Synthesis and characterization of novel forward osmosis membranes based on layer-by-layer assembly

Forward osmosis (FO) has received considerable interest for water- and energy-related applications in recent years. FO does not require an applied pressure and is believed to have a low fouling tendency. However, a major challenge in FO is the lack of high performance FO membranes. In the current work, novel nanofiltration (NF)-like FO membranes with good magnesium chloride retention were synthesized using layer-by-layer (LbL) assembly. The membrane substrate was tailored (high porosity, finger-like pores, thin cross-section, and high hydrophilicity) to achieve a small structural parameter of 0.5 mm. Increasing the number of polyelectrolyte layers improved the selectivity of the LbL membranes while reducing their water permeability. The more selective membrane 6#LbL (with 6 polyelectrolyte layers) had much lower reverse solute transport compared to 3#LbL and 1#LbL. Meanwhile, the FO water flux was found to be strongly affected by both membrane water permeability and solute reverse transport. Severe solute reverse transport was observed for the active-layer-facing-draw-solution membrane orientation, likely due to the suppression of Donnan exclusion as a result of the high ionic strength of the draw solution. In contrast, the active-layer-facing-feed-solution orientation showed remarkable FO performance (15, 20, and 28 L/m2.h at 0.1, 0.5, and 1.0 M MgCl?, respectively, for membrane 3#LbL using distilled water as feed solution), superior to other NF-like FO membranes reported in the literature. To the best of the knowledge of the authors, this is the first work on the synthesis and characterization of LbL based FO membranes.     

High performance thin-film composite forward osmosis hollow fiber membranes with macrovoid-free and highly porous structure for sustainable water production

The development of high-performance and well-constructed thin-film composite (TFC) hollow fiber membranes for forward osmosis (FO) applications is presented in this study. The newly developed membranes consist of a functional selective polyamide layer formed by highly reproducible interfacial polymerization on a polyethersulfone (PES) hollow fiber support. Using dual-layer coextrusion technology to design and effectively control the phase inversion during membrane formation, the support was designed to possess desirable macrovoid-free and fully sponge-like morphology. Such morphology not only provides excellent membrane strength, but it has been proven to minimize internal concentration polarization in a FO process, thus leading to the water flux enhancement. The fabricated membranes exhibited relatively high water fluxes of 32-34 LMH and up to 57-65 LMH against a pure water feed using 2 M NaCl as the draw solution tested under the FO and pressure retarded osmosis (PRO) modes, respectively, while consistently maintaining relatively low salt leakages below 13 gMH for all cases. With model seawater solution as the feed, the membranes could display a high water flux up to 15-18 LMH, which is comparable to the best value reported for seawater desalination applications.     

Adverse impact of feed channel spacers on the performance of pressure retarded osmosis

This article analyzes the influence of feed channel spacers on the performance of pressure retarded osmosis (PRO). Unlike forward osmosis (FO), an important feature of PRO is the application of hydraulic pressure on the high salinity (draw solution) side to retard the permeating flow for energy conversion. We report the first observation of membrane deformation under the action of the high hydraulic pressure on the feed channel spacer and the resulting impact on membrane performance. Because of this observation, reverse osmosis and FO tests that are commonly used for measuring membrane transport properties (water and salt permeability coefficients, A and B, respectively) and the structural parameter (S) can no longer be considered appropriate for use in PRO analysis. To accurately predict the water flux as a function of applied hydraulic pressure difference and the resulting power density in PRO, we introduced a new experimental protocol that accounts for membrane deformation in a spacer-filled channel to determine the membrane properties (A, B, and S). PRO performance model predictions based on these determined A, B, and S values closely matched experimental data over a range of draw solution concentrations (0.5 to 2 M NaCl). We also showed that at high pressures feed spacers block the permeation of water through the membrane area in contact with the spacer, a phenomenon that we term the shadow effect, thereby reducing overall water flux. The implications of the results for power generation by PRO are evaluated and discussed.     

Performance of forward (direct) osmosis process: membrane structure and transport phenomenon

The performance of a forward (direct) osmosis (FO) process was investigated using a laboratory-scale unit to elucidate the effect of membrane structure and orientation on waterflux. Two types of RO membrane and a FO membrane were tested using ammonium bicarbonate, glucose, and fructose as the draw solution to extract water from a saline feed solution. The FO membrane was able to achieve higher water flux than the RO membranes under the same experimental conditions while maintaining high salt rejection of greater than 97%. Increasing operating temperature increased the water flux in FO process. To investigate the effect of membrane orientation on water flux, the FO membrane was tested normally (dense selective layer facing draw solution) and reversely (dense selective layer facing feed solution). Explanations on transport phenomenon in FO process were proposed which explain the observation that the FO membrane, when used in the normal orientation, performed better due to lesser internal concentration polarization. This study suggests that an ideal FO membrane should consist of a thin dense selective layer without any loose fabric support layer.     

Membrane fouling during filtration of milk––a microstructural study

The surface morphology and internal microstructure of a membrane are the result of membrane manufacturing processes and subsequent use during fluid processing in industry. Both these structural factors have a great effect on fouling and filtration performance.

In this study, scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy were used to comprehensively characterise the surface of unused microfiltration and ultrafiltration membranes, the fouled layer on the surface of membranes used for milk filtration and the internal fouling within the used membranes.

A simple and effective sample preparation method was developed to study the internal membrane structure using high-resolution field emission SEM with low-accelerating voltage. Various methods of structural characterisation were compared and the results showed that for flat sheet membranes AFM is an appropriate and convenient technique for examining the surface topography of membranes. In contrast SEM is a very appropriate technique for examining the cross-sectional and internal structure of a membrane, either unused or fouled.

Using these complimentary techniques it has been shown that internal fouling, during filtration of skim milk, proceeds by protein–polymer and protein–protein interactions. A gel layer forms on the surface of the membrane and has been imaged using SEM. This layer is slightly compressible and densifies as it grows. Fouling initiation commences after a very short filtration time.     

Boric acid permeation in forward osmosis membrane processes: modeling, experiments, and implications

Forward osmosis (FO) is attracting increasing interest for its potential applications in desalination. In FO, permeation of contaminants from feed solution into draw solution through the semipermeable membrane can take place simultaneously with water diffusion. Understanding the contaminants transport through and rejection by FO membrane has significant technical implications in the way to separate clean water from the diluted draw solution. In this study, a model was developed to predict boron flux in FO operation. A strong agreement between modeling results and experimental data indicates that the model developed in this study can accurately predict the boron transport through FO membranes. Furthermore, the model can guide the fabrication of improved FO membranes with decreased boron permeability and structural parameter to minimize boron flux. Both theoretical model and experimental results demonstrated that when membrane active layer was facing draw solution, boron flux was substantially greater compared to the other membrane orientation due to more severe internal concentration polarization. In this investigation, for the first time, rejection of contaminants was defined in FO processes. This is critical to compare the membrane performance between different membranes and experimental conditions.     

Comprehensive bench- and pilot-scale investigation of trace organic compounds rejection by forward osmosis

Forward osmosis (FO) is a membrane separation technology that has been studied in recent years for application in water treatment and desalination. It can best be utilized as an advanced pretreatment for desalination processes such as reverse osmosis (RO) and nanofiltration (NF) to protect the membranes from scaling and fouling. In the current study the rejection of trace organic compounds (TOrCs) such as pharmaceuticals, personal care products, plasticizers, and flame-retardants by FO and a hybrid FO-RO system was investigated at both the bench- and pilot-scales. More than 30 compounds were analyzed, of which 23 nonionic and ionic TOrCs were identified and quantified in the studied wastewater effluent. Results revealed that almost all TOrCs were highly rejected by the FO membrane at the pilot scale while rejection at the bench scale was generally lower. Membrane fouling, especially under field conditions when wastewater effluent is the FO feed solution, plays a substantial role in increasing the rejection of TOrCs in FO. The hybrid FO-RO process demonstrated that the dual barrier treatment of impaired water could lead to more than 99% rejection of almost all TOrCs that were identified in reclaimed water.     

正渗透技术浓缩苹果汁过程中反向溶质扩散的研究

反向渗透扩散（RSF）是正渗透技术中的一大挑战,本实验立足于研究正渗透技术浓缩苹果汁性能以及功能性汲取液（乙酸钠、碳酸氢钠、柠檬酸钠溶液）的溶质扩散规律。首先,利用NaCl溶液为汲取液研究正渗透膜的基础特性,通过改变NaCl溶液浓度、进水流速以及膜操作模式,探究正渗透体系的水通量、反向溶质扩散及截留率,分析对去离子水和苹果汁的浓缩能力及溶质扩散规律;其次,对比不同功能性汲取液对苹果汁浓缩的效果和对RSF的影响,以期达到将RSF化弊为利的目的。结果表明,汲取液浓度和膜操作模式影响浓缩效率和RSF;采用压力延迟渗透（PRO）模式,苹果汁浓缩倍数和RSF均比正渗透（FO）模式高,PRO模式下,5 mol·L?1 NaCl汲取液RSF达87.34±6.32 g·m?2·h?1;不同种类功能性汲取液浓缩苹果汁的能力不同,汲水能力:碳酸氢钠>氯化钠>乙酸钠>柠檬酸钠。RSF:乙酸钠>碳酸氢钠>氯化钠>柠檬酸钠。2 mol·L?1柠檬酸钠汲取液的RSF为29.61±2.19 g·m?2·h?1,仅为同浓度NaCl汲取液的一半,与传统的NaCl汲取液相比,柠檬酸钠汲取液可有效控制RSF。     

Microscopic observation of multispecies biofilm of various structures on whey concentration membranes

The objective of this study was to evaluate biofilm formation on polyamide reverse osmosis (RO) whey concentration membranes. Biofilms were observed with scanning electron and fluorescence microscopy. For scanning electron microscopy, pieces of 6-, 12-, and 14-mo-old membranes were allowed to air dry at room temperature (22°C) for 24 h followed by sputter coating with a 5-nm layer of gold and microscopic observations. Scanning electron microscopy images revealed that the hydrophilic layer, used to prevent membrane plugging, was not evenly distributed on the surface. Although this hydrophilic layer seemed to prevent the attachment of proteins, it supported biofilm formation. Three different structures of multispecies biofilm were observed on the retentate side of the membrane: 1) a mono layer, 2) a 3-dimensional structure of a dense matrix of extracellular polymeric substances where different types of bacterial cells were embedded, and 3) cell aggregates. In some of the biofilms, a smooth layer (shell) covered cell aggregates. In the 6-mo-old membranes, part of the shell layer was broken off. Biofilms as observed on the RO membrane were described as having a hill-and-valley type of structure, with hills showing a mushroom-like appearance and valleys comprising dense matrices of extracellular polymers with embedded bacterial cells. Fluorescence microscopy showed live cells on the surface of the biofilm. It is concluded that both cells in the deep layers of biofilm and surface cells may resist cleaning and sanitation. The extent of biofilm formation and the presence of live cells on RO membranes after regular clean in place cycles indicate the need for a more effective cleaning regimen customized for dairy separation systems.     

正渗透膜污染的影响因素及清洗效果研究

为研究正渗透(FO)浓缩过程中的膜通量衰减规律,本文以牛血清白蛋白(BSA)为特征污染物,研究了正渗透过程中原料液的离子强度及BSA浓度、膜方位等参数不同时FO膜的污染规律,以提高膜通量和截留率为目标,对驱动液的种类、浓度,料液流速进行了优化,并优化了适宜的膜清洗方案.结果表明:原料液中离子强度越大,FO膜的初始通量越...     

中空纤维超滤膜在水处理中的应用

为了测试不同的中空纤维超滤膜在污水处理方面的性能,本文采用三种不同材料的中空纤维超滤膜：聚醚砜、聚丙烯和聚丙烯腈超滤膜,分别过滤了自来水和污水。讨论了过滤前后,水的色度、浊度、PH值、电导率、氨氮含量、化学需氧量和水流量的变化,并通过扫描电镜观察过滤前后三种超滤膜的表面和横截面微观形态的变化。试验结果表明：在三种超滤膜中,聚醚砜超滤膜在污水处理中表现出了最好的综合应用性能。三种超滤膜对污水中的氨氮和化学需氧量的去除率有限,采用二级或三级过滤可以提高过滤效果。电镜实验还说明,指状结构比孔状结构的防污性能好。     

溶剂交换法制备纳米纤维素多孔薄膜及其结构表征

以纳纤化纤维素(NFC)为原料,通过真空抽滤制备NFC湿膜,采用多步法完成溶剂交换并干燥制得具有纳米孔洞结构的NFC多孔薄膜;探讨了不同溶剂和干燥方法对NFC多孔薄膜表面形貌的影响。结果表明,经真空抽滤得到的NFC湿膜先用乙醇后用叔丁醇溶剂交换并冷冻干燥制得的NFC多孔薄膜的表面孔径分布均匀(孔径100～200 nm)、孔隙率为42.95%;通过该方法制得的NFC多孔薄膜的孔径大、孔隙率高,具有良好的润湿性和热稳定性,在高效过滤、生物医药和分析检测等领域的应用前景广泛。     

Use of reverse osmosis membranes to remove perfluorooctane sulfonate (PFOS) from semiconductor wastewater

Perfluorooctane sulfonate (PFOS) and related substances are persistent, bioaccumulative, and toxic, and thus of substantial environmental concern. PFOS is an essential photolithographic chemical in the semiconductor industry with no substitutes yet identified. The industry seeks effective treatment technologies. The feasibility of using reverse osmosis (RO) membranes for treating semiconductor wastewater containing PFOS has been investigated. Commercial RO membranes were characterized in terms of permeability, salt rejection, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and membrane surface zeta potential (streaming potential measurements). Filtration tests were performed to determine the membrane flux and PFOS rejection. Over a wide range of feed concentrations (0.5 - 1500 ppm), the RO membranes generally rejected 99% or more of the PFOS. Rejection was better for tighter membranes, but was not affected by membrane zeta potential. Flux decreased with increasing PFOS concentration. While the flux reduction was severe for a loose RO membrane probably due to its higher initial flux, very stable flux was maintained for tighter membranes. At a very high feed concentration (about 500 ppm), all the membranes exhibited an identical stable flux. Isopropyl alcohol, present in some semiconductor wastewaters, had a detrimental effect on membrane flux. Where present it needs to be removed from the wastewater prior to using RO membranes.     

CLSM技术在纤维表面形态和纸张结构研究中的应用

采用共聚焦激光扫描显微镜(CLSM),可以在不损伤样品的前提下获得样品的三维图像,具有较高的横向(XY向)分辨率和信噪比。综述了CLSM的工作原理,重点讨论了其在纤维表面分析(纤维形态和表面的木素)、纤维形变性研究(柔韧性和压溃性能)以及纸张结构观察(表面结构和内部结构)中的应用。     

超声处理对马铃薯淀粉结构特性及理化性质的影响

为了研究超声对马铃薯淀粉微观结构及理化性质的影响,以马铃薯淀粉为原料,通过扫描电子显微镜、激 光共聚焦显微镜、X射线衍射、傅里叶变换红外光谱、差示扫描量热法、快速黏度分析、偏光显微镜观察等方法, 研究了超声对马铃薯淀粉微观结构及理化性质的影响,揭示超声对马铃薯淀粉的机械力化学效应。结果表明：由于 马铃薯淀粉颗粒内部“狭长的脐点区”结构比较疏松,超声处理对马铃薯淀粉可产生显著机械力化学效应;随着超 声时间延长,马铃薯淀粉颗粒内部依次经过受力阶段、聚集阶段、团聚阶段;同时由于超声处理引起了马铃薯淀粉 颗粒结构变化,故导致了马铃薯淀粉理化性质显著变化。     

Evaluation of hybrid pressure-driven and osmotically-driven membrane process for non-thermal production of apple juice concentrate

This study investigates the potential of hybrid membrane processes including microfiltration (MF), ultrafiltration (UF) and forward osmosis (FO) for non-thermal concentration of apple juice. The process performance and characteristics (physicochemical properties, nutritional and aroma components and microbiological quality) of apple juice were studied. The clarity of apple juice was significantly promoted as pore size of membrane reduced. MF and UF can also ensure microbiological safety in pre-treated apple juice. According to its efficiency of filtration as well as performance of simultaneous clarification and cold-sterilization, 0.22 μm MF membrane was identified as the optimal membrane for the pre-treatment. The pre-treated apple juice can be concentrated up to 65°Brix by subsequent single stage FO. FO retained nutritional and volatile compounds of apple juice while significant reductions were found in the juice concentrated by vacuum evaporation. Hybrid MF-FO can be a promising non-thermal technology to produce apple juice concentrate with high quality.     

Ultrafiltration fouling of amylose solution: Behavior, characterization and mechanism

Applications of ultrafiltration membrane often deal with feed streams containing amylose starch. This paper describes a detailed investigation of amylose fouling during ultrafiltration. Commercial membranes made of polysulfone and fluoro polymer were used. Both adsorptive and ultrafiltration fouling were investigated. Experiments using different membrane characteristics, feed concentrations and trans-membrane pressures were carried out. The resulting fouling was characterized by water flux and contact angle measurements and was visualized by scanning electron microscopy (SEM). The results suggest that solute adsorption has occurred as noticed by significant water flux reductions as well as changes in membrane characteristics. Further, both reversible and irreversible fouling have occurred during ultrafiltration with irreversible fouling was more dominant. Apparently, cake layer formation initiated by either adsorption due to hydrophobic–hydrophobic interactions or pore blocking is the dominant fouling mechanism. However, pore narrowing instead of pore blocking was also observed for the membrane having large and relative uniform pore structure or for the ultrafiltration using low trans-membrane pressure or low solute concentration. Membrane autopsy using SEM confirmed the formation of solute layer on the membrane surface.     

Superhydrophobic nanofibre membranes: effects of particulate coating on hydrophobicity and surface properties

Superhydrophobic electrospun polyacrylonitrile nanofibre membranes have been prepared by surface coating of silica nanoparticles and fluorinated alkyl silane. The coated membranes were characterised by scanning electron microscopy, water contact angle, thermogravimetry analysis, Brunauer–Emmett–Teller, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. It was shown that the loading of nanoparticle on the nanofibre membrane was controlled by the particle concentration in the coating solution, which played a critical role in the formation of superhydrophobic surface. Increased particle loading led to higher surface roughness and WCA. The nanoparticle coating had little influence on the porosity of the nanofibre membranes. However, overloading of the particles would affect the specific surface area of the nanofibre membrane.     

Characterization of cake layer in submerged membrane bioreactor

Cake layer formation on the membrane surface has been a major challenge in the operation of membrane bioreactors (MBRs). In this study, the cake layer formation mechanism in an MBR used for synthetic wastewater treatment was investigated. The major components of cake layer were systematically examined by particle size analyzer (PSA), scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), X-ray fluorescence (XRF), energy-diffusive X-ray analyzer (EDX), and Fourier transform infrared (FTIR) spectroscopy. The results indicate that the small particles in sludge suspension had a strong deposit tendency on the membrane surface. The SEM and CLSM analysis exhibited that bacterial clusters and polysaccharides were significant contributors to membrane fouling. The main components of biopolymers were identified as proteins and polysaccharide materials by the FTIR. The examination by EDX and XRF demonstrated that Mg, Al, Ca, Si, and Fe were the major inorganic elements in fouling cake. Furthermore, the results suggest that bridging between deposited biopolymers and inorganic compounds could enhance the compactness of fouling layer. During the operation of MBRs, the biopolymers and inorganic elements in the bioreactor should be controlled to minimize membrane fouling.     

Effect of fractal flocculation behavior on fouling layer resistance during apple juice microfiltration

The colloidal material in juice obtained from Red Delicious apples held in cold storage for over 6 months was observed to spontaneously aggregate to form ordered flocculation patterns. These flocculation patterns could be altered by heat treatment and by gelatin or antioxidant addition. The specific resistance of the fouling layer produced by each juice during microfiltration could be qualitatively predicted by examining the structure of macroscopic flocculation patterns. Environmental scanning electron microscopy (ESEM) photos of the various hydrated fouling layers indicated that loose flocs appeared to compress and rearrange on the membrane surface, producing smooth, low porosity secondary membranes. Densely packed cross-linked aggregate networks also formed low porosity structures, once again creating a fouling layer with high resistance. The lowest resistance fouling layers were produced when the flocs were dense enough to resist compression but porous enough to provide pathways for permeate flow. Field emission scanning electron microscopy (FESEM) and ESEM images indicated that, in many cases, the spacial distribution of solids in the microscopic aggregates were reminiscent of those in the macroscopic flocs. Such scaling relationships are typical of fractal systems.