Characterization of multilayer nanofiltration membranes using positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) coupled with a slow positron beam was used to characterize in situ the layer structure and depth profile of the cavity size in thin film composite (TFC) polyamide nanofiltration (NF) membranes prepared by the interfacial polymerization method. Two techniques, using PAS coupled with a slow positron beam of Doppler broadening energy spectra (DBES) and positron annihilation lifetime spectroscopy (PALS) designed to reveal the layer structure and the cavity sizes contained in a multilayer thin film composite NF membrane, were assessed. To the best knowledge of the authors, a characterization of the depth profile of cavities in NF membranes using PAS coupled with a slow positron beam has never been reported. The membranes selected have a composite structure containing three layers: a selective polyamide layer, a transition layer, and a porous support prepared by the phase inversion technique. Furthermore, the cavity size distribution in the selective top layer plays an important role in determining the performance of the NF membranes.     

Biofouling in spiral wound membrane systems: Three-dimensional CFD model based evaluation of experimental data

A three-dimensional (3D) computational model describing fluid dynamics and biofouling of feed channels of spiral wound reverse osmosis and nanofiltration membrane systems was developed based on results from practice and experimental studies. In the model simulations the same feed spacer geometry as applied in practice and the experimental studies was used. The 3D mathematical model showed the same trends for (i) feed channel pressure drop, (ii) biomass accumulation, (iii) velocity distribution profile, resulting in regions of low and high liquid flow velocity also named channeling. The numerical model predicted a dominant biomass growth on the feed spacer, consistent with direct in situ observations on biofouling of spiral wound membrane modules and monitors using Magnetic Resonance Imaging (MRI). The model confirms experimental results that feed spacer fouling is more important than membrane fouling. The paper shows that mathematical modeling techniques have evolved to a stage that they can be used hand-in-hand with experiments to understand the processes involved in membrane fouling.     

The effect of non-contact heating (microwave irradiation) and contact heating (annealing process) on properties and performance of polyethersulfone nanofiltration membranes

In this work the effect of microwave irradiation on morphology and performance of polyethersulfone (PES) membranes was investigated. The membranes were prepared with 20 wt.% of PES by phase inversion method. N,N-dimethylformamide (DMF) and mixture of water and ethyl alcohol (90/10 vol.%) were employed as solvent and coagulant respectively. Polyvinylpirrolidone (PVP) with the concentration of 2 wt.% was selected as pore former. The effects of irradiation time (10, 30, 60, 90, 120 s) and microwave power (180, 360, 720 and 900 W) on structure and performance of membranes were studied. Increasing the irradiation time and power caused variation in permeate flux and ion rejection. Moreover, the effects of annealing processes (60, 70, 80 °C) were studied. Transmembrane pressure was selected around 1.5 MPa for all experiments. Scanning electron microscope (SEM) and atomic force microscope (AFM) were employed to describe the surface morphology of the prepared membranes. The effect of microwave irradiation time in different power revealed alterations in membrane surface morphology and AFM images represented that surface parameters (such as surface roughness) have been changed. The membrane exhibited moderate rejection (47%) and low permeate flux (4.5 kg/m² h) at 80 °C for NaCl solution. The SEM images indicate that the dense skin layer is formed at 80 °C annealing.     

Ion transport modelling through nanofiltration membranes

The aim of this work is to study the transport mechanism of ions through nanofiltration membranes. A model based on extended Nernst–Planck and film theory equations is reported. This model can be characterized by three transport parameters: the water permeability L_p, the salt transmittance Φ and the effective salt transfer coefficient K_eff. The knowledge of the feed and permeate concentration and of the permeate volumetric flux enable us to calculate these transport parameters. The model is used to estimate cadmium salts rejection by a NANOMAX 50 membrane. Experimental and calculated results are shown to be in good agreement. The model is then successfully extended to experimental data reported in the literature.     

中空纤维复合膜

中空纤维复合膜是分离膜的一种,它是由两种(或两种以上)不同的材料采用一定的制备工艺复合而成的,其优点是将中空纤维的结构特点(如自支撑等)和复合膜的分离优势(如高选择性高通量等)有机结合.本文首先介绍了中空纤维复合膜的基膜及复合层所用到的材料(或添加材料),并按照中空纤维复合膜的结构特点对其进行了简单的分类,并重点论述了中空纤维复合膜的制备设备及工艺.最后论述了中空纤维复合膜在渗透汽化、气体分离和纳滤等领域的研究进展和应用情况,指出中空纤维复合膜需要继续深入的研究内容.     

纳滤膜在水处理中的最新应用进展

纳滤膜(NF)是新的分离膜品种,对溶质的截留性能介于超滤膜(UF)和反渗透膜(RO)之间。纳滤膜的特性是表面带有电荷并具有纳米级的微孔,能够去除高价离子和分子量大于200的溶解性的有机物。特殊的分离效果使纳滤技术单独或者和其它技术联用应用于水处理领域,主要包括:(1)饮用水制备和深度净化;(2)海水淡化;(3)废水处理,例如生活污水、垃圾渗滤液等等。本文综述了纳滤技术在上述水处理领域中的应用进展及现存问题。     

Nanofiltration studies of larger organic microsolutes in methanol solutions

Multistep organic solvent-based pharmaceutical syntheses of larger organic microsolutes having molecular weights (MW) in the range of 300–1000 generally require athermal separation processes because the active molecules and the intermediates are thermally labile. To that end, nanofiltration (NF) of methanol solutions of three selected solutes, safranin O (MW 351), brilliant blue R (MW 826) and vitamin B₁₂ (MW 1355) has been studied in a batch stirred cell for a dilute solution of each individual solute at 3034 kPa (440 psig). The solvent-resistant membranes investigated and their manufacturer-specified molecular weight cut-offs (MWCO) are MPF-44 (250), MPF-50 (700) and MPF-60 (400). During an initial transient period, the solvent flux decreased with time and the solute rejection increased with time for every membrane reaching a steady state after about 12 h. This behavior resulting from membrane compaction and pore size reduction was partially reversible. Additional studies using higher feed solute concentrations (1 and 3 wt.%) show considerable reduction in solvent flux and increase in solute rejection; the effect appears to be far more than that due to an increase in osmotic pressure and possible reasons for such a behavior have been suggested. The observed solute rejection values are generally significantly lower than the manufacturer-specified MWCO values. Additional studies varying the feed solution pressure through the membrane MPF-60 indicate that the variation of the percent rejection of solutes safranin O and brilliant blue R with the solvent flux tends to follow the relation suggested by the Finely Porous Model.     

Molecularly Mixed Composite Membranes for Advanced Separation Processes

Porous organic cages (POCs) are individual soluble, porous molecules. When fabricated into mixed‐matrix membranes (MMMs), the soluble POC molecules have the potential to exhibit intimate molecular‐level mixing with the polymer matrix. POCs have only recently been incorporated into mixed matrix membrane materials, but this process has not yet resulted in significant improvements of membrane performance. Now, vertex‐functionalized amorphous scrambled porous organic cages (ASPOCs) have been utilized as membrane performance enhancers and the amorphous ASPOC mixtures are observed to distribute throughout the matrix without any indication of particle formation or agglomeration, creating unique, molecularly mixed composite membranes. Overall, the molecularly mixed composite membrane provide significant increases in both membrane permeability and selectivity, offering new avenues for creation of membranes with unique properties in industrially relevant separations.     

Natural organic matter removal by coagulation during drinking water treatment: A review

Natural organic matter (NOM) is found in all surface, ground and soil waters. An increase in the amount of NOM has been observed over the past 10-20 years in raw water supplies in several areas, which has a significant effect on drinking water treatment. The presence of NOM causes many problems in drinking water and drinking water treatment processes, including (i) negative effect on water quality by causing colour, taste and odor problems, (ii) increased coagulant and disinfectant doses (which in turn results in increased sludge volumes and production of harmful disinfection by-products), (iii) promoted biological growth in distribution system, and (iv) increased levels of complexed heavy metals and adsorbed organic pollutants. NOM can be removed from drinking water by several treatment options, of which the most common and economically feasible processes are considered to be coagulation and flocculation followed by sedimentation/flotation and sand filtration. Most of the NOM can be removed by coagulation, although, the hydrophobic fraction and high molar mass compounds of NOM are removed more efficiently than hydrophilic fraction and the low molar mass compounds. Thus, enhanced and/or optimized coagulation, as well as new process alternatives for the better removal of NOM by coagulation process has been suggested. In the present work, an overview of the recent research dealing with coagulation and flocculation in the removal of NOM from drinking water is presented.     

Influence of operating conditions on the rejection of cobalt and lead ions in aqueous solutions by a nanofiltration polyamide membrane

The potential use of nanofiltration polyamide membrane for removing cobalt and lead ions from wastewater was investigated. Rejection experiments were conducted with Pb(NO₃)₂ and Co(NO₃)₂ in both single-salt solutions and mixtures. Experimental rejection rates were corrected for concentration polarization phenomenon by means of film theory. The structural features of the membrane (pore radius and thickness-to-porosity ratio) were first estimated from the fitting of glucose rejection rates. Its surface charge properties were then investigated in single-salt solutions at pH values between 3 and 7. Rejection of both heavy metal ions was found to be influenced by operating conditions such as permeate flux, solution pH and feed salt concentration. In single-salt solutions, rejection of lead was higher than that of cobalt at pH ≥ 5. This behavior may be explained by (i) higher normalized volume charge density in the Pb(NO₃)₂ than in the Co(NO₃)₂ solution and (ii) lower ionic strength of the Pb(NO₃)₂ solution as compared with the Co(NO₃)₂ solution. At pH < 5, the dielectric exclusion would be more important for Co(NO₃)₂ than for Pb(NO₃). Lead rejection was almost the same in both single-salt solutions and ternary mixtures, whereas cobalt rejection was strongly affected by the presence of lead. Cobalt was found to be rejected much more than lead in mixtures at equal mass concentrations, the difference between rejections of the two cations being greater as pH increased.