Nanofiltration-like forward osmosis membranes on in-situ mussel-modified polyvinylidene fluoride porous substrate for efficient salt/dye separation

Here, polyvinylidene fluoride (PVDF) membranes were fabricated via non-solvent induced phase separation (NIPS) using dopamine (DA) and polyethyleneimine (PEI) as the hydrophilic additives, which has a loose surface and somewhat improved hydrophilicity. Then nanofiltration (NF)-like thin-film composite forward osmosis (TFC FO) membrane with a loose polyamide (PA) active layer on the blend membrane was synthesized via the interfacial polymerization. The as-prepared NF-like TFC FO membrane exhibited a high water flux (J_w) of 29.98 L m⁻² h⁻¹ and a much low specific salt flux (J_s/J_w) of 0.018 g/L, when 0.6 M NaCl was used as draw solution (DS). It had a superior rejection of malachite green (99.6% ± 0.1%) and a low rejection of NaCl (27.4% ± 4.2%), when filtrated malachite green/NaCl mixture solution in active layer-facing draw solution (AL-FS) mode. The results provide new insights on the design and preparation of FO membranes of selective separation for dyes from salty water.     

Channel regulation of TFC membrane with hydrophobic carbon dots in forward osmosis

Zero-dimensional carbon dots have emerged as important nanofillers for the separation membrane due to their small specific size and rich surface functional groups. This study proposed a strategy based on hydrophobic carbon dots (HCDs) to regulate water channels for an efficient forward osmosis (FO) membrane. Thin-film composite (TFC) membranes with superior FO performance are fabricated by introducing HCDs as the nanofiller in the polyacrylonitrile support layer. The introduction of HCDs promotes the formation of the support layer with coherent finger-like hierarchical channels and micro-convex structure and an integrated polyamide active layer. Compared to the original membrane, TFC-FO membrane with 10 wt% HCDs exhibits high water flux (15.47 L m⁻² h⁻¹) and low reverse salt flux (2.9 g m⁻² h⁻¹) using 1 mol/L NaCl as the draw solution. This improved FO performance is attributed to the lower structural parameters of HCDs-induced water channels and alleviated internal concentration polarization. Thus, this paper provides a feasible strategy to design the membrane structure and boost FO performance.     

Removal of Natural Organic Matter by Forward Osmosis Membrane: Flux Behaviour and Foulant Characterization

Fouling of cellulose triacetate(CTA) forward osmosis(FO) membranes by natural organic matter(NOM) was studied by means of a cross-flow flat-sheet forward osmosis membrane system. The NOM solution was employed as the feed solution(FS), and a sodium chloride solution(3 mol/L) was used for the draw solution(DS). The process was conducted at various temperatures and cross-flow velocities. The flux decline was investigated with 3 h forward osmosis operation. The substances absorbed on the membranes were cleaned by ultrasonic oscillation of the fouled membranes and were characterized by methodologies including fluorescence excitation-emission matrices (EEMs) and liquid chromatography with an organic carbon detector(LC-OCD), and the variations of membrane properties were also investigated by Fourier transform infrared spectrometer(FTIR) and a contact angle meter. It was noted that the rejection efficiency of NOM is remarkable and that ultrasonic oscillation is an effective method to extract the NOM fouled on the CTA membranes after FO process. A higher cross-flow velocity and lower temperature benefit the anti-fouling capacity of the membrane significantly. Although humic substances accounted for the majo- rity of the NOM, aromatic proteins and amino acids were the main fouling components on the membranes, with symbolic FTIR peaks at 2355, 1408 and 873 cm^-1. The present surface foulant made the membranes becoming more hydrophilic, as demonstrated by a significant decrease in contact angle(ranging from 20% to 46%) under all the operation conditions.     

Membranes for power generation by pressure-retarded osmosis

A model has been developed for obtaining the projected performance of membranes in pressure-retarded osmosis (PRO) from direct osmosis and reverse osmosis measurements. The model shows that concentration polarization within the porous substrate of the membrane markedly lowers the water flux under PRO conditions. The model has been used along with experimental data obtained with a variety of reverse osmosis membranes to project PRO performance with several water—brine sources. Some literature data on PRO have been similarly examined. Based on these results and a simple economic analysis we conclude that membranes with significantly improved performance will be needed if PRO is to become an economically feasible method for power generation using seawater—fresh water as the salinity gradient resource. However, the economics of a brine/fresh water system appear competitive with conventional power generation technologies.     

Thin-film nanocomposite forward osmosis membranes modified with Zr-based metal–organic framework to improve desalination performance

In present work, first, the water-stable metal–organic framework (MOF) nanocrystals, UiO-66-(F)₄, were synthesized under green reaction condition and then some PES/PA thin-film nanocomposite (TFN) membranes were prepared using this synthesized nanocrystals (as modifier) and polyethersulfone (as the substrate). The obtained MOF and membranes were characterized by various characterization techniques such as FE-SEM, AFM, PXRD, contact angle measurements and FT-IR spectroscopy. Finally, the forward osmosis performance of the resultant membranes was evaluated by using different concentrations of NaCl as a draw solution and deionized water as a feed solution. Among all used membranes, the membrane with 0.1 wt% loading of UiO-66-(F)₄ (TFN-2) was found to be an efficient composite membrane in the FO performance with high J_w and low J_s/J_w.     

基于海水淡化的正渗透膜分离技术的发展

近年来,水资源危机日益加重。在海水淡化技术中,与反渗透分离技术相比,正渗透分离技术作为一种低能耗、绿色的解决方案,具有明显优势,在国际上得到了广泛的重视,是膜分离技术的研究热点之一。本文从正渗透膜材料结构与性能关系的角度,对膜材料的最新进展进行了综述,同时分析了汲取液的常见类型,并简述了正渗透分离技术的新应用,展望了其未来的发展前景。     

聚多巴胺/聚乙烯亚胺共沉积中间层增强薄膜复合正渗透膜性能

以聚多巴胺/聚乙烯亚胺(PDA/PEI)共沉积于三醋酸纤维素(CTA)多孔支撑膜表面形成中间层,再结合界面聚合法获得聚酰胺薄膜,构建了PDA/PEI共沉积中间层改性薄膜复合(TFC)正渗透(FO)膜.通过傅里叶变换衰减全反射红外光谱法、扫描电子显微镜、原子力显微镜、溶质截留法、水接触角仪等研究了PDA/PEI共沉积中间层对CTA膜和TFC膜的表面结构和性质的影响.研究结果表明,PDA/PEI共沉积使得CTA膜表面变得更为平滑,表面孔径减小至(30.0±4.1) nm,且表面孔径分布趋于均一.同时,在PDA/PEI共沉积改性CTA膜表面界面聚合得到的聚酰胺层呈现出更均匀的叶片状结构和优异的亲水性.基于此,具有PDA/PEI共沉积中间层的TFC正渗透膜显著提高了水通量(FO模式:(7.1±2.3) L/(m^2·h)),较空白TFC膜提升了57.6%.同时,中间层改性TFC膜具有更低的反向盐通量(FO模式:1.4±0.1 g/(m^2·h))和"净盐通量"(FO模式:(0.2±0.06) g/L),与空白TFC膜相比分别下降了83.9%和90.6%.说明PDA/PEI共沉积中间层不仅能有效提升TFC正渗透膜的水渗透性,而且大幅提升了膜的截盐性和渗透选择性.     

Mixed Polymer‐Coated Magnetic Nanoparticles as Forward Osmosis Draw Agents of Tuned Hydrophilicity

We recently reported a polymer‐coated magnetic nanoparticle (MNP) draw agent for the forward osmosis (FO) water desalination process. The water flux was found to increase when the polymer poly(sodium acrylate) (PSA) was anchored to the MNP surface as compared to the polymer (or polyelectrolyte solution) alone, due to the polymer chains being stretched out and most of the hydrophilic groups on the polymer contributing to water flux. We herein report the use of a secondary polymer poly(N‐isopropylacrylamide) PNIPAM to manipulate the PSA polymer conformation and influence inter‐ and intrachain interactions to enhance the efficiency of the FO draw agent. These PSA–PNIPAM‐coated MNPs generated a much higher water flux of ～11.66 LMH when compared to the 100 % PSA‐coated MNPs featuring a value of ～5.32 LMH under identical FO conditions. The osmotic pressure and water flux driven by the mixed polymer‐coated MNPs were found to be a strong function of the net polymer coverage on MNPs, that is, net available hydrophilic groups. Our new draw agent demonstrates potential for use in the water industry due to its improved efficiency and cost effectiveness as it uses only ～0.062 % (w/v) of the draw agent solution.     

Forward Osmosis Membranes for Water Reclamation

This review addressed the fundamental principles, advantages and challenges of forward osmosis (FO) membrane processes. FO is receiving more and more research attractions because it can concurrently produce clean water with low energy input and generate hydraulic energy (pressure retarded osmosis). FO typically requires zero or low hydraulic driving pressure, therefore the fouling potential of the FO membranes is much lower than conventional pressure-driven membrane processes. However, concentration polarization (CP), especially the internal CP significantly reduces the effective osmotic pressure across the FO membrane, the major driving force for the filtration process. As a result, innovative FO membrane materials like electrospun nanofibers have been explored to make low tortuosity, high porosity, and thin FO membranes with a high rejection rate of solutes and low or zero diffusion of the draw solute. The orientation of the FO membrane with active layer-facing-feed solution has less fouling than active layer-facing-draw solution. In addition, to further decrease the fouling potential, a hydrophilic and more negatively charged membrane is preferred when filtration of natural organic matter (NOM) or alginate in the absence of multivalent cations.     

Polybenzimidazole (PBI) nanofiltration hollow fiber membranes applied in forward osmosis process

For the first time, the potential of polybenzimidazole (PBI) nanofiltration membrane as a forward osmosis membrane has been investigated. PBI was chosen mainly because of its unique nanofiltration characteristics, robust mechanical strength and excellent chemical stability. The MgCl₂ solutions with different concentrations and other different salt solutions were employed as draw solutions to test the water permeation flux through the PBI membrane during forward osmosis. High water permeation flux and excellent salt selectivity were achieved by using the PBI nanofiltration membrane which has a narrow pore size distribution. Effects of membrane morphology, operation conditions and flowing patterns of two feed streams within the membrane module on water transport performance have been investigated. It may conclude that PBI nanofiltration membrane is a promising candidate as a forward osmosis (FO) membrane.     

Chitosan‐modified graphene oxide as a modifier for improving the structure and performance of forward osmosis membranes

Chitosan (CS) with good hydrophilicity and charged property was used to modify graphene oxide (GO), the obtained GO‐CS was used as a novel modifier to fabricate thin film composite forward osmosis (FO) membranes. The results revealed that the amino groups on CS reacted with carboxyl groups on GO, and the lamellar structure of the GO nanosheets was peeled off by CS, resulting in the reducing of their thicknesses. The GO‐CS improved the hydrophilicity of polyethersulfone (PES) substrate, and their contact angles decreased to 64° with the addition of GO‐CS in the substrate. GO‐CS also increased the porosity of the substrate and surface roughness of FO membrane, thereby optimizing the water flux and reverse salt flux of FO membrane. The average water flux of the FO membrane reached the optimal flux of 21.34 L/(m² h) when GO‐CS addition was 0.5 wt%, and further addition of GO‐CS to the substrate would decrease the water flux of FO membrane, and the reverse salt flux also decreased to the lowest value of 2.26 g/(m² h). However, the salt rejection of the membrane increased from 91.4% to 95.1% when GO‐CS addition increased from 0.5 to 1.0 wt% under FO mode using 1 mol/L sodium chloride (NaCl) solution as draw solution (DS). In addition, high osmotic pressure favored water permeation, and at the same concentration of DS, magnesium chloride (MgCl₂) exhibited better properties than NaCl. These results all suggested that GO‐CS was a good modifier to fabricate FO membrane, and MgCl₂ was a good DS candidate.     

木质素磺酸钠改性聚砜膜的制备及其在正渗透膜中的应用

采用木质素磺酸钠作为亲水添加剂,通过浸没沉淀相转化法制备了木质素磺酸钠共混改性聚砜膜,以改善聚砜膜的亲水性,并用作正渗透膜的支撑层,以降低内浓差极化效应.利用扫描电子显微镜、衰减全反射傅里叶变换红外光谱仪、水接触角仪等研究了不同木质素磺酸钠添加量对聚砜膜的结构和表面性质的影响.结果表明,添加木质素磺酸钠后,聚砜膜的指状孔变得规整且狭长.水接触角实验证实添加木质素磺酸钠能改善聚砜膜的亲水性,当木质素磺酸钠含量为0.4 wt%时,聚砜膜的表面水接触角可降低至65°.正/反渗透测试装置分别用于表征正渗透膜的传质性质和结构参数.结果表明,以0.4 wt%木质素磺酸钠改性聚砜膜为支撑层的正渗透膜的水渗透性能(A=3.12×10~(-5) LMH×Pa~(-1))优于纯聚砜基底正渗透膜(0.76×10~(-5)LMH×Pa~(-1)),而且前者的结构参数(S=2010mm)远小于后者(3450mm),说明木质素磺酸钠改性聚砜膜有效弱化了正渗透膜的内浓差极化效应.     

正渗透过程中纳米碳管膜的盐水通道行为

以纳米碳管(CNT)仿生构筑正渗透(FO)膜, 采用分子动力学模拟的方法考察水和盐在由CNT(6,6)、CNT(7,7)、CNT(8,8)、CNT(9,9)、CNT(10,10)、CNT(11,11)等不同尺寸纳米碳管构筑膜中, 于2.5、3.75、5.0mol·L^-1等不同汲取液浓度下的传递行为. 纳秒级的模拟得到水分子在不同尺寸纳米碳管膜内的分布, 水通量的变化以及盐截留等情况. 模拟结果表明, 由CNT(8,8)构筑的正渗透膜表现出优异的通水阻盐性能.     

Water,salt, and ethanol diffusion through membranes for water recovery by forward (direct) osmosis processes

Measurements of the diffusive permeances of water, NaCl, and ethanol through several, unoptimized membranes are presented. Such data can facilitate analysis and development of water recovery from highly impaired sources using hybrid processes based on forward (direct) osmosis (FO) with aqueous ethanol solutions as the “osmotic” agent. The membranes we have studied include anion and cation exchange materials and cross-linked poly(vinylalcohol) (PVA) gels, the latter being a membrane chemistry commercially used for ethanol dehydration via pervaporation. The measured transport properties are reported and suitability of these materials for an FO-based water recovery process is discussed in the context of process simulations.     

正渗透驱动液的发展状况与趋势

正渗透具有低能耗、低污染、高回收等特点,其应用范围非常广泛。具有高渗透压的驱动液是正渗透技术的核心问题。正渗透驱动液的类型有直接利用型和循环利用型,后者包括NH3/CO2汲取液、磁性驱动液、有机化合物驱动液和无机化合物驱动液。随着正渗透驱动技术的发展,驱动液已从简单的盐类、糖类转变为功能性物质。正渗透技术发展空间巨大,将成为解决困扰人类可持续发展的水资源和能源问题的关键技术之一。     

Chemical and physical aspects of organic fouling of forward osmosis membranes

The growing attention to forward osmosis (FO) membrane processes from various disciplines raises the demand for systematic research on FO membrane fouling. This study investigates the role of various physical and chemical interactions, such as intermolecular adhesion forces, calcium binding, initial permeate flux, and membrane orientation, in organic fouling of forward osmosis membranes. Alginate, bovine serum albumin (BSA), and Aldrich humic acid (AHA) were chosen as model organic foulants. Atomic force microscopy (AFM) was used to quantify the intermolecular adhesion forces between the foulant and the clean or fouled membrane in order to better understand the fouling mechanisms. A strong correlation between organic fouling and intermolecular adhesion was observed, indicating that foulant–foulant interaction plays an important role in determining the rate and extent of organic fouling. The fouling data showed that FO fouling is governed by the coupled influence of chemical and hydrodynamic interactions. Calcium binding, permeation drag, and hydrodynamic shear force are the major factors governing the development of a fouling layer on the membrane surface. However, the dominating factors controlling membrane fouling vary from foulant to foulant. With stronger intermolecular adhesion forces, hydrodynamic conditions for favorable foulant deposition leading to cake formation are more readily attained. Before a compact cake layer is formed, the fouling rate is affected by both the intermolecular adhesion forces and hydrodynamic conditions. However, once the cake layer forms, all three foulants have very similar flux decline rates, and further changes in hydrodynamic conditions do not influence fouling behavior.     

HPLC study of the physicochemical characteristics of reverse-osmosis separation on a polyamide membrane material

Summary The fundamental characteristics of reverse osmosis on a polymer membrane have been correlated with HPLC experimental conditions by using the membrane material as a column packing. Twelve formulas were used to calculate the physicochemical characteristics of the reverse-osmosis separation process and it was found that these characteristics can be determined on the basis of retention (t _R, V_R) and partition coefficient (K) of the solute in HPLC. It seems that HPLC is an effective tool for studying the physicochemical nature of reverse-osmosis separations and the characteristics of the polymer membrane.     

Changes of antioxidative enzymes and cell membrane osmosis in tomato colonized by arbuscular Mycorrhizae under NaCl stress

Salinity toxicity is a worldwide agricultural and eco-environmental problem. Many literatures show that arbuscular mycorrhizal fungi (AMF) can enhance salt tolerance of many plants and some physiological changes occurred in AM symbiosis under salt stress. However, the role of ROS-scavenging enzymes in AM tomato is still unknown in continuous salt stress. This study investigated the effect of Glomus mosseae on tomato growth, cell membrane osmosis and examined the antioxidants (superoxide-dismutase, SOD; catalase, CAT; ascorbate peroxidase, APX; peroxidase, POD) responses in roots of mycorrhizal tomato and control under different NaCl stress for 40 days in potted culture. NaCl solution (0, 0.5 and 1%) was added to organic soil in the irrigation water after 45 days inoculated by AMF (Glomus mosseae). (1) AMF inoculation improved tomato growth under salt or saltless condition and reduced cell membrane osmosis, MDA (malonaldehyde) content in salinity. So the salt tolerance of tomato was enhanced by AMF; (2) SOD, APX and POD activity in roots of AM symbiosis were significantly higher than corresponding non-AM plants in salinity or saltless condition. However, CAT activity was transiently induced by AMF and then suppressed to a level similar with non-AM seedlings; (3) higher salinity (1% level) and long stress time suppressed the effect of AMF on SOD, APX, POD and CAT activity; (4) this research suggested that the enhanced salt tolerance in AM symbiosis was mainly related with the elevated SOD, POD and APX activity by AMF which degraded more reactive oxygen species and so alleviated the cell membrane damages under salt stress. Whereas, the elevated SOD, POD and APX activity due to AMF depended on salinity environment.     

Study on preparation and performances of cellulose acetate forward osmosis membrane

Cellulose acetate (CA) forward osmosis (FO) membranes were prepared via a phase inversion process. CA was used as membrane material for FO. Acetone and 1,4-dioxane were employed as solvent. Polyvinylpyrrolidone (PVP), maleic acid, and methanol were applied as additives. An orthogonal experiment was performed to optimize the ratio of every component in the casting solution. The membrane with best performance was selected to concentrate an anthocyanin solution. Saturated sucrose solution (about 60°Brix) was fit for using as draw solution in the concentration experiment. Water flux, porosity, and rejection rate were measured to evaluate the membrane properties. Reverse water rinsing was used in cleaning membrane that was fouled by anthocyanin solution. Results showed that under membrane thickness of 100 μm, coagulation temperature at room temperature, and evaporation time of 30 s, the optimum components in casting solution were 13% CA, 45% 1,4-dioxane, 31% acetone, 2% maleic acid, 3% PVP, and 6% methanol. In the concentration experiment, the prepared FO membrane showed water flux of 2.04 L m^?2 h^?1 and rejection rate of 98.61%. In the membrane cleaning experiment, the water flux of the FO membrane recovered 87.51% after rinsing for 1 h. The prepared membranes and previously published membranes were compared which showed the prepared membrane could significantly improve the rejection rate for anthocyanin solution.