymer for Cr treatment due to abundant amine groups and cost-effectiveness. However, PEI as a photoactive polymer has not been previously explored for Cr treatment. Toward this, we demonstrate the significant photoactive potential of PEI for Cr sorption and reduction [to Cr (III)], delineating amine structure-function relationships. Overall, Cr(VI) photoreduction is enhanced dramatically as a function of amine group density. Additionally, among the types of amine groups evaluated, tertiary amines are observed to be the most reactive followed by secondary and primary amines. When PEI is incorporated onto graphene oxide (GO) as a stabilizing scaffold, mass loading of PEI is a key variable for Cr(VI) treatment performance, which is observed to be a function of PEI molecular weight and oxidation extent of GO. Finally, when combined with magnetic nanomaterials, low energy, high efficiency separation, and reuse are demonstrated. For these, sorption-based separation processes maintained excellent performance over five cycles. Taken together, this work demonstrates that PEI can serve as highly effective, multifunctional material coating(s) not only for chromium treatment but potentially for other oxidized aqueous species as well.

Photoactive cetyltrimethylammonium bromide-oleic acid (CTAB-OA) bilayer coated iron oxide nanocrystals (IONCs) were precisely synthesized and demonstrated for photo-enhanced hexavalent chromium (Cr(VI)) treatment. For these materials, an outward facing, CTAB layer offers specific affinity for Cr(VI), while the core IONC (8 – 25 nm, as Fe3O4 single domain) surface participates in Fe(II)/Fe(III) redox cycling. Further, at the coating-particle interface, the OA layer alters material photoactive dynamics (near UV and visible light) via a ligand-to-metal charge transfer (LMCT) mechanism, significantly enhancing Cr(VI) reduction. To delineate process variables, we first quantified photo-catalytic Cr(VI) reduction rates under UVA irradiation (sunlight mimic intensities) as a function of IONC core size, which correlates with the amount of surface available ferrous ion (Fe(II)). It was observed that 25 nm CTAB functionalized IONC showed outstanding Cr(VI) sorption and photoreduction performance compared with other IONC explored and controls (i.e. TiO2 nanocrystals). We next systematically explored the photoreduction mechanism(s) through a matrix of control experiments, considering solution pH, dissolved gas (air, O2, and CO2), along with hydroxyl and superoxide radicals. Results indicate that Fe(II) (at the IONC interface) acts as the primary Cr(VI) reductant and that Fe(III) (oxidized from Fe(II)) (re)cycles via a photocatalyzed reaction. Taken together, this work presents a clear physical and mechanistic description of CTAB-OA bilayer IONCs which, when optimized, are highly effective for Cr(VI) treatment under UVA. In addition to potential broad application for other contaminant targets which are reactive with Fe(II), which are many, this work also highlights the need for precise interfacial understanding of nano composite materials towards process optimization.

In this work, we have developed and characterized flux-enhanced graphene oxide laminate (GOL) membranes by increasing interlayer (layer-to-layer) spacing using multibranched polyethylenimine (PEI) polymers with varied molecular weights and by controlling the graphene oxide (GO) oxidation extent. For these assemblies, water flux was demonstrated to increase by as much as ca. 30-fold compared to GO only laminate controls. PEI-embedded GOL membranes also had better methyl orange (MO) rejection performance than GO laminate controls due to the dilution effects (i.e., water is transported through the assembly much faster than MO). Further, boron removal is demonstrated via functionalized PEI with d-glucono-1,5-lactone, containing a high density of boron chelating groups, which can also be recycled/recovered with high efficiency.

Inorganic–organic nanoscale materials have recently received significant interest as tunable, next generation, sorbents for the separation of metal and metalloid contaminants, including arsenic (As) and chromium (Cr), among others. In this work, we have designed and synthesized IONCs coated with specific functionalized organic materials with the goal of variable explicit evaluation. Specifically, single domain, superparamagnetic, monodisperse IONCs were synthesized and transferred into water via surface functionalization by ligand exchange and encapsulation methods. The as synthesized hybrid materials showed high performance for both As(V) and Cr(VI) sorption when nanocrystals are coated with positively charged organic surface coatings such as polyethyleneimine (PEI) and cetyltrimethylammonium bromide (CTAB). IONC cores coated with negatively charged organic coating materials (polyethyleneglycol (PEG), oleic acid (OA), sodium dodecyl sulfate (SDS)) and silica (SiO2) demonstrated significantly lower sorption capacities. When silica coated IONCs (Fe3O4@SiO2, core–shell materials) were surface coated with PEI, sorption capacities for As(V) and Cr(VI) of Fe3O4@SiO2@PEI are comparable to Fe3O4@PEI, underscoring the importance of surface coating functionality. To complement these studies, real-time sorption behavior of As(V) and Cr(VI) with PEI was explored by quartz crystal microbalance with dissipation (QCM-D).

Developments in nanoscale engineering now allow for molecular scale optimization of reactivity, sorption, and magnetism, among other properties, for advanced, material-based environmental applications, including sorption, separation, and sensing of radionuclides. Herein, we describe novel, monodisperse nanoscale manganese ferrite crystals (MnFe2O4) for ultra-high capacity environmental sorption and subsequent separation of uranyl in water. System optimization was explored as a function of nanocrystal (core) composition, surface coating(s), and water chemistry. 11 nm MnFe2O4 nanocrystals, which were colloidally stabilized via engineered oleyl-based surface bilayers, demonstrate extreme, yet specific, uranium binding capacities while remaining monomerically stable under environmentally relevant conditions (water chemistries), which are key for application. In particular, MnFe2O4 cores with oleyl phosphate (as the outer facing layer) bilayers demonstrate preferential uranium binding of >150% (uranium weight)/(particle system weight) while being highly water stable in elevated ionic strengths/types and pH (up to 235.4 ppm (10.24 mM) of NaCl and 51.3 ppm (1.28 mM) of CaCl2, in addition to 60 ppm of uranyl, pH 5–9). Further, when normalized for size and surface coatings, MnFe2O4 nanocrystals had significantly enhanced sorption capacities compared to Mn2FeO4, Fe3O4 and manganese oxide core analogs. Mechanistically, we demonstrate that observed uranium sorption enhancement is due not only to thermodynamically favorable interfacial interactions (for both particle and selected bilayer coatings), but also due to significant uranyl reduction at the particle interface itself. Uranium sorption capacities for optimized systems described are the highest of any material reported to date.

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 mechanism(s) 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.

Here, we evaluate H2 generation, via NaBH4 hydrolysis, for a rationally designed library of cobalt based metal oxide nanocrystals. Specifically, we developed highly surface active wusite cobalt oxide nanorods (wz-CoO-NRs) with varied dimensions through the thermal decomposition of cobalt oleate (Co-OL) and cobalt stearate (Co-ST) – yielding multiple ‘nail’ structures grown along (002) direction. The length of wz-CoO-NRs (from 30 to 200 nm) was controlled by the ratio of Co-OL to Co-ST, temperature ramp rate, and reaction time. Further, by adjusting oleic acid concentration (as a surfactant) and/or (other) metal co-precursors (e.g. iron oleate (Fe-OL) and nickel oleate (Ni-OL)), shape (e.g. nanorod to urchin-like, irregular cube, flower, or sphere), crystal structure(s), and composition (from wz-CoO to cubic type CoO (cu-CoO) or ferrite metal oxides (Fe3O4, CoFe2O4, and Co0.5Ni0.5Fe2O4)) were serially tuned. The library of cobalt based metal oxide nanocrystals was compared, as catalysts, for H2 generation via NaBH4 decomposition, and performance was observed to be a function of crystal length for wz-CoO-NRs. A maximum hydrogen production rate of 10,367 mL min−1 g−1 at 293 K with an activation energy of 27.4 kJ mol-1 was observed for 200 nm wz-CoO-NRs, which is among the lowest reported activation energies for this reaction. Enhanced reactivity is likely due to increased favorable surface chemistries (multiple catalytically active (002) sites on the edge of multiple nails) and lowered charge transfer energy for longer (e.g. 200 nm) wz-CoO-NRs.

The physical and chemical factors affecting boron solute flux behavior and membrane transport mechanisms in forward osmosis (FO) have been systematically investigated. Boron solute flux behavior in FO was further compared with that in reverse osmosis (RO) by employing identical plate-and-frame cells and membranes under the same filtration conditions. The influence of draw solution pH, draw solution type, and membrane orientation on boron solute flux was examined for FO, and the effects of water flux, cross-flow velocity, feed water boron concentration, and solution pH on boron solute flux were examined for both FO and RO. Results show that reverse salt diffusion, a unique feature of FO, is a key mechanism governing boron solute flux in FO. Boron solute flux through the FO membrane was inversely proportional to the degree of reverse salt diffusion by draw solution. The higher boron rejection observed in FO compared to RO is also attributed to reverse salt diffusion in FO. It is also shown that membrane orientation in FO plays an important role, affecting boron solute flux due to different degrees of internal concentration polarization. In both FO and RO, boron solute flux increased with increasing water flux. However, the influence of water flux on boron solute flux was less significant in FO than RO. Furthermore, boron solute flux decreased with increasing feed water pH due to the conversion of the neutral boric acid to borate anions. The findings provide new insight into the mechanisms and factors controlling boron solute transport in FO. © 2012 Elsevier B.V.

Soft phase magnetic Fe-B nanoparticles were prepared by coprecipitation and the hard phase magnetic Nd-Fe-B nanoparticles were obtained from mechanical ball mill of the bulk Nd-Fe-B with a shaker mill. Also mechanical ball mill technique was applied to the mixture of Nd-Fe-B and Fe-B nanoparticles to build up an exchange-coupled nanocomposite magnet. It was loaded in electronic furnace and annealed under a flow of hydrogen at 953 K for 30 min. The exchange coupled interaction of isotropic magnets was characterized by smooth hysteresis loop with a high value of remanence (Mr/Ms > 0.5).