In Situ Photocatalytic Synthesis of Ag Nanoparticles (nAg) by Crumpled Graphene Oxide Composite Membranes for Filtration and Disinfection Applications.

Graphene oxide (GO) materials have demonstrated considerable potential in next-generation water treatment membrane-based technologies, which include antimicrobial applications. GO antimicrobial properties can be further enhanced by preloading or chemically generating surface-associated nanoscale silver particles (nAg). However, for these systems, enhanced antimicrobial functionality decreases over time as a function of Ag mass loss via dissolution (as Ag(+)). In this work, we demonstrate facile photocatalytic in situ synthesis of nAg particles by crumpled GO-TiO2 (GOTI) nanocomposites as an approach to (re)generate, and thus maintain, enhanced antimicrobial activity over extended operation times. The described photocatalytic formation process is highly efficient and relatively fast, producing nAg particles over a size range of 40 to 120 nm and with active (111) planes. Additionally, we show in situ surface-based photocatalyzed synthesis of nAg particles at the surface of GOTI nanocomposite membrane assemblies, allowing for simultaneous filtration and disinfection. With ca. 3 log inactivation for both Escherichia coli and Bacillus subtilis, the described membrane assemblies with in situ formed nAg demonstrate enhanced antimicrobial activity compared to the GOTI membrane surface or the support membrane alone. Under typical conditions, the working and operational time (Ag dissolution time) is calculated to be over 2 orders of magnitude higher than the loading (synthesis) time (e.g., 123 h versus 0.5 h, respectively). Taken together, results highlight the described material-based process as a potentially novel antifouling membrane technology.

Engineered crumpled graphene oxide nanocomposite membrane assemblies for advanced water treatment processes.

In this work, we describe multifunctional, crumpled graphene oxide (CGO) porous nanocomposites that are assembled as advanced, reactive water treatment membranes. Crumpled 3D graphene oxide based materials fundamentally differ from 2D flat graphene oxide analogues in that they are highly aggregation and compression-resistant (i.e., π-π stacking resistant) and allow for the incorporation (wrapping) of other, multifunctional particles inside the 3D, composite structure. Here, assemblies of nanoscale, monomeric CGO with encapsulated (as a quasi core-shell structure) TiO2 (GOTI) and Ag (GOAg) nanoparticles, not only allow high water flux via vertically tortuous nanochannels (achieving water flux of 246 ± 11 L/(m(2)·h·bar) with 5.4 µm thick assembly, 7.4 g/m(2)), outperforming comparable commercial ultrafiltration membranes, but also demonstrate excellent separation efficiencies for model organic and biological foulants. Further, multifunctionality is demonstrated through the in situ photocatalytic degradation of methyl orange (MO), as a model organic, under fast flow conditions (tres < 0.1 s); while superior antimicrobial properties, evaluated with GOAg, are observed for both biofilm (contact) and suspended growth scenarios (>3 log effective removal, Escherichia coli). This is the first demonstration of 3D, crumpled graphene oxide based nanocomposite structures applied specifically as (re)active membrane assemblies and highlights the material's platform potential for a truly tailored approach for next generation water treatment and separation technologies.

Aqueous aggregation and surface deposition processes of engineered superparamagnetic iron oxide nanoparticles for environmental applications.

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).

Reduction of hydroxylated fullerene (fullerol) in water by zinc: reaction and hemiketal product characterization.

Water-soluble, hydroxylated fullerene (fullerol) materials have recently gained increasing attention as they have been identified as the primary product(s) during the exposure of fullerenes (as water stable, nanoscale aggregated C60) to UV light in water. The physical properties and chemical reactivity of resulting fullerols, however, have not been thoroughly studied. In this paper, we identified and characterized the reductive transformation of fullerol (C60(OH)x(ONa)y) by solid zinc metal (Zn(0)) through a series of batch reaction experiments and product characterization, including (13)C NMR, FTIR, XPS, UV-vis, DLS, and TEM. Results indicated the facile formation of water stable, pH sensitive hemiketal functionality as part of a relatively reduced fullerol product. Further, aqueous physical behavior of the product fullerol, as measured by octanol partitioning and surface deposition rates, was observed to significantly differ from the parent material and is consistent with a relative increase in molecular (product) hydrophobicity.

Harnessing the potential of in-situ, electrically generated microbubbles via nickel foam for enhanced, low energy membrane fouling control.

For membrane-based, water treatment technologies, fouling remains a significant challenge for pressure-driven processes. While many antifouling strategies have been proposed, there remains significant room for improved efficiency. Direct application of microbubbles (MBs) at a membrane surface offers a promising approach for managing interfacial fouling through continuous physical interaction(s). Despite such potential, to date, integration and optimization of in-situ generated MBs at the membrane interface that are both highly antifouling with minimal energy inputs and unwanted side reactions remains mostly outstanding. Here we demonstrate the application of conductive, porous nickel foam for electrolysis-based generation of hydrogen microbubbles at an ultra-filtration (UF) membrane interface, which significantly mitigates membrane fouling for a range of model foulants. System characterization and optimization includes comparison of metal foams (Ni, Cu, Ti), faradic efficiencies, hydrogen evolution reaction (HER) curves, cyclic voltammetry, and quantification of hydrogen gas flux and bubble size, as a function of applied current. When optimized, we report rapid (<5 min) and near complete (∼99 %) flux recovery for three classes of foulants, including calcium alginate, humic acid (HA), and SiO2 particles. For all, the described MB-based approach is orders of magnitude more energy efficient when compared to conventional cleaning strategies. Finally, we demonstrate the MB-based regeneration/cleaning process is stable and repeatable for ten cycles and also highly effective for a challenge water (as a model oilfield brine). Taken together, this work presents a novel and efficient approach for the application of in-situ electrically generated MBs to support sustainable pressure-driven membrane processes.

Ultralow protein adsorbing coatings from clickable PEG nanogel solutions: benefits of attachment under salt-induced phase separation conditions and comparison with PEG/albumin nanogel coatings.

Clickable nanogel solutions were synthesized by using the copper catalyzed azide/alkyne cycloaddition (CuAAC) to partially polymerize solutions of azide and alkyne functionalized poly(ethylene glycol) (PEG) monomers. Coatings were fabricated using a second click reaction: a UV thiol-yne attachment of the nanogel solutions to mercaptosilanated glass. Because the CuAAC reaction was effectively halted by the addition of a copper-chelator, we were able to prevent bulk gelation and limit the coating thickness to a single monolayer of nanogels in the absence of the solution reaction. This enabled the inclusion of kosmotropic salts, which caused the PEG to phase-separate and nearly double the nanogel packing density, as confirmed by quartz crystal microbalance with dissipation (QCM-D). Protein adsorption was analyzed by single molecule counting with total internal reflection fluorescence (TIRF) microscopy and cell adhesion assays. Coatings formed from the phase-separated clickable nanogel solutions attached with salt adsorbed significantly less fibrinogen than other 100% PEG coatings tested, as well as poly(L-lysine)-g-PEG (PLL-g-PEG) coatings. However, PEG/albumin nanogel coatings still outperformed the best 100% PEG clickable nanogel coatings. Additional surface cross-linking of the clickable nanogel coating in the presence of copper further reduced levels of fibrinogen adsorption closer to those of PEG/albumin nanogel coatings. However, this step negatively impacted long-term resistance to cell adhesion and dramatically altered the morphology of the coating by atomic force microscopy (AFM). The main benefit of the click strategy is that the partially polymerized solutions are stable almost indefinitely, allowing attachment in the phase-separated state without danger of bulk gelation, and thus producing the best performing 100% PEG coating that we have studied to date.

Measuring the grafting density of nanoparticles in solution by analytical ultracentrifugation and total organic carbon analysis.

Many of the solution phase properties of nanoparticles, such as their colloidal stability and hydrodynamic diameter, are governed by the number of stabilizing groups bound to the particle surface (i.e., grafting density). Here, we show how two techniques, analytical ultracentrifugation (AUC) and total organic carbon analysis (TOC), can be applied separately to the measurement of this parameter. AUC directly measures the density of nanoparticle-polymer conjugates while TOC provides the total carbon content of its aqueous dispersions. When these techniques are applied to model gold nanoparticles capped with thiolated poly(ethylene glycol), the measured grafting densities across a range of polymer chain lengths, polymer concentrations, and nanoparticle diameters agree to within 20%. Moreover, the measured grafting densities correlate well with the polymer content determined by thermogravimetric analysis of solid conjugate samples. Using these tools, we examine the particle core diameter, polymer chain length, and polymer solution concentration dependence of nanoparticle grafting densities in a gold nanoparticle-poly(ethylene glycol) conjugate system.

In-situ sequestration of perfluoroalkyl substances using polymer-stabilized ion exchange resin.

Remediation of groundwater impacted by per- and polyfluoroalkyl substances (PFAS) is challenging due to the strength of the carbon-fluorine bond and the need to achieve nanogram per liter drinking water targets. Previous studies have shown that ion exchange resins can serve as effective sorbents for the removal of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in conventional water treatment systems. The objectives of this study were to evaluate the in situ delivery and PFAS sorption capacity of a polymer-stabilized ion exchange resin (S-IXR) consisting of Amberlite® IRA910 beads and Pluronic® F-127 in a quartz sand. At concentrations below 100 µg/L, individual and mixed PFAS adsorption on resin beads exhibited linear isotherms with no apparent competitive effects. However, at concentrations up to 100 mg/L, PFAS adsorption isotherms were non-linear and a mixture of six PFAS exhibited strong competitive effects. In columns packed with 40-50 mesh Ottawa sand, injection of the S-IXR suspension created a uniform sorptive zone that increased PFOA or PFOS retention by more than five orders-of-magnitude compared to untreated control columns. Multi-solute column studies revealed earlier breakthrough of shorter-chain length PFAS, which was consistent with the mixed PFAS adsorption data. These findings indicate that injectable ion exchange resins could provide an effective in situ remediation strategy for PFAS-impacted groundwater plumes.

Graphene oxide/mussel foot protein composites for high-strength and ultra-tough thin films.

Graphene oxide (GO)-based composite materials have become widely popular in many applications due to the attractive properties of GO, such as high strength and high electrical conductivity at the nanoscale. Most current GO composites use organic polymer as the matrix material and thus, their synthesis suffers from the use of organic solvents or surfactants, which raise environmental and energy-consumption concerns. Inspired by mussel foot proteins (Mfp) secreted by the saltwater mussel, Mytilus galloprovincialis and by recent advances in microbial protein production, we developed an aqueous-based green synthesis strategy for preparing GO/Mfp film composites. These GO/Mfp films display high tensile strength (134-158 MPa), stretchability (~ 26% elongation), and high toughness (20-24 MJ/m3), beyond the capabilities of many existing GO composites. Renewable production of Mfp proteins and the facile fabrication process described provides a new avenue for composite material synthesis, while the unique combination of mechanical properties of GO/Mfp films will be attractive for a range of applications.

Intrapore energy barriers govern ion transport and selectivity of desalination membranes.

State-of-the-art desalination membranes exhibit high water-salt selectivity, but their ability to discriminate between ions is limited. Elucidating the fundamental mechanisms underlying ion transport and selectivity in subnanometer pores is therefore imperative for the development of ion-selective membranes. Here, we compare the overall energy barrier for salt transport and energy barriers for individual ion transport, showing that cations and anions traverse the membrane pore in an independent manner. Supported by density functional theory simulations, we demonstrate that electrostatic interactions between permeating counterion and fixed charges on the membrane substantially hinder intrapore diffusion. Furthermore, using quartz crystal microbalance, we break down the contributions of partitioning at the pore mouth and intrapore diffusion to the overall energy barrier for salt transport. Overall, our results indicate that intrapore diffusion governs salt transport through subnanometer pores due to ion-pore wall interactions, providing the scientific base for the design of membranes with high ion-ion selectivity.

Real-time evaluation of natural organic matter deposition processes onto model environmental surfaces.

Natural organic matter (NOM) is ubiquitous in aqueous systems and dynamically partitions onto/from environmental surfaces. However, such interfacial processes have not been uniformly quantified in situ and in real time. In this work, adsorption and deposition processes of Suwannee River humic acid (SRHA) and Suwannee River fulvic acid (SRFA), as model NOM, were evaluated for a series of environmentally relevant interfaces. Real-time, interfacial phenomenon, including deposition, release, and adlayer viscoelastic properties, were quantified over a variety of water chemistries via quartz crystal microbalance with dissipation monitoring (QCM-D). Specifically, adlayer mass and deposition rates of SRHA and SRFA were evaluated as a function of NOM concentration/molecular weight (fraction), pH, electrolyte composition (type and concentration), and collector surface type. For these, the adsorption of SRHA onto aluminum oxide (Al2O3) and polystyrene (PS) surfaces follows the Langmuir isotherm model. Rapid, near-monolayer formation of SRHA/SRFA adlayers were observed on Al2O3, hydroxyapatite (HAP), and poly (l-lysine) (PLL) surfaces, but not on PS or iron oxide (Fe3O4) surfaces. The presence of divalent cations (Ca2+/Mg2+) at relatively low concentrations (0.5-5.0 mM) significantly enhances the mass of SRHA/SRFA deposited onto the surfaces of silica (SiO2), Al2O3, and PS. Viscoelastic properties of the adsorbed layer based on the ratio of dissipation to frequency revealed a relatively unique adlayer structure for SRHA in the presence of 5.0 mM Ca2+.