Maximizing the right stuff Park, Ho Bum; Kamcev, Jovan; Robeson, Lloyd M. ...
Science,
06/2017, Volume:
356, Issue:
6343
Journal Article
Peer reviewed
Open access
Increasing demands for energy-efficient separations in applications ranging from water purification to petroleum refining, chemicals production, and carbon capture have stimulated a vigorous search ...for novel, high-performance separation membranes. Synthetic membranes suffer a ubiquitous, pernicious trade-off: highly permeable membranes lack selectivity and vice versa. However, materials with both high permeability and high selectivity are beginning to emerge. For example, design features from biological membranes have been applied to break the permeability-selectivity trade-off. We review the basis for the permeability-selectivity trade-off, state-of-the-art approaches to membrane materials design to overcome the trade-off, and factors other than permeability and selectivity that govern membrane performance and, in turn, influence membrane design.
Fundamental water and salt transport properties of polymers are critical for applications such as reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO), pressure-retarded osmosis (PRO), and ...membrane capacitive deionization (MCDI) that require controlled water and salt transport. Key developments in the field of water and salt transport in polymer membranes are reviewed, and a survey of polymers considered for such applications is provided. Many polymers considered for such applications contain charged functional groups, such as sulfonate groups, that can dissociate in the presence of water. Water and ion transport data from the literature are reviewed to highlight the similarities and differences between charged and uncharged polymers. Additionally, the influence of other polymer structure characteristics, such as cross-linking and morphology in phase separated systems, on water and salt transport properties is discussed. The role of free volume on water and salt transport properties is discussed. The solution–diffusion model, which describes the transport of water and ions in nonporous polymers, is used as a framework for discussing structure/property relations in polymers related to water and salt transport properties. Areas where current knowledge is limited and opportunities for further research are also noted.
Membrane gas separation is a mature and expanding technology. However, the availability of better membrane materials would promote faster growth. In this Perspective we analyze the state of the art ...of membrane materials, including polymers and hybrid materials, as well as the current issues and barriers, and finally, we outline future research directions in membrane science. Development of new membrane materials for large scale separations will rely on a multidisciplinary approach that embraces the broad fields of chemical and materials engineering, polymer science, and materials chemistry.
Polymeric membranes are an energy‐efficient means of purifying water, but they suffer from fouling during filtration. Modification of the membrane surface is one route to mitigating membrane fouling, ...as it helps to maintain high levels of water productivity. Here, a series of common techniques for modification of the membrane surface are reviewed, including surface coating, grafting, and various treatment techniques such as chemical treatment, UV irradiation, and plasma treatment. Historical background on membrane development and surface modification is also provided. Finally, polydopamine, an emerging material that can be easily deposited onto a wide variety of substrates, is discussed within the context of membrane modification. A brief summary of the chemistry of polydopamine, particularly as it may pertain to membrane development, is also described.
Cleaning up: The efficiency of polymeric water purification membranes, which are capable of removing many impurities from water but frequently suffer from fouling, can be improved by surface modification. Common techniques used to modify membrane surfaces, including grafting, coating, chemical treatment, UV irradiation, plasma treatment, and polydopamine application, are reviewed.
Biological ion channels have remarkable ion selectivity, permeability and rectification properties, but it is challenging to develop artificial analogues. Here, we report a metal-organic ...framework-based subnanochannel (MOFSNC) with heterogeneous structure and surface chemistry to achieve these properties. The asymmetrically structured MOFSNC can rapidly conduct K
, Na
and Li
in the subnanometre-to-nanometre channel direction, with conductivities up to three orders of magnitude higher than those of Ca
and Mg
, equivalent to a mono/divalent ion selectivity of 10
. Moreover, by varying the pH from 3 to 8 the ion selectivity can be tuned further by a factor of 10
to 10
. Theoretical simulations indicate that ion-carboxyl interactions substantially reduce the energy barrier for monovalent cations to pass through the MOFSNC, and thus lead to ultrahigh ion selectivity. These findings suggest ways to develop ion selective devices for efficient ion separation, energy reservation and power generation.
Manning’s counterion condensation theory, originally developed for polyelectrolyte solutions, was used to predict ion activity coefficients in charged (i.e., ion exchange) membranes with no ...adjustable parameters. Equilibrium sodium and chloride ion concentrations in negatively and positively charged membranes were determined experimentally as a function of external NaCl concentration, and ion activity coefficients in the membranes were obtained via a thermodynamic treatment. Theoretical values for membrane ion activity coefficients obtained via Manning’s model were compared with those obtained experimentally. Good agreement was observed between the experimental and theoretical values for membrane ion activity coefficients, especially at higher external NaCl concentrations. However, some deviation between experimental and theoretical values was observed in the dilute regime. Manning’s model was also used to obtain activity coefficients for various electrolytes in ion exchange resins using ion sorption data from the literature, and these values were compared to those obtained experimentally.
Over the past three decades, polymeric gas separation membranes have become widely used for a variety of industrial gas separations applications. This review presents the fundamental scientific ...principles underpinning the operation of polymers for gas separations, including the solution-diffusion model and various structure/property relations, describes membrane fabrication technology, describes polymers believed to be used commercially for gas separations, and discusses some challenges associated with membrane materials development. A description of new classes of polymers being considered for gas separations, largely to overcome existing challenges or access applications that are not yet practiced commercially, is also provided. Some classes of polymers discussed in this review that have been the focus of much recent work include thermally rearranged (TR) polymers, polymers of intrinsic microporosity (PIMs), room-temperature ionic liquids (RTILs), perfluoropolymers, and high-performance polyimides.
Display omitted
Controllable fabrication of angstrom-size channels has been long desired to mimic biological ion channels for the fundamental study of ion transport. Here we report a strategy for fabricating ...angstrom-scale ion channels with one-dimensional (1D) to three-dimensional (3D) pore structures by the growth of metal-organic frameworks (MOFs) into nanochannels. The 1D MIL-53 channels of flexible pore sizes around 5.2 × 8.9 Å can transport cations rapidly, with one to two orders of magnitude higher conductivities and mobilities than MOF channels of hybrid pore configurations and sizes, including Al-TCPP with 1D ~8 Å channels connected by 2D ~6 Å interlayers, and 3D UiO-66 channels of ~6 Å windows and 9 - 12 Å cavities. Furthermore, the 3D MOF channels exhibit better ion sieving properties than those of 1D and 2D MOF channels. Theoretical simulations reveal that ion transport through 2D and 3D MOF channels should undergo multiple dehydration-rehydration processes, resulting in higher energy barriers than pure 1D channels. These findings offer a platform for studying ion transport properties at angstrom-scale confinement and provide guidelines for improving the efficiency of ionic separations and nanofluidics.
Membrane fouling is often characterized in the laboratory by flux decline experiments, where an increase in transport resistance due to accumulation of foulants on and/or in a membrane is manifested ...as a decrease in permeate flux with filtration time at fixed transmembrane pressure. However, many industrial microfiltration and ultrafiltration applications operate at constant permeate flux, and there are few reports comparing these modes of operation. In this study, emulsified oil fouling of polysulfone ultrafiltration membranes was studied using both constant permeate flux and constant transmembrane pressure experiments. Mass transfer resistance changes during fouling were compared between constant flux experiments and constant transmembrane pressure experiments performed at an initial flux equal to the flux imposed during the constant flux experiment. At low fluxes, the transport resistance and its change with permeate volume per unit area agreed within experimental error regardless of operational mode. In contrast, at high fluxes, the change in membrane resistance with permeate volume per unit area was much higher in constant flux than in constant transmembrane pressure experiments. The threshold flux, defined recently as the flux at which the rate of fouling begins to increase rapidly, separates the regimes of good and poor agreement between the two types of experiments. The weak form of the critical flux, below which spontaneous adsorption is the only significant resistance imposed by foulant, was also observed.
Ion exchange membranes are used in various membrane-based processes (e.g., electrodialysis, fuel cells). Charged solute transport is largely governed by the charged groups on the polymer backbone. In ...this review, fundamental relationships describing salt permeability and ionic conductivity, as well as water permeability, in charged polymers are developed within the framework of the Nernst-Planck and solution-diffusion models. The influence of fixed charge groups and polymer structure on water sorption and diffusion is discussed. Current understanding of ion partitioning in charged polymers, focusing on the use of thermodynamic models (i.e., Donnan theory) to describe such phenomena, is summarized. Ion diffusivity data from the literature are interpreted using a model developed by Mackie and Meares to assess relative and absolute effects of the polymer and fixed charge groups on ion diffusivity. Furthermore, membrane requirements for several important technologies are listed. Knowledge gaps and opportunities for fundamental research are also discussed.
PDF
