New Class Of Membranes Research Articles

Highly permeable and selective graphene oxide-enabled thin film nanocomposite for carbon dioxide separation

Membrane technology appeals as an environmental friendly approach for CO2 capture application due to its modest energy requirement and scalability but high performing membranes are constantly sought after to make this technology economically competitive to the conventional amine absorption process. This study focuses on a relatively new class of membrane known as the thin film nanocomposite (TFN) which is characterized with extremely thin selective layer for high productivity. Using the interfacial polymerization (IP) technique, TFNs embedded with graphene oxide (GO) were successfully fabricated to investigate the effects of GO loading on the morphology and gas separation performance of the resultant membrane. Permeance and selectivity of the TFN is a positively correlated with GO content. The optimum sample, TFN-0.5 exhibited more than twice the CO2 permeance of neat thin film composite (TFC) while achieving CO2/N2 and CO2/CH4 separation factor of 41 (26% increment) and 25 (26% increment) respectively. The enhancement in TFN performance was attributed to the increased tortuosity in polymer matrix and the introduction of rapid diffusion passages resulted from incorporation of rigid GO sheets. GO-TFN demonstrated attractive potential for low pressure post-combustion CO2 separation to reduce emission of the greenhouse gas and mitigate the global warming issue.

Modeling of Internal Filtration in Theranova Hemodialyzers.

High retention onset (HRO) is the designation for a new class of hemodialysis membranes. A unique characteristic of this class is the highly selective and controlled porosity resulting in sieving properties that provide a clinically desirable balance between middle/large molecular weight solute removal and albumin loss. Another defining feature of this membrane class is the relatively small fiber diameter, which produces high convective volumes in the form of internal filtration. The aim of the present study was to estimate, by semi-empirical methods, convective volumes for 2 new HRO dialyzers: Theranova 400 and Theranova 500 (Baxter International Inc., Deerfield, IL, USA). Axial blood and dialysate compartment pressure drop along with transmembrane pressure, measured in vitro with blood (Qb = 300 or 400 mL/min; Qd = 500 mL/min; net ultrafiltration rate = 0), served as input parameters for 3 different models: linear, geometric, and (non-linear) mathematical. Based on the most rigorous mathematical model, the estimated convective volumes were 1,661 mL/h (Qb = 300 mL/min) and 1,911 mL/h (Qb = 400 mL/min) for Theranova 400 and 1,864 mL/h (Qb = 300 mL/min) and 1,978 mL/h (Qb = 400 mL/min) for Theranova 500. These results suggest that the unique fiber characteristics of this new class of membranes provide substantial convective volumes without the need for exogenous substitution fluid. As such, HRO membranes are a major end-stage renal disease treatment advance in the quest to enhance the removal of larger-sized uremic toxins.

Expanded Hemodialysis: A New Therapy for a New Class of Membranes.

A wide spectrum of molecules is retained in end-stage kidney disease, normally defined as uremic toxins. These solutes have different molecular weights and radii. Current dialysis membranes and techniques only remove solutes in the range of 50-15,000 Da, with limited or no capability to remove solutes in the middle to high molecular weight range (up to 50,000 Da). Improved removal has been obtained with high cut-off (HCO) membranes, with albumin loss representing a limitation to their practical application. Hemodiafiltration (HDF) at high volumes (>23 L/session) has produced some results on middle molecules and clinical outcomes, although complex hardware and high blood flows are required. A new class of membrane has been recently developed with a cut off (MWCO) close to the molecular weight of albumin. While presenting negligible albumin loss, these membranes have a very high retention onset (MWRO), allowing high clearances of solutes in a wide spectrum of molecular weights. These membranes originally defined (medium cut off) are probably better classified as high retention onset. The introduction of such membranes in the clinical routine has allowed the development of a new concept therapy called "expanded hemodialysis" (HDx). The new therapy is based on a special hollow fiber and dialyzer design. Its simple set-up and application offer the possibility to use it even in patients with suboptimal vascular access or even with an indwelling catheter. The system does not require a particular hardware or unusual nursing skill. The quality of dialysis fluid is, however, mandatory to ensure a safe conduction of the dialysis session. This new therapy is likely to modify the outcome of end-stage kidney disease patients, thanks to the enhanced removal of molecules traditionally retained by current dialysis techniques.

Multidimensional Classification of Dialysis Membranes.

Hemodialysis is a process of mass separation by a semipermeable membrane, utilized to cleanse blood from waste products retained in case of kidney failure. Traditionally, dialysis membranes have been classified based on composition and hydraulic conductance, creating the net differentiation between cellulosic versus non-cellulosic on one hand and low-flux versus high-flux on the other. With the evolution of biomaterials and improved spinning technology, new membranes have been introduced in the market with specific characteristics and refined individual properties. Therefore, we should consider new parameters to classify dialysis membranes including polymer blending, surface functionalization, molecular weight cut-off (MWCO), hydrophilic/hydrophobic properties, thickness and architecture, adsorption capacity, and electric potential. All these parameters may be utilized to characterize a membrane alone or in combination. Recently, a new parameter has been identified as an important element to characterize a new class of membranes. Beyond the classic MWCO describing the molecular weight at which the sieving value in pure convection = 0.1, the molecular weight retention onset (MWRO) is a new parameter that defines membrane sieving properties. The retention onset value is the molecular weight at which the sieving value = 0.9. The relationship between MWCO and MWRO describes the steepness of the sieving curve and the membrane pore size distribution with important consequences on the final mass separation process and solute removal.

EFFECT OF PMMA-MWNTS LOADING ON CO2 SEPARATION PERFORMANCE OF THIN FILM NANOCOMPOSITE MEMBRANE

Nanocomposite membrane, especially the thin film nanocomposite (TFN) fabricated via interfacial polymerization (IP) is a relatively new class of membrane which features good separation performance and practical processing. This study investigated on the effects of multi-walled carbon nanotubes (MWNTs) loading on the gas separation performance of the resultant TFNs. TFNs were tested with pure CO2, N2 and CH4 gases at feed pressure of 2 bar. The findings from this study suggested that the optimum fillers loading was around 0.25 g/L in the coating solution which gives TFN with CO2 permeance of 53.5 gas permeation unit (GPU) (12% higher than base membrane without filler), CO2/N2 selectivity of 61 and CO2/CH4 selectivity of 35. The enhancement in CO2 permeance without sacrificing the membrane selectvities was attributed to the good dispersion and compatibility of the MWNTs with the polymer matrix while the nanotubes serve as rapid diffusion channels to facilitate transport of gases. TFN embedded with polymethyl methacrylate (PMMA)-MWNTs showed potential for low pressure carbon capture and storage application.

Pervaporation dehydration of acetone using P84 co-polyimide flat sheet membranes modified by vapor phase crosslinking

In this work, a unique vapor crosslinking method is applied for the first time to fabricate polyimide-based pervaporation membranes for the dehydration of acetone. By controlling the temperature and duration of the crosslinking reaction using ethylenediamine (EDA) vapor and using the right dope formulation to cast P84 co-polyimide membranes with balanced macrovoid and sponge-like morphology, we have optimized the crosslinking parameters to obtain a membrane with a separation factor of 53 (i.e., a water permeate concentration of 90%) and a high flux of 1.8kgm−2h−1 for pervaporation dehydration of acetone at 50°C from a feed of 85/15 acetone/water. With the aid of characterizations for surface chemistry and physics of the modified membranes, such as FTIR-ATR, XPS, and XRD, the crosslinking between EDA and P84 is confirmed at the selective layer of the membrane, as well as structure–performance relationship. This new modification method for pervaporation membranes is attractive for creating a new class of membranes for the separation of organic solvents and biofuel.

Extended Characterization of a New Class of Membranes for Blood Purification: The High Cut-off Membranes

High cut-off membranes are a new class of blood purification membranes whose particular characteristics present challenges for commonly-used characterization methods. Dextran sieving curves for representative blood purification membranes of the high-flux and high cut-off types were measured and compared to curves for the glomerular filtration barrier (GFB). The performance was also determined after blood exposure for the most permeable synthetic membranes. High cut-off membranes were observed to be more open than the GFB before blood exposure, but become tighter and more selective after the exposure, keeping the permeation for low and middle molecules while restraining the filtration of large species. Based on dextran sieving experiments for a variety of commercially available blood purification membranes, we present a novel method for classifying blood purification membranes. By using a well-established technique and introducing a new characteristic parameter for the sieving curve--the molecular weight retention onset (MWRO)--a graph of molecular weight cut-off versus molecular weight retention onset provides the landscape of dialysis membrane types. This meaningful representation is based on only one in vitro method, and allows the membrane classification by indirectly considering two structural parameters: pore size and pore size distribution. In this way, the families of low-flux, high-flux, protein leaking, and high cut-off membranes are clearly differentiated. The differentiation allows for the definition of MWCO/MWRO regions for the different types, so that further classification of newly developed membranes can be easily achieved.

Graphene – a platform technology for a new class of membranes ,

Scientists in the USA are taking the initial step to practically realising graphene-based membranes, as this brief feature shows.

Enhanced conductivity and electrochemical properties for class of hybrid systems via sol–gel techniques

Enhanced high proton-conducting hybrid membranes were prepared via the sol–gel technique using phosphotungsticacid (PWA) mixed with polyvinylalcohol (PVA) and phosphosilicate (P2O5/SiO2). These new class of membranes showed excellent proton conductivities and were confirmed to be promising electrolytes for fuel cells. Moreover, their mechanical, thermal, swelling and structural properties were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermal analysis, and swelling measurements. Also proton conductivity and electrochemical studies were performed. The materials were thermally stable up to 300°C, and the swelling capability was found to be increased by addition of the SiO2 function. The phosphosilicate mixed with poly(vinylalcohol) (PVA) and heteropolyacid thus gave rise to the creation of a strong PVA/PWA/P2O5/SiO2 hybrid membrane network through Keggin ions.

PVDF/Nanosilica Dual-Layer Hollow Fibers with Enhanced Selectivity and Flux as Novel Membranes for Ethanol Recovery

In this work, we have demonstrated the design and engineering of poly(vinylidene fluoride) (PVDF)/nanosilica dual-layer hollow fibers as novel pervaporation membranes for ethanol recovery. The newly developed dual-layer hollow fiber membrane can exhibit a high separation factor of up to 29 with a sustainable high flux of 1.1 kg m–2 h–1, which is equivalent to the separation performance regime of inorganic membranes. Central to this performance achievement is the synergy of (1) desirable membrane morphology, nanopore size, and high surface porosity of a thin-PVDF/nanosilica composite on a fully porous substrate accomplished by the dual-layer coextrusion technology, and (2) optimal operating downstream pressure with the aid of controlled pervaporation transport. The membrane selectivity-downstream pressure dependence of PVDF/nanosilica hybrid membranes is comprehensible via a modified pore-flow model. This study may represent a new class of membranes for ethanol–water separation.

Energy Storage: A Percolating Membrane with Superior Polarization and Power Retention for Rechargeable Energy Storage (Adv. Mater. 1/2012)

An entirely new class of membranes with superior polarization is presented by Xian Ning Xie and co-workers, on page 76. The polarization of such a membrane is up to a million times that of conventional polarizable materials and, in practice, the membrane is immediately applicable for energy storage and associated technologies, such as hybrid/electric vehicles, solar panels, power grids, and consumer electronics.

Transport behavior of aromatic hydrocarbons through high density polyethylene/ ethylene propylene diene terpolymer blends

Membrane based separation technology is currently regarded as a new frontier of chemical engineering and widely used for the purification, concentration and fractionation of fluid mixtures. Polymer blend membranes are promising materials that can overcome the difficulties associated with homopolymer systems and hence the selection of polymer blend membrane as a novel material for various applications is worth probing. Transport properties of aromatic hydrocarbons through a new class of membranes from blends of high density polyethylene (HDPE) and ethylene propylene diene terpolymer rubber (EPDM) have been investigated at different temperatures to understand the effect of physical and chemical nature of the polymer blend on the transport phenomena. The effects of blend ratio, temperature and penetrant size on the sorption properties were studied. The equilibrium solvent uptake decreases with an increase in concentration of HDPE, in the blends. Relationship between transport behavior and the morphology of the system was examined. The mechanism of transport has been analyzed and found that the mode of transport is close to Fickian. The sorption data have been used to estimate the transport coefficients and various kinetic parameters of sorption.

Gas separation properties of functionalized carbon nanotubes mixed matrix membranes

The synergistic combinations of organic polymers for separation applications with inorganic substances such as multi-walled carbon nanotubes (MWNTs), have resulted in a new class of membrane material called mixed matrix membrane (MMM) for the separation of CO 2/CH 4 gases. Mixed matrix membrane incorporated functionalized MWNTs (f-MWNTs) were fabricated by the solution casting method, in which the f-MWNTs were embedded into the polyimide membrane and the resulting membranes were characterized. The effect of nominal MWNTs content between 0.5 and 1.0 wt% on the gas separation properties was investigated. The mixed matrix membranes showed 100% enhancement for the selectivity of CO 2/CH 4 compared to the corresponding neat polymer membrane. This new class of mixed matrix membrane has the ability to separate gases at the molecular level and has the potential to ultimately reduce the energy consumed in present-day separation operations. This study has shown that addition of CNTs to polymeric membranes has improved separation properties of the membranes to a certain extent.

Preparation and characterization of multi-walled carbon nanotube/PBNPI nanocomposite membrane for H 2/CH 4 separation

By combining organic polymers normally used to make membrane filters with inorganic substances, multi-walled carbon nanotube (MWCNTs), an extraordinary ability to separate H 2 from CH 4 was developed in this study. A series of MWCNTs/PBNPI nanocomposite membrane with a nominal MWCNTs content between 1 and 15 wt% were prepared by solution casting method, in which the very fine MWCNTs were embedded into glassy polymer membrane. Detailed characterizations, such as morphology, thermal stability and crystalline structure have been conducted to understand the structures, composition and properties of nanocomposite membranes. The results found that this new class of membrane had increased permeability and enhanced selectivity, and a useful ability to filter gases and organic vapours at the molecular level.

Interactions of Membrane-Active Peptides with Thick, Neutral, Nonzwitterionic Bilayers

Alamethicin is a well-studied channel-forming peptide that has a prototypical amphipathic helix structure. It permeabilizes both microbial and mammalian cell membranes, causing loss of membrane polarization and leakage of endogenous contents. Antimicrobial peptide-lipid systems have been studied quite extensively and have led to significant advancements in membrane biophysics. These studies have been performed on lipid bilayers that are generally charged or zwitterionic and restricted to a thickness range of 3-5 nm. Bilayers of amphiphilic diblock copolymers are a relatively new class of membranes that can have significantly different physicochemical properties compared with those of lipid membranes. In particular, they can be made uncharged, nonzwitterionic, and much thicker than their lipid counterparts. In an effort to extend studies of membrane-protein interactions to these synthetic membranes, we have characterized the interactions of alamethicin and several other membrane-active peptides with diblock copolymer bilayers. We find that although alamethicin is too small to span the bilayer, the peptide interacts with, and ruptures, thick polymer membranes.

Protein-Leaking Membranes for Hemodialysis

A new class of membranes that leak protein has been developed for hemodialysis. These membranes provide greater clearances of low molecular weight proteins and small protein-bound solutes than do conventional high-flux dialysis membranes but at the cost of some albumin loss into the dialysate. Protein-leaking membranes have been used in a small number of clinical trials. The results of these trials suggest that protein-leaking membranes improve anemia correction, decrease plasma total homocysteine concentrations, and reduce plasma concentrations of glycosylated and oxidized proteins. However, it is not clear yet that routine use of protein-leaking membranes is warranted. Specific uremic toxins that are removed by protein-leaking membranes but not conventional high-flux membranes have not been identified. It is also unclear whether protein-leaking membranes offer benefits beyond those obtained with conventional high-flux membranes used in convective therapies, such as hemofiltration and hemodiafiltration. Finally, the amount of albumin loss that can be tolerated by hemodialysis patients in a long-term therapy has yet to be determined. Protein-leaking membranes offer a new approach to improving outcomes in hemodialysis, but whether their benefits will outweigh their disadvantages will require more basic and clinical research.

Potential use of nanofiltration membranes in treatment of industrial wastewater from Ni-P electroless plating

Nanofiltration (NF) membranes are a relatively new class of membranes which have properties in between those of ultrafiltration (UF) membranes and reverse-osmosis (RO) ones. Their separation mechanisms involve both steric (sieving) effects and electrical (Donnan) effects. The significance of this membrane, besides having small pores is the membrane's surface charges, which allows charged solutes that are smaller than the membrane pores to be rejected along with bigger neutral solutes and salts. Furthermore it is capable of rejecting multivalent ions effectively, and on the other hand lets the monovalent ions pass through. These are the characteristics that make the nanofiltration membrane a potential process to reject heavy metal ions, which in general are multivalent ions. In this study, the potential use of a nanofiltration membrane for rejection of heavy metal ions such as Ni 2+ from wastewater from Ni-P electroless plating industry was studied. A composite NF membrane designated as HL membrane was used. The characteristics of the membranes were analyzed using single salt rejection studies of NaCl, MgCl 2, Na 2SO 4, NiSO 4. The wastewater from the Ni-P electroless plating industry was found to contain various cations such as nickel, zinc and sodium. In order to ascertain the effect of ion rejection in a multicomponent system, the rejection behaviour of Ni 2+ ion in mixed solutions of Na 2SO 4 and NiSO 4 was tested and the rejections were then compared to those obtained using real solutions. The results show that in general the rejection of Ni is high regardless of the presence of other ions. The experimental data were also analyzed using two predictive models, namely Donnan steric pore model (DSPM) and ion transport model. The models are based on the extended Nerrnst-Planck equation but solved using different approaches and parameters. The results from the models were compared to find which model gives better agreement with the experimental findings.

Template‐Synthesized Nanotubes for Chemical Separations and Analysis.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Template-synthesized nanotubes for chemical separations and analysis.

We have developed a new class of synthetic membranes that consist of a porous polymeric support that contains an ensemble of gold nanotubes that span the thickness of the support membrane. The support is a commercially-available microporous polycarbonate filter with cylindrical nanoscopic pores. The gold nanotubes are prepared by electroless deposition of Au onto the pore walls, that is, the pores acts as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (<1 nm) can be prepared. These nanotube membranes can be used to cleanly separate small molecules on the basis of molecular size. Furthermore, use of these membranes as a novel electrochemical sensor is also discussed. This new sensing scheme involves applying a constant potential across the Au nanotube membrane and measuring the drop in the transmembrane current upon the addition of the analyte. This paper reviews our recent progress on size-based based transport selectivity and sensor applications in this new class of membranes.

Molecular sieving and sensing with gold nanotube membranes.

We have developed a new class of synthetic membranes that consist of a porous polymeric support that contains an ensemble of gold nanotubes that span the thickness of the support membrane. The support is a commercially available microporous polycarbonate filter with cylindrical nanoscopic pores. The gold nanotubes are prepared via electroless deposition of Au onto the pore walls; i.e., the pores act as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (<1 nm) can be prepared. These nanotube membranes can be used to cleanly separate small molecules on the basis of molecular size. Furthermore, use of these membranes as a novel electrochemical sensor is also discussed. This new sensing scheme involves applying a constant potential across the Au nanotube membrane and measuring the drop in the transmembrane current upon the addition of the analyte. This paper reviews our recent progress on size-based transport selectivity and sensor applications in this new class of membranes.