Chlorine-resistance of reverse osmosis (RO) polyamide membranes

Polyamide (PA) reverse osmosis (RO) membranes suffer performance decay when exposed to oxidizing species, limiting their lifetime and increasing operation costs. This article aims at reviewing the effect of chlorine species on the performance and characteristics of PA-membranes. Experimental evidence supporting different competing mechanisms for chlorine-polymer interaction will be presented, together with the influence of operational parameters. Additionally, an overview of different modification methods that exist to render PA-membranes more chlorine-resistant is given.     

Surface-tailoring chlorine resistant materials and strategies for polyamide thin film composite reverse osmosis membranes

Polyamide thin film composite membranes have dominated current reverse osmosis market on account of their excellent separation performances compared to the integrally skinned counterparts. Despite their very promising separation performance, chlorine-induced degradation resulted from the susceptibility of polyamide toward chlorine attack has been regarded as the Achilles’s heel of polyamide thin film composite. The free chlorine species present during chlorine treatment can impair membrane performance through chlorination and depolymerization of the polyamide selective layer. From material point of view, a chemically stable membrane is crucial for the sustainable application of membrane separation process as it warrants a longer membrane lifespan and reduces the cost involved in membrane replacement. Various strategies, particularly those involved membrane material optimization and surface modifications, have been established to address this issue. This review discusses membrane degradation by free chlorine attack and its correlation with the surface chemistry of polyamide. The advancement in the development of chlorine resistant polyamide thin film composite membranes is reviewed based on the state-of-the-art surface modifications and tailoring approaches which include the in situ and post-fabrication membrane modifications using a broad range of functional materials. The challenges and future directions in this field are also highlighted.     

Study of the structure and transport of asymmetric polyamide membranes for reverse osmosis using the electron spin resonance (ESR) method

The electron spin resonance technique (ESR) was used to study the structure and transport of asymmetric aromatic polyamide membranes. TEMPO (2,2,6,6-tetramethyl-1-piperridinyloxy free radical) was used as a spin probe that was brought into the membrane either by (a) immersion ofthe membranes in aqueous TEMPO solutions, (b) reverse osmosis (RO) experiments with feed solutions involving TEMPO or (c) blending TEMPO in casting solutions. The membranes were further tested for the separation of sodium chloride and TEMPO from water by RO. It was concluded that aromatic polyamide membranes contain water channels in the polymer matrix like cellulose acetate membranes. The presence of such water channels allows aromatic polyamide membranes to be used as RO membranes. The diffusion of organic solutes through the water channels seems much slower in aromatic polyamide membranes than in cellular acetate membranes, which probably causes a higher separation of organic solutes by aromatic polyamide membranes than cellulose acetate membranes. A comparison was made with other RO membranes (cellulose acetate, CA) and ultrafiltration membranes (polyethersulphone, PES). It was observed that the ESR technique can be used to study the structure of OF and RO membranes. The presence of water channels in the polymer matrix seems indispensable for the RO membrane.     

Synthesis and evaluation of aromatic polyamide membranes for desalination in reverse-osmosis technique

Reverse-osmosis membrane-grade aromatic polyamides have been synthesized by reacting 3,5-diaminobenzoic acid separately with three different acylchlorides, viz. isophthaloyl chloride, terephthaloyl chloride, and 4,4′-diphenyldicarboxylic acid chloride. Using these polyamides, asymmetric membranes were developed and characterized for various physical parameters, such as Staverman coefficient, membrane potential, and percent salt rejection using sodium chloride solution under high pressure. The effects of pressure, feed concentration, and feed flow rate have been studied on membrane transport parameters, viz. pure water permeability constant, product rate, solute transport parameter, and separation factor. The effects of annealing temperature and solvent evaporation time on the performance of the membranes were also studied. The analysis of the reverse-osmosis data revealed that the membranes prepared from the 3,5-diaminobenzoic acid and 4,4′-diphenyldicarboxylic acid chloride are superior to the membranes prepared from other polymeric materials. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 643–653, 1997     

反渗透膜的污染及其化学清洗

反渗透膜的污染是反渗透系统应用中最大的问题之一,及时的化学清洗可有效地恢复反渗透膜的性能、延长其使用寿命。对反渗透膜的污染及其化学清洗方法进行了综述,主要内容包括反渗透膜污染物的种类,污染物形成的原因,以及清洗时机的选取,清洗药品的选择,影响清洗效果的主要因素、清洗系统的构成,化学清洗的方法和清洗效果的评价指标。     

Improved separation performance of polyamide based reverse osmosis membrane incorporated with poly(dopamine) coated carbon nanotubes

A series of polyamide thin-film nanocomposite (PA TFN) membranes have been fabricated by incorporating hydrophilic poly(dopamine) (PDA) coated carbon nanotubes (CNTs@PDA) into the PA selective layer via interfacial polymerization. The effects of PDA coating thickness on surface characteristics and separation performances of membranes are studied in detail. The PDA coating makes the surface of PA TFN membrane more hydrophilic, smoother and less electronegative. The desalination performance is obviously influenced by the coating thickness of PDA and the loading concentration of PDA@CNTs. The water fluxes of PDA@CNTs incorporated PA TFN membranes have been improved without sacrificing NaCl rejections. When the loading concentration is 0.0010%, the maximum water flux is 48.1 L m⁻² h increasing by 45% compared with that of pristine PA membrane. Meanwhile, the NaCl rejection is up to 99.8%. The CNTs@PDA incorporated PA TFN membranes exhibit better anti-fouling property and separation performance durability. This work proves that CNTs@PDA has great potential application in PA TFN membranes.     

优化聚酰胺分离层制备高选择透过性反渗透膜

反渗透(RO)膜分离技术由于具有高效、低耗和产水水质高等优点,已成为现阶段解决水资源短缺的有效手段。进一步提高RO膜的选择透过性能有利于降低产水成本和提高产水质量,因此制备高选择透过性能的RO膜一直是膜领域研究的重点。从优化界面聚合工艺、优化基膜及开发新型制膜工艺三方面对近年来改善RO膜选择透过性能的研究进行了综述。通过优化界面聚合工艺和开发新型制膜工艺可以直接改变分离层的结构和性质,通过调节基膜的孔径、孔隙率及亲疏水性可以影响分离层的结构,从而改善RO膜的性能。最后对制备高选择透过性能的反渗透膜的研究方向与发展前景进行了总结与展望。     

Permselectivity of polyamide composite membrane modified by solvent

Asymmetric reverse osmosis (RO) polyamide (PA) composite membrane may be modified by means of a solvent‐processing technique into pervaporation (PV) membrane for separating organic aqueous solution. Formic acid, acetic acid, phosphoric acid, hydrochloric acid, phenol, and so forth, better solvents for PA, were selected as the modifying agents in this study. The effect of the modifying conditions (type and concentration of the modifying agent, processing time of membrane in liquid medium) on the sorption of modified membranes was investigated. After the PA composite membrane was treated with 8 wt % acetic acid for 1 h, the swelling rate for water (S_w) increased, whereas that for isopropanol (S_IPA) decreased, compared with that of the nonmodified membrane. The difference between S_w and S_IPA of the membrane treated with acetic acid was greater than that with other modifying agents. The separation factor and flux of the modified membrane were correspondingly maximal when it was used in a PV separating isopropanol aqueous solution. The contact of solvent molecules in liquid medium with polymer chains in the PA membrane and the intranodular chains in the PA membrane tending to diffuse by reptation toward the solvent molecules were the principal causes leading to a change of structure of asymmetric PA composite membrane. The mechanism of changing membrane structure by means of the solvent‐processing technique may better interpret the modification of PA composite RO membrane. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1005–1010, 2004     

PA/PEG交联共聚反渗透膜的制备及耐污染性能

以聚丙烯腈（PAN）超滤膜为基底,在均苯三甲酰氯（TMC）和间苯二胺（MPD）界面聚合反应过程中引入聚乙二醇（PEG）,制备聚酰胺（PA）/PEG反渗透复合膜。利用傅里叶红外光谱（FTIR）、X射线光电子能谱（XPS）、场发射扫描电镜（FE-SEM）和原子力显微镜（AFM）等对膜的结构和物化性质进行了表征。考察了PEG分子量、PEG加入量、热处理温度及时间对膜反渗透性能的影响,并系统考察了膜对十二烷基硫酸钠（SDS）、十二烷基三甲基溴化铵（DTAB）以及牛血清蛋白（BSA）三种不同荷电性典型污染物的耐受性能。实验结果表明,相比于未改性的PA反渗透膜,PEG交联共聚反渗透膜的盐截留率和通量恢复率均有所提高。其中PA/PEG1000-0.1膜渗透通量为3.96kg/(m²?h),NaCl表观截留率为97.5%,并且对于SDS、DTAB和BSA都表现出较高的耐污染性,其中,SDS污染清洗后膜的通量恢复率可达89.4%。     

Experimental study and modeling of organic solvent reverse osmosis separations through organosilica membranes

Exceptionally stable, mechanically robust, and highly methanol-selective organosilica membranes, including Bis(triethoxysiyl)acetylene (BTESA), fluorine-doped bis(triethoxysiyl) methane (F-BTESM), and Cetyltrimethylammonium chloride-etched bis(trimethoxysiyl)hexane (CTAC-BTMSH), were prepared and utilized for organic solvent reverse osmosis (OSRO) separations. The BTESA membrane showed optimal separation performance regarding methanol/toluene and possessed the highest levels of both permeation flux and rejection. Continuous measurements were performed to highlight the molecule size/shape discrimination of BTESA membranes using compounds such as methanol/methyl acetate, methanol/dimethyl carbonate (DMC) and methanol/methyl tert-butyl ether (MTBE). Also, a generalized solution-diffusion model was successful in predicting the permeation behaviors through organosilica membranes when used in an OSRO modality, and proved to be capable of accurate predictions on pressure-dependent permeation flux and rejection for a wide range of feed concentrations (0–55 wt%) and pressures (2–14 MPa). This study lends important insight for the development of organosilica membranes and process design for the energy-efficient OSRO separation of organic liquids.     

Distribution and diffusion coefficients of NaCl in polyamide (nylon‐6,6 and polyxylyleneadipamide) membranes

Thin membranes of an aliphatic polyamide (nylon‐6,6) and an aromatic polyamide (polyxylyleneadipamide) (PXAP) were prepared, and their distribution (K) and overall diffusion (D) coefficients of sodium chloride were measured with the unsteady‐state and steady‐state dialysis method. The overall diffusion coefficients at a zero concentration [D⁽⁰⁾] of sodium chloride for nylon‐6,6 and PXAP were 1.3–0.8 μm²/s (from 2 min of interfacial polymerization to 4 min) and 0.078, respectively. D⁽⁰⁾ for PXAP was about 3 times greater than that of a cellulose acetate (CA) membrane (0.024 μm²/s). The K values for nylon‐6,6 and PXAP were 0.7–0.5 from 2 to 4 min and 0.05, respectively. K for PXAP was almost the same as K for CA (0.06). A two‐part (dense and porous) model of the membrane structure was applied to obtain D_d (the diffusion coefficient in the dense part of the membrane) and D_p (the diffusion coefficient in the porous part of the membrane) for CA, PXAP, and nylon‐6,6 thin membranes. The values of D_d were almost the same for both nylon‐6,6 and PXAP (0.05–0.061 μm²/s) and about 10 times greater than the value for the CA membrane (5.6 × 10^?3 μm²/s). D_p for PXAP was almost the same as D_p for CA. However, D_p for the nylon‐6,6 membrane was 10–16 times greater than D_p for the PXAP membrane. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2605–2612, 2002     

Pesticides removal performance by low‐pressure reverse osmosis membranes

Pressure driven techniques (viz. reverse osmosis and nanofiltration) have the potentiality to remove the pesticides from water. The observations revealed that pesticides removal mostly depends upon the molecular weight (size exclusion) and hydrophobicity (log P) of the pesticides. Interfacial polymerization of m‐phenylene diamine (MPD) and trimesoyl chloride (TMC) on the polysulfone membranes impart the salt rejection property in it. It is shown that with the greater salt rejection property, the performance removal of pesticides also is in increasing trend. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3575–3579, 2006     

沸石/聚酰胺反渗透复合膜的制备

为了提高反渗透膜的通量,通过在界面聚合反应过程中添加NaA型纳米沸石分子筛制备了沸石/聚酰胺反渗透复合膜,采用SEM及复合膜性能测试的方法比较了沸石分子筛添加在水相或者油相中时对膜结构及分离性能的影响.SEM图谱结果表明:沸石分子筛添加在油相中时,沸石在聚酰胺基质中分散均匀,膜结构比较均一;但当沸石分子筛添加在水相中时...     

纳滤/反渗透膜处理重金属废水的性能

The performance of different nanofiltration （NF） and reverse osmosis （RO） membranes was studied in treating the toxic metal effluent from metallurgical industry. The characteristics and filtration behavior of the processes including the wastewater flux, salt rejection and ion rejection versus operating pressure were evaluated. Then the wastewater flux of RO membrane was compared with theoretical calculation using mass transfer models, and good consistency was observed. It was found that a high rejection rate more than 95% of metal ions and a low Chemical Oxygen Demand （COD） value of 10 mg·L^-1 in permeate could be achieved using the RO composite membrane, while the NF rejection of the salt could be up to 78.9% and the COD value in the permeate was 35 mg·L^-1. The results showed that the product water by both NF and RO desalination satisfied the State Reutilization Qualification, but NF would be more suitable for large-scale industrial practice, which offered significantly higher permeate flux at low operating pressure.     

Hydration,porosity and water dynamics in the polyamide layer of reverse osmosis membranes: A molecular dynamics study

This study employs molecular dynamics to gain insight into the mechanisms underlying the formation, hydration and functioning of aromatic polyamide membrane used for water purification by reverse osmosis. A polyamide membrane structure was successfully generated with a chemical composition and degree of crosslinking that closely match recent experimental data. The density of dry polymer was found to be slightly lower than in previous studies but closer to recent experimental measurements. The volume expansion and mass gain upon hydration show marked variations due to the presence of a significant fraction of permanent voids, revealed by dynamically averaged water concentration maps. The mass hydration and porosity were also larger than previously obtained, but water self-diffusivity was similar to previous simulations, presumably, since the larger porosity was offset by the higher cross-linking. Using a radial distribution function of water within polyamide, strong next-neighbor correlations and random long-distance water–water correlations could be differentiated, which could be assigned to small network and large aggregate pores. Unfortunately, this dual porosity picture could not be linked to the experimentally found bimodal distribution of the pKa values of the COOH groups, since the distribution of the local water content around COOH was found to be unimodal.     

抗污染芳香聚酰胺反渗透膜研究进展

聚酰胺反渗透膜具有选择透过性高、化学稳定性好等优点,在水处理领域应用广泛。但膜污染导致的通量下降、寿命降低等问题严重制约了其发展与应用,开发抗污染反渗透膜是缓解膜污染的重要手段。本文根据抗污染膜作用机理将抗污染反渗透膜分为抗黏附型、污染驱除型和杀菌型,综述了近年来相关方面的研究成果,并对合理组合多种机制制备抗污染反渗透膜的进展进行简要概括,最后对抗污染反渗透膜的发展前景进行了展望。     

反渗透膜的技术经济指标评测

反渗透膜过滤技术是一种高效、低能和易操作的液体分离技术,比传统的水处理方法效果好,可实现海水淡化、废水的循环利用及对有用物质有效回收。综述了反渗透膜技术的原理、特点及应用,重点总结了反渗透膜的技术指标,如水通量、脱盐率等,并介绍了其评测方法,同时对反渗透膜的经济指标及计算方法进行了介绍。     

硼酸、硼酸盐及其络合物在反渗透膜内扩散过程的分子模拟

利用分子模拟的方法建立了交联的反渗透膜模型,采用分子动力学模拟方法研究了水、氯化钠、硼酸、硼酸盐、山梨醇-硼酸盐络合物在反渗透膜和溶液中的扩散过程,以及其他几种硼酸盐与多羟基化合物（D-甘露醇、N-甲基D-葡糖胺和D-葡萄糖酸钠）的络合物在4种反渗透膜中的扩散过程。分析各粒子的均方位移曲线可得,各粒子在溶液中的扩散系数均大于在膜体系的扩散系数,水分子的扩散系数远大于各溶质分子和离子。与硼酸分子相比,硼酸盐离子与其体积相差不大,但受电荷效应的影响,其扩散系数小于硼酸分子一个数量级。与单络合物相比,双络合物由于分子体积增大,扩散系数更小。因此在反渗透络合强化脱硼过程中应保持过量的山梨醇添加量,以提高其双络合物浓度。在1,3,5-环己烷三甲酰氯/4-甲基间苯二胺（HT/MMPD）反渗透膜中,N-甲基D-葡糖胺-硼酸盐络合物的扩散系数是5种络合物中最低的,D-葡萄糖酸钠-硼酸盐络合物在含有间苯二胺（MPD）单体的膜类型中扩散系数较低,而D-甘露醇-硼酸盐络合物在含有MMPD单体的膜类型中扩散系数较低。由各粒子的轨迹图可知,各粒子在溶液中做较稀疏的无规则扩散运动,在膜内粒子由于受到聚酰胺膜结构的限制做较密集的扩散运动。     

Preparation and testing of polyvinyl alcohol composite membranes for reverse osmosis

Thin film composite membranes were prepared by coating porous polysulfone membranes with a polyvinyl alcohol layer and further cross-linking its surface. The thin layer of cross-linked polyvinyl alcohol served as a selective membrane. The membranes were prepared under various conditions and tested for sodium chloride separation. A high sodium chloride separation was achieved but the permeation rate was low compared with commercially available thin film composite membranes. Resistance against the flow of solvent water and sodium chloride solute were determined for individual component barrier layers.