William A. Phillip

The Future of Seawater Desalination: Energy, Technology, and the Environment

In recent years, numerous large-scale seawater desalination plants have been built in water-stressed countries to augment available water resources, and construction of new desalination plants is expected to increase in the near future. Despite major advancements in desalination technologies, seawater desalination is still more energy intensive compared to conventional technologies for the treatment of fresh water. There are also concerns about the potential environmental impacts of large-scale seawater desalination plants. Here, we review the possible reductions in energy demand by state-of-the-art seawater desalination technologies, the potential role of advanced materials and innovative technologies in improving performance, and the sustainability of desalination as a technological solution to global water shortages.

High Performance Thin-Film Composite Forward Osmosis Membrane

Recent studies show that osmotically driven membrane processes may be a viable technology for desalination, water and wastewater treatment, and power generation. However, the absence of a membrane designed for such processes is a significant obstacle hindering further advancements of this technology. This work presents the development of a high performance thin-film composite membrane for forward osmosis applications. The membrane consists of a selective polyamide active layer formed by interfacial polymerization on top of a polysulfone support layer fabricated by phase separation onto a thin (40 mum) polyester nonwoven fabric. By careful selection of the polysulfone casting solution (i.e., polymer concentration and solvent composition) and tailoring the casting process, we produced a support layer with a mix of finger-like and sponge-like morphologies that give significantly enhanced membrane performance. The structure and performance of the new thin-film composite forward osmosis membrane are compared with those of commercial membranes. Using a 1.5 M NaCl draw solution and a pure water feed, the fabricated membranes produced water fluxes exceeding 18 L m(2-)h(-1), while consistently maintaining observed salt rejection greater than 97%. The high water flux of the fabricated thin-film composite forward osmosis membranes was directly related to the thickness, porosity, tortuosity, and pore structure of the polysulfone support layer. Furthermore, membrane performance did not degrade after prolonged exposure to an ammonium bicarbonate draw solution.

Reverse draw solute permeation in forward osmosis: modeling and experiments.

Osmotically driven membrane processes are an emerging set of technologies that show promise in water and wastewater treatment, desalination, and power generation. The effective operation of these systems requires that the reverse flux of draw solute from the draw solution into the feed solution be minimized. A model was developed that describes the reverse permeation of draw solution across an asymmetric membrane in forward osmosis operation. Experiments were carried out to validate the model predictions with a highly soluble salt (NaCl) as a draw solution and a cellulose acetate membrane designed for forward osmosis. Using independently determined membrane transport coefficients, strong agreement between the model predictions and experimental results was observed. Further analysis shows that the reverse flux selectivity, the ratio of the forward water flux to the reverse solute flux, is a key parameter in the design of osmotically driven membrane processes. The model predictions and experiments demonstrate that this parameter is independent of the draw solution concentration and the structure of the membrane support layer. The value of the reverse flux selectivity is determined solely by the selectivity of the membrane active layer.

Thin-film composite pressure retarded osmosis membranes for sustainable power generation from salinity gradients

Pressure retarded osmosis has the potential to produce renewable energy from natural salinity gradients. This work presents the fabrication of thin-film composite membranes customized for high performance in pressure retarded osmosis. We also present the development of a theoretical model to predict the water flux in pressure retarded osmosis, from which we can predict the power density that can be achieved by a membrane. The model is the first to incorporate external concentration polarization, a performance limiting phenomenon that becomes significant for high-performance membranes. The fabricated membranes consist of a selective polyamide layer formed by interfacial polymerization on top of a polysulfone support layer made by phase separation. The highly porous support layer (structural parameter S = 349 μm), which minimizes internal concentration polarization, allows the transport properties of the active layer to be customized to enhance PRO performance. It is shown that a hand-cast membrane that balances permeability and selectivity (A = 5.81 L m(-2) h(-1) bar(-1), B = 0.88 L m(-2) h(-1)) is projected to achieve the highest potential peak power density of 10.0 W/m(2) for a river water feed solution and seawater draw solution. The outstanding performance of this membrane is attributed to the high water permeability of the active layer, coupled with a moderate salt permeability and the ability of the support layer to suppress the undesirable accumulation of leaked salt in the porous support. Membranes with greater selectivity (i.e., lower salt permeability, B = 0.16 L m(-2) h(-1)) suffered from a lower water permeability (A = 1.74 L m(-2) h(-1) bar(-1)) and would yield a lower peak power density of 6.1 W/m(2), while membranes with a higher permeability and lower selectivity (A = 7.55 L m(-2) h(-1) bar(-1), B = 5.45 L m(-2) h(-1)) performed poorly due to severe reverse salt permeation, resulting in a similar projected peak power density of 6.1 W/m(2).

Self-Assembled Block Copolymer Thin Films as Water Filtration Membranes

Nanoporous membranes containing monodisperse pores of 24 nm diameter are fabricated using poly(styrene-b-lactide) block copolymers to template the pore structure. A 4 mum thin film of the block copolymer is cast onto a microporous membrane that provides mechanical reinforcement; by casting the copolymer film from the appropriate solvents and controlling the solvent evaporation rate, greater than 100 cm(2) of a thin film with polylactide cylinders oriented perpendicular to the thin dimension is produced. Exposing the composite membrane to a dilute aqueous base selectively etches the polylactide block, producing the porous structure. The ability of these pores to reject dissolved poly(ethylene oxide) molecules of varying molecular weight matches existing theories for transport through small pores.

Forward with Osmosis: Emerging Applications for Greater Sustainability

Many conventional practices in the production and use of water, energy, and food are unsustainable. Existing technologies and concepts can be improved with the integration of forward osmosis, a membrane-based technology that uses osmosis as its driving force. This Feature highlights five emerging applications of forward osmosis that elegantly bypass the difficult step of draw solution regeneration and make common processes more sustainable. These applications enhance the efficiency of the production and use of water, energy, and food; utilize wastes and abundant, low value resources; and better protect the environment.

Tuning Structure and Properties of Graded Triblock Terpolymer-Based Mesoporous and Hybrid Films

Despite considerable efforts toward fabricating ordered, water-permeable, mesoporous films from block copolymers, fine control over pore dimensions, structural characteristics, and mechanical behavior of graded structures remains a major challenge. To this end, we describe the fabrication and performance characteristics of graded mesoporous and hybrid films derived from the newly synthesized triblock terpolymer, poly(isoprene-b-styrene-b-4-vinylpyridine). A unique morphology, unachievable in diblock copolymer systems, with enhanced mechanical integrity is evidenced. The film structure comprises a thin selective layer containing vertically aligned and nearly monodisperse mesopores at a density of more than 10(14) per m(2) above a graded macroporous layer. Hybridization via homopolymer blending enables tuning of pore size within the range of 16 to 30 nm. Solvent flow and solute separation experiments demonstrate that the terpolymer films have permeabilities comparable to commercial membranes, are stimuli-responsive, and contain pores with a nearly monodisperse diameter. These results suggest that moving to multiblock polymers and their hybrids may open new paths to produce high-performance graded membranes for filtration, separations, nanofluidics, catalysis, and drug delivery.

Bidirectional Permeation of Electrolytes in Osmotically Driven Membrane Processes

Osmotically driven membrane processes (ODMP) are emerging water treatment and energy conversion technologies. In this work, we investigated the simultaneous forward and reverse (i.e., bidirectional) solute fluxes that occur in ODMP. Numerous experiments were conducted using ternary systems (i.e., systems containing three distinct ions) and quaternary systems (i.e., systems containing four distinct ions) in conjunction with a membrane in a forward osmosis orientation. Ten different combinations of strong electrolyte salts constitute the ternary systems; common anion systems studied included KCl-NaCl, KBr-NaBr, KNO(3)-NaNO(3), KCl-CaCl(2), and KCl-SrCl(2); and common cation systems explored were KCl-KH(2)PO(4), NaCl-NaClO(4), NaCl-Na(2)SO(4), NaCl-NaNO(3), and CaCl(2)-Ca(NO(3))(2). For each combination, two experiments were conducted with each salt being used once in the draw solution and once in the feed solution. Quaternary systems studied were NaCl-KNO(3), NaCl-MgSO(4), MgSO(4)-KNO(3), and NaCl-K(2)SO(4). Experimental fluxes of the individual ions were quantified and compared to a set of equations developed to predict bidirectional electrolyte permeation for ODMP in a forward osmosis orientation. Results demonstrate that ion fluxes from the draw solution to the feed solution are well predicted; however, ion fluxes from the feed solution to the draw solution show slight deviations from the model that can be rationalized in terms of the electrostatic interactions between charged ions. The model poorly predicts the flux of nitrate containing solutions; however, several unique mass transfer mechanisms are observed with implications for ODMP process design.

Diffusion and flow across nanoporous polydicyclopentadiene-based membranes.

We report gas and liquid transport measurements through membranes that have 40% voids made of 14 nm pores. A reactive polylactide-polynorbornenylethylstyrene block polymer is used as a structural template in the polymerization of dicyclopentadiene during the membrane formation process. After the membrane is cast, the pore structure is formed by etching the polylactide component using dilute aqueous base. The pore structure is isotropic; therefore, there is no need for special alignment techniques. Knudsen diffusion experiments and water flow experiments show pores with a tortuosity of 1.81 and a size of 14 nm, a diameter consistent with nitrogen adsorption and small-angle X-ray scattering measurements. These membranes are effective for ultrafiltration, with molecular weight cutoffs (MWCO) consistent with theoretical predictions with no adjustable parameters. These MWCOs can be tuned by changing the size of the constituent blocks in the templating copolymer.

Mixed Mosaic Membranes Prepared by Layer-by-Layer Assembly for Ionic Separations

Charge mosaic membranes, which possess distinct cationic and anionic domains that traverse the membrane thickness, are capable of selectively separating dissolved salts from similarly sized neutral solutes. Here, the generation of charge mosaic membranes using facile layer-by-layer assembly methodologies is reported. Polymeric nanotubes with pore walls lined by positively charged polyethylenimine moieties or negatively charged poly(styrenesulfonate) moieties were prepared via layer-by-layer assembly using track-etched membranes as sacrificial templates. Subsequently, both types of nanotubes were deposited on a porous support in order to produce mixed mosaic membranes. Scanning electron microscopy demonstrates that the facile deposition techniques implemented result in nanotubes that are vertically aligned without overlap between adjacent elements. Furthermore, the nanotubes span the thickness of the mixed mosaic membranes. The effects of this unique nanostructure are reflected in the transport characteristics of the mixed mosaic membranes. The hydraulic permeability of the mixed mosaic membranes in piezodialysis operations was 8 L m(-2) h(-1) bar(-1). Importantly, solute rejection experiments demonstrate that the mixed mosaic membranes are more permeable to ionic solutes than similarly sized neutral molecules. In particular, negative rejection of sodium chloride is observed (i.e., the concentration of NaCl in the solution that permeates through a mixed mosaic membrane is higher than in the initial feed solution). These properties illustrate the ability of mixed mosaic membranes to permeate dissolved ions selectively without violating electroneutrality and suggest their utility in ionic separations.