Influence of inorganic electrolytes on the retention of polyethyleneglycol by a nanofiltration ceramic membrane

Retention properties of a nanofiltration ceramic membrane were investigated with single polyethyleneglycol (PEG) solutions and mixed PEG/inorganic electrolyte solutions. The rejection coefficient of PEGs was found to decrease in the presence of ions. It was shown that the effect of ions on the retention of neutral solutes increases with the electrolyte concentration. This phenomenon was ascribed to the partial dehydration of PEG molecules induced by the surrounding ions. This argument was confirmed by using various electrolytes (KCl, LiCl and MgCl₂). It was found that the lowering of the PEG rejection coefficients follows the Hofmeister series, i.e. Mg²⁺ > Li⁺ > K⁺. Experimental data were used to compute the resulting decrease in the Stokes radius of PEG molecules in the presence of the various electrolytes.     

A critical flux to avoid biofouling of spiral wound nanofiltration and reverse osmosis membranes: Fact or fiction?

The relation between biofouling and membrane flux in spiral wound nanofiltration and reverse osmosis membranes in drinking water stations with extensive pretreatment such as ultrafiltration has been studied. The flux – water volume flowing through the membrane per unit area and time – is not influencing the development of membrane biofouling. Irrespective whether a flux was applied or not, the feed spacer channel pressure drop and biofilm concentration increased in reverse osmosis and nanofiltration membranes in a monitor, test rigs, a pilot scale and a full-scale installation. Identical behavior with respect to biofouling and feed channel pressure drop development was observed in membrane elements in the same position in a nanofiltration installation operated with and without flux. Calculation of the ratio of diffusive and convective flux showed that the diffusive flux is considerably larger than the convective flux, supporting the observations that the convective flux due to permeate production is playing an insignificant role in biofouling. Since fouling occurred irrespective of the actual flux, the critical flux concept stating that “below a critical flux no fouling occurs” is not a suitable approach to control biofouling of spiral wound reverse osmosis and nanofiltration membranes.     

纳滤过程原理综述

本文介绍了纳滤的基本概念和特性,研究探讨了纳滤过程机理,并对纳滤技术的应用和发展前景做了展望。     

Nanofiltration membranes synthesized from hyperbranched polyethyleneimine

Four nanofiltration membranes, two negatively and two positively charged, were fabricated by interfacial polymerization. Three different amines, ethylenediamine (EDA), diethylenetriamine (DETA), and hyperbranched polyethyleneimine (PEI) were selected to react with two acyl chlorides, trimesoyl chloride (TMC) and terephthaloyl chloride (TPC). The two membranes containing hyperbranched PEI, PEI/TPC and PEI/TMC, are positively charged at the operational pH. But the other two membranes, EDA/TMC and DETA/TMC, are negatively charged. It is found that the two PEI membranes own special rejection characters during nanofiltration. The PEI/TPC membrane has a similar pore size to the EDA/TMC membrane but owns simultaneously the higher salt rejection and permeation flux. The PEI/TMC has a pore size as large as 1.5 nm and still has a higher NaCl rejection than the EDA/TMC membrane of which the pore size as small as 0.43 nm. We consider that the special rejection characters are derived from the special structure of PEI. The hyperbranched structure allows some of the charged amine groups drifting inside the pores and interacting with the ions in the pathway. The drifting amines increase salt rejection but have little effect on water permeation. It implies that a high flux and high rejection membrane for desalting can be obtained by attaching freely rotating charged groups.     

Comparison of redox initiated graft polymerisation and sulfonation for hydrophilisation of polyethersulfone nanofiltration membranes

Three polyethersulfone nanofiltration membranes, including two commercial membranes (NFPES10 and N30F) and one lab-made membrane (N71), were hydrophilised in view of an increased water flux and a reduction of membrane fouling.Non-modified as well as modified membranes were characterized using contact angle measurements, water fluxes and retention of sugars.At first, redox initiated graft polymerization was applied, and the influence of reaction time, initiator concentration and monomer concentration were investigated. It was observed that grafting always resulted in a significant reduction of the contact angle. The higher the reaction time, the lower was the contact angle, but the initiator and monomer concentration had no effect when above a threshold value. Grafting results in a decrease of the water flux, while the retention of raffinose increased.A comparison was made with the addition of sulfonated polyethersulfone to the polymer solution. By blending polyethersulfone and sulfonated polyethersulfon, a sufficiently high viscosity could be achieved. The influence of the blending ratio was investigated. The larger the fraction of sulfonated polyethersulfon, the more hydrophilic the membrane became. The addition of sulfonated polyethersulfon, however, has also an influence on the water flux and on the retention of sugars. An important conclusion is also that the membranes containing sulfonated polyethersulfone are significantly more hydrophilic than the ones that have been modified through the grafting technique.     

Influence of membrane fouling by (pretreated) surface water on rejection of pharmaceutically active compounds (PhACs) by nanofiltration membranes

The effects of surface water pretreatment on membrane fouling and the influence of these different fouling types on the rejection of 21 neutral, positively and negatively charged pharmaceuticals were investigated for two nanofiltration membranes. Untreated surface water was compared with surface water, pretreated with a fluidized anionic ion exchange and surface water, pretreated with ultrafiltration. Fouling the nanofiltration membranes with anionic ion exchange resin effluent, resulted in the deposition of a mainly colloidal fouling layer, with a rough morphology. Fouling the nanofiltration membranes with ultrafiltration permeate, resulted in the deposition of a smooth fouling layer, containing mainly natural organic matter. The fouling layer on the nanofiltration membranes, caused by the filtration of untreated surface water, was a combination of both colloids and natural organic matter.Rejection of pharmaceuticals varied the most for the membranes, fouled with the anionic ion exchange effluent, and variations in rejection were caused by a combination of cake-enhanced concentration polarisation and electrostatic (charge) effects. For the membranes, fouled with the other two water types, variations in rejection were smaller and were caused by a combination of steric and electrostatic effects.Changes in membrane surface hydrophobicity due to fouling, changed the extent of partitioning and thus the rejection of hydrophobic, as well as hydrophilic pharmaceuticals.     

Fabrication and development of interfacial polymerized thin-film composite nanofiltration membrane using different surfactants in organic phase; study of morphology and performance

The effects of addition of cationic cetyltrimethylammonium bromide (CTAB), non-ionic (Triton X-100) and anionic sodium dodecyl sulfate (SDS) surfactants in organic phase for preparing the composite nanofiltration membranes were investigated. The interfacial polymerization technique was employed by applying trimesoyl chloride (TMC) and piperazine (PIP) as the reagents for the preparation of poly(piperazineamide) on a UF support. The obtained thin layer membranes were placed in oven for 2 min at 70 °C. Water permeation performance, salt rejection, membrane surface charge, chemical structure and membrane morphology including top surface and cross-section were investigated for characterization of the prepared membranes using IR-ATR, SEM, filtration and zeta potential measurement. The prepared membranes using SDS showed higher flux compared to the other membranes. SEM surface images demonstrate some defects and cracks on the thin layer surface of the membrane prepared with SDS. For membrane containing CTAB, the salt rejection increased in the order of Na₂SO₄ > NaCl > MgCl₂ with variation around 50–90%.     

荷电高聚物纳滤膜

本文对荷电聚合物纳纳滤膜的制备、性能及应用进行了全面的介和评述，反映了近年来荷电聚合笺纳滤膜研究的最新进展和动向。     

Effect of membrane pretreatment on performance of solvent resistant nanofiltration membranes in methanol solutions

This paper reports the effect of membrane pretreatment using different organic solvents on the performance of polyamide, polyimide and polydimethylsiloxane (PDMS) membranes in methanol solutions. Membrane pretreatment using acetone, methanol and toluene results in significant changes of membrane flux and rejection for polyamide- and polyimide-based membranes (Desal-DK and STARMEM 228) due to membrane swelling. The Performance of a polydimethylsiloxane (PDMS)-based membrane (MPF-50) in methanol solutions was not significantly affected by membrane pretreatment.