High-flux Reverse Osmosis Membranes Research Articles

Development of high-flux reverse osmosis membranes with MIL-101(Cr)/Fe3O4 interlayer

Development of high-flux reverse osmosis membranes with MIL-101(Cr)/Fe3O4 interlayer

Fabrication of high selectivity and high flux reverse osmosis membranes

Fabrication of high selectivity and high flux reverse osmosis membranes

Cavitating substrates to boost water permeance of reverse osmosis membranes

Reverse osmosis membranes have been shown to achieve reliable freshwater supply by means of water desalination. Nevertheless, breaking the long-standing trade-off is a major challenge for realizing high-efficient desalination. Herein, we show that selective swelling of substrates made by a block copolymer, polysulfone-block-poly(ethylene glycol) (PSF-b-PEG), is very promising in the fabrication of high-flux reverse osmosis membranes. Polyamide layers are first formed on relatively dense PSF-b-PEG substrates by interfacial polymerization. Then, the substrates are cavitated into highly porous substrates by selective swelling, which follows the mechanism of selective swelling-induced pore generation. The substrate porosity can be well tuned over a wide range via adjusting swelling duration. Selective swelling does not compromise the structural integrity and surface properties of polyamide layers atop the substrates. Importantly, the boosted porosity reduces the water transport resistance in substrates markedly, which in turn enables fast water permeation in reverse osmosis membranes. Thus, the optimal membrane shows exceptional water permeance of 50.4 L m−2 h−1 MPa−1 and a high NaCl rejection of 99.2%. Our work not only offers a novel strategy to enhance the water permeance of reverse osmosis membranes, but also demonstrates that the transport resistance of substrates also significantly influences water permeance of reverse osmosis membranes.

High flux reverse osmosis membranes fabricated with hyperbranched polymers via novel twice-crosslinked interfacial polymerization method

In this study, two kinds of carboxyl-terminated hyperbranched polyesters (HBPs) with special three-dimensional structure were proposed to act as the substrates to synthesize two new kinds of acyl chloride-terminated hyperbranched polyesters (HBPACs) that were used as the key functional materials to participate in the interfacial polymerization reaction to fabricate the novel thin film composite membranes (TFC) with high water flux and monovalent salt rejection. First, the two HBPACs were synthesized from two HBPs with different molecule weights (Mn ≈ 3289 g/mol and 1964 g/mol) through acylation reaction with thionyl chloride. Then, the resultant HBPACs were used to modify the conventional reverse osmosis (RO) membrane by a new twice-crosslinked interfacial polymerization method. That is, the m-phenylenediamine (MPD) coated on the polysulfone supporting was successively reacted with HBPAC and trimesoyl chloride (TMC) which were solved in different organic solutions, respectively. The resultant HBPAC-based TFC membranes presented smoother surface and showed few change in surface hydrophilicity. Especially, one of the HBPAC-based membrane fabricated with the mixture of two HPBACs (a mass ratio of 1:1), showed a ~50% increase in water flux without sacrificing the salt rejection and exhibited excellent long-term stability compared with the conventional TFC RO membrane. Obviously, the HBPAC-based composite PA layer showed a certain cross-linking network, which not only maintained the salt retention rate but also enhanced the compatibility of HBPAC in PA main body, and as a result promoted the membrane stability. Furthermore, the residual carboxyl groups of HBPAC can act as hydrophilic sites between polyamide chains, which contribute to the rapid diffusion of water molecules through the PA layer of the membrane. Thus, this work provides a new approach to enhance the flux and stability of the membrane via the incorporation of new acyl chloride-terminated hyperbranched polymer.

High-flux reverse osmosis membranes incorporated with NaY zeolite nanoparticles for brackish water desalination

Thin film nanocomposite (TFN) membranes incorporated with NaY zeolite nanoparticles were prepared via interfacial polymerization (IP) of trimesoyl chloride and m-phenylenediamine on nanoporous polysulfone supports. The isolated zeolite-filled polyamide layer was observed by transmission electron microscopy, and the nanoparticles dispersed in the dense nodular polyamide on the polysulfone side. The effects of IP reaction time and zeolite loading on membrane separation performance were investigated. The results showed that a longer IP reaction time was necessary to form a denser zeolite–polyamide layer for higher salt rejection, and the optimum zeolite loading was determined to be 0.15wt%. Under the optimum conditions, the water flux increased from 0.95 to 1.78m3/m2/day (23.3 to 43.7gal/ft2/day (gfd)) with the incorporation of the zeolite nanoparticles, while providing a high salt rejection of 98.8% (2000ppm NaCl solution, 225psi (1.55MPa), 25°C). The TFN membranes were then post-treated with aqueous solutions containing glycerol, camphorsulfonic acid-triethylamine salt, and sodium lauryl sulfate to further improve the water flux. By optimizing the post-treatment solution composition, an improved brackish water desalination performance was achieved with 2.06m3/m2/day (50.6gfd) water flux, which was more than double compared to that of the TFC membrane without the zeolite nanoparticles, along with 98.4% salt rejection.

High-flux reverse osmosis membranes incorporated with hydrophilic additives for brackish water desalination

Novel high-flux reverse osmosis membranes with hydrophilic additives were synthesized and characterized under brackish water desalination conditions. Four selected hydrophilic additives, o-aminobenzoic acid-triethylamine salt, m-aminobenzoic acid-triethylamine salt, 2-(2-hydroxyethyl) pyridine, and 4-(2-hydroxyethyl) morpholine, were added into m-phenylenediamine (MPD) solution to react with trimesoyl chloride (TMC) during the interfacial polymerization between MPD and TMC. The effects of different concentrations for each hydrophilic additive were evaluated using brackish water desalination tests with 2000ppm NaCl solution at 225psi and 25°C. After the additive concentration was optimized, the synthesized membranes were post-treated to further improve the water flux using aqueous solutions containing glycerol, sodium lauryl sulfate, and camphorsulfonic acid-triethylamine salt. The resulting membrane showed a flux of 52.6gal/ft2/day and a salt rejection of about 98.8% or greater. This membrane outperformed the other membranes evaluated, and it also exhibited a good stability in terms of water flux and salt rejection during the entire period of a 30-day test.

High-Flux Reverse Osmosis Membranes for Brackish Water Desalination

High-Flux Reverse Osmosis Membranes for Brackish Water Desalination

Fouling of reverse osmosis membranes using electrical impedance spectroscopy: Measurements and simulations

The use of modern high flux reverse osmosis membranes with low salinity feeds at high recovery rates, makes the measurement of fouling by flux decline difficult in practice. Early detection of fouling is important, because the onset of fouling occurs at a critical flux which is difficult to predict a priori. An alternative method, measuring the electrical properties of the membrane is proposed here. The fouling of a reverse osmosis membrane was measured with an INPHAZE™ Electrical Impedance Spectroscope from frequencies of 10 −1 to 10 5 Hz. Both the conductance and impedance showed dramatic changes when the reverse osmosis membrane was fouled by a small amount of precipitated divalent salts. From these experimental results, electrical properties of the reverse osmosis membrane system were determined. This paper takes electrical properties determined in previous work and models the electrical properties of the first and second stages of a sea water desalination process. The decrease in the electrical conductivity of the membrane skin layer, which would accompany fouling, is significant at frequencies below 100 Hz. Such a method has great potential for online measurement of membrane fouling and more effective operation of membrane filtration systems.

Flux enhancement of reverse osmosis membranes by chemical surface modification

We propose a novel method for dramatically increasing the flux by chemically modifying the surface of commercially available thin film composite reverse osmosis membranes. SW 30HR (Filmtec) membranes show a large increase in flux, up to an order of magnitude, with no decrease in ion rejection properties on treatment with hydrofluoric and fluosilicic acids. Surface studies by scanning electron microscopy and X-ray photoelectron spectroscopy revealed significant morphological and chemical changes in the active skin layer of the chemically treated membrane compared to that of the fresh (unexposed) membrane. The surface treatment method proposed here may have significant implications for the development of high flux reverse osmosis membranes in the future.

Poly(vinyl alcohol) gel sub-layers for reverse osmosis membranes. III. Insolubilization by crosslinking with potassium peroxydisulphate

Both flat-sheet and tubular composite reverse osmosis (RO) membranes were prepared by depositing aqueous solutions of poly(vinyl alcohol) [PVA] and potassium peroxydisulphate [K 2S 2O 8] on asymmetric poly(arylether sulphone) [PES] substrate membranes. Heat treatment of the PVA coatings resulted in crosslinking through carbon-carbon bond formation. The crosslinked PVA gels were highly hydrophilic and maintained their integrity in acidic and alkaline environments. The effect of PVA concentration, molecular mass and degree of hydrolysis, K 2S 2O 8 concentration, curing temperature and curing time on RO properties was studied. A simple manipulation of one or more variables resulted in membranes with widely differing RO properties. Long-term exposure of membranes to a CaSO 4 slurry feed indicated a remarkable resistance to abrasion. RO tests on a real industrial effluent have underscored the stability and chemical resistance of these PVA membranes. The insolubilized PVA coatings were intended to serve as hydrophilic gel sub-layers on which salt-retention barriers could be formed by interfacial polycondensation. For this purpose, high permeate fluxes are required. The latter can be obtained by adjusting coating solution compositions and fabrication conditions. These PES-PVA gel-layer composite membranes could also function as medium-retention, high-flux RO membranes, even in the abasence of an interfacially formed salt-retention barrier.

The effect of precipitating media on the performance of porous cellulose acetate reverse osmosis membranes

A two-stage method of making semi-permeable high flux reverse osmosis membranes was developed using water-ethanol mixtures to precipitate the cellulose acetate. This eliminated the need for heat treatment and produced membranes with fluxes up to 5 m 3/m 2 day and sodium chloride rejections up to 85 %. Their properties are compared with membranes made by the three-stage method involving heat treatment and show more sensitivity to the effects of pressure.