动电法研究磺化聚醚砜纳滤膜界面电现象

采用动电法对表面具有功能基团解离的磺化聚醚砜纳滤膜(NTR-7450)界面电现象进行探索。其中,在Zeta电位测试过程中引入相关措施,例如采用电化学工作站测定体系总电导(膜体电导、膜表面电导和电解质溶液电导)和变化流道高度等,以便获得更为真实的Zeta电位,进而根据Gouy-Chapman双电层模型系统地考察了离子强度、阴离子种类(KCl,K2SO4和K3PO4)对膜表面荷电性能的影响。实验结果表明,在较低浓度(0.1～0.5mmol/L)电解质溶液中,磺酸基的解离是NTR-7450纳滤膜荷电的主要原因;而在较高浓度下(1.0～10mmol/L),NTR-7450纳滤膜荷电则是由特性吸附引起,并且膜体积电荷密度与电解质溶液浓度之间符合Freundlich吸附等温式:在KCl,K2SO4和K3PO4溶液中分别为:ln|X|(mmol/L)=2.3337+0.772lnC(mmol/L),ln|X|(mmol/L)=3.584+1.119lnC(mmol/L)和ln|X|(mmol/L)=2.988+1.067lnC(mmol/L)。     

用流动电位法表征纳滤膜的结构及性能

利用测量流动电位的方法考察了纳滤膜的表面电学性能对纳滤膜的截留性能的影响.首先,采用不同功能层材料制备了复合纳滤(NF)膜,考察功能层的交联时间、单体结构等对表面电性能的影响,研究纳滤膜对不同无机盐的选择截留性能与表面电性能的关系.通过流动电位法测定纳滤膜的表面电学参数,如流动电位(ΔE)、zeta电位(ζ)和表面电荷密度(σd).实验表明,这些电学参数的变化与功能层交联时间和纳滤膜截留率的变化一致,在交联时间为45 s时,3种电学参数的绝对值均最大,而纳滤膜对无机盐的截留率也最大.复合纳滤膜zeta电位的绝对值(|ζ|)按照Na2SO4>MgSO4>MgCl2变化,同截留率的变化相同.带侧基单体交联后得到的纳滤膜的表面电性能参数的绝对值小于不带侧基单体的.因此,流动电位法可用于研究复合纳滤膜的截留机理和功能层结构.     

聚酰胺纳滤膜对黄连中生物碱的截留性能

以盐酸小檗碱为模型分子,考察循环时间、操作压力、料液浓度和离子强度等因素对聚酰胺纳滤膜截留盐酸小檗碱性能的影响。实验表明:聚酰胺纳滤膜对盐酸小檗碱的截留率80min后基本稳定;随着操作压力的增加,膜通量和截留率都增大;随着料液浓度的增加,聚酰胺纳滤膜通量下降,对盐酸小檗碱的截留率先增大后下降;随着溶液中离子强度的增加,膜通量和截留率都减小。在黄连提取液中生物碱含量为0.025g/L、操作压力为0.4MPa条件下,聚酰胺纳滤膜5min可使黄连提取液中生物碱浓缩6.27倍。     

纳滤膜对电解质溶液分离特性的理论研究(II): 混合电解质溶液

假定纳滤膜具有狭缝状孔, 使用纯水透过系数、膜孔径及膜表面电势来表征纳滤膜的分离特征, 用流体力学半径和无限稀释扩散系数表征了离子特性. 采用扩展Nernst-Planck方程、Donnan平衡模型和Poisson-Boltzmann理论描述了混合电解质溶液中离子在膜孔内的传递现象, 计算了三种商用纳滤膜(ESNA1-LF, ESNA1和LES90)对同阴离子、同阳离子和含四种离子的混合电解质体系中离子的截留率, 并与实验数据进行了比较. 计算结果表明, 电解质溶液中离子在纳滤膜孔内传递的主要机理是离子的扩散和电迁移, 纳滤膜对混合电解质溶液中离子的分离效果主要由空间位阻和静电效应决定. 该模型在低浓度时对含一价离子的混合电解质溶液通过纳滤膜的截留率计算结果比较准确, 但对高浓度或含高价离子的混合电解质溶液则偏差较大.     

紫外辐照接枝制备亲水性荷正电纳滤膜

通过在酚酞基聚芳醚酮超滤膜表面紫外辐照接枝亲水性单体二烯丙基二甲基氯化铵(DADMAC)制备了一种表面荷正电的纳滤膜. ATR-FTIR和表面水接触角的研究结果表明膜表面的接枝率和亲水性随着辐照时间和单体在接枝溶液中的浓度的增加而增加. 荷正电纳滤膜对盐溶液有很好的截留, 对盐溶液中的高价阳离子和低价阳离子的截留率分别为95%和65%. 但当溶液中存在高价负离子时, 膜的截留性能会明显下降. 表明静电效应在荷电纳滤膜的分离过程中起了重要的作用.     

氮气等离子体改性壳聚糖-聚丙烯腈复合纳滤膜

以氮气低温等离子体对壳聚糖-聚丙烯腈复合纳滤膜进行表面改性。用接触角、扫描电镜、扫描探针显微镜、X射线光电子能谱观察膜表面的亲水性和形貌特征,分析膜表面化学组成;以γ-氨基丁酸为分离对象表征膜的纳滤性能。结果表明:经50 W、20 Pa的氮气等离子体作用2 min,壳聚糖膜表面的亲水性大幅改善,其接触角由102.0°下降至44.3°,平整度明显提升;膜表面中的C—C、C—O和酰胺基团均减少,而胺基和羰基相应增加;在pH=6.15的水溶液中对w=1.0%的γ-氨基丁酸进行纳滤,液体通量由原来的1.12 L/(m~2·h)提高至1.75 L/(m~2·h),且对氨基酸的截留率从28%提升至83%。     

解读纳滤:一种具有纳米尺度效应的分子分离操作

纳滤膜是20世纪80年代末期发展起来的一种广泛用于液体分离的新型分离膜。早期研究中,先后提出的基于筛分效应的细孔模型,基于静电效应的电荷模型,以及同时考虑上述两种效应的静电位阻模型和道南位阻模型等为人们更好地理解纳滤膜分离机理和指导纳滤膜过程应用发挥了十分重要的作用。然而由于这些具有“疏松型反渗透膜”特点的纳滤膜没有相应的膜性能预测评价软件,使得针对具体应用过程的纳滤膜的大规模标准化应用受到了一定的制约。为此,结合上述模型,根据一些特定实验拟合确定混合盐体系同号离子间的竞争作用和异号离子间的调节作用,提出了一个适于混合盐体系的纳滤膜分离性能评价模型,促进了纳滤膜技术在水处理过程的大规模推广。最近,根据纳滤膜对离子选择性分离性能及其伴随的动电性质的细致而深入的实验研究,发现仅考虑筛分效应和静电效应并不能完全合理地解释纳滤膜的分离性能,且在动电性质的解析上也存在一定缺陷,进而对纳滤膜纳米级孔径引起的特殊效应和溶液体系中复杂相互作用引起的荷电性质变化有了更为深刻的认识和理解,提出并定量分析了离子透过纳滤膜时存在的介电排斥效应。     

氮气等离子体改性壳聚糖-聚丙烯腈复合纳滤膜

以氮气低温等离子体对壳聚糖-聚丙烯腈复合纳滤膜进行表面改性。用接触角、扫描电镜和扫描探针显微镜观察膜表面的亲水性和形貌特征,X射线光电子能谱分析膜表面化学组成,以γ-氨基丁酸为分离对象表征膜的纳滤性能。结果表明:经50W、20Pa的氮气等离子体作用2min,壳聚糖膜表面获得最大程度的亲水性改善,其接触角由102.0°下降至44.3°,平整度明显提升;膜表面C—C、C—O和酰胺基团均有减少,而胺基和羰基相应增加;在pH=6.15的水溶液中对w=1.0%的γ-氨基丁酸进行纳滤,液体的通量由原来的1.12L/(m2·h)提高至1.75L/(m2·h),且对氨基酸的截留率从28%提升至83%。     

纳滤膜新型材料研究

纳滤是一种介于超滤与反渗透之间的重要膜分离过程,具有工作压力低、无相转变及分离效率高等独特优势。膜污染及渗透性/选择性之间的平衡是纳滤膜在使用和研发过程中面临的亟待解决的两个主要问题。膜材料是膜与膜分离技术的核心,开发新型的纳滤膜材料是解决上述问题的重要手段。本文从新型纳滤膜材料的设计与选择的角度出发,总结归纳了近年来新型材料在纳滤膜的制备与应用研究现状,包括新型有机纳滤膜材料、新型无机纳滤膜材料和新型有机-无机杂化纳滤膜材料三个方面,拓展了对纳滤膜材料的认知,探讨了新型纳滤膜材料的共性及其存在的主要问题,并对未来高性能纳滤膜材料的研制方向进行了展望。     

纳滤膜对电解质溶液分离特性的理论研究(I): 单一电解质溶液

电解质溶液在纳滤膜中的截留率对于膜法海水淡化和重金属离子的脱除非常重要. 本文假定膜具有狭缝状孔, 采用扩展Nernst-Planck方程、Donnan平衡模型和Gouy-Chapman理论来描述电解质溶液中离子在膜孔内的传递现象. 使用纯水透过系数、膜孔径及膜表面电势来表征纳滤膜的分离特征, 这三个参数可通过Levenberg-Marquardt方法由实验数据关联得到. 本文使用该模型计算了两种商用纳滤膜(NF45和SU200)对1-1型(NaCl, KCl, LiCl), 2-1型(K₂SO₄)和2-2型(MgSO₄)单一电解质溶液的截留率, 并与实验数据进行了比较, 两者吻合较好. 计算结果表明电解质溶液中离子在纳滤膜孔内传递的主要机理是离子的扩散和电迁移, 纳滤膜对电解质溶液中离子的分离效果主要由空间位阻和静电效应决定. 该模型在低浓度时对电解质溶液通过纳滤膜的截留率计算结果较准确, 但对高浓度电解质溶液则偏差较大.     

Effect of operating variables on rejection of indium using nanofiltration membranes

Indium and its compounds exhibit excellent semiconductor properties however they are suspected carcinogenic to human beings. For the first time, we applied nanofiltration (NF) technology to the separation of indium from a synthetic wastewater as a literature review revealed little information on the treatment of such a waste. In this research, three types of nanofiltration membranes, NTR7450, ES10 and ES10C, were employed to compare their performances under various operating conditions. With increasing indium concentration in the feed solution, the rejection rates decreased in all the membranes, which could be ascribed to concentration polarization and ion-shielding effects. The changes of indium concentration in the permeate (C_p) were then correlated to the concentration factor (CF) during nanofiltration of the feed solution. The experimental results were well predicted by the theoretical analysis. Increase of operating pressure enhanced their rejection rates of indium, which might be attributed to the “dilute effect”. The real rejection (f_r) of indium by nanofiltration was found permeate flux dependent. Based on the results obtained, the nanofiltration mechanisms of multivalent cations such as In³⁺ were delineated and discussed. It was found that most of the models developed from nanofiltration of univalent and divalent cations were still valid for the nanofiltration process of trivalent cations. However, the strong chemical potential of trivalent cations to form complexes in the solution around neutral pH exerted a significant impact on indium rejection rates of the NF membranes. The experimental results suggest a stable performance of nanofiltration when applied to the semiconductor wastewater, however, acidic conditions should be avoided.     

The electrostatic and steric-hindrance model for the transport of charged solutes through nanofiltration membranes

Nanofiltration (NF) membranes possess the intermediate molecular weight cut-off between reverse osmosis membranes and ultrafiltration membranes, and also have rejection to inorganic salts. So one can assume that NF membranes have charged pore structure. We have developed the electrostatic and steric-hindrance (ES) model from the steric-hindrance pore (SHP) model and the Teorell-Meyer-Sievers (TMS) model (Wang et al., J. Chem. Eng. Japan, 28 (1995) 372) to predict the transport performance of charged solutes through NF membranes based on their charged pore structure. In this article, by doing the permeation experiments of aqueous solutions of neutral solutes and sodium chloride, the structural parameters (the pore radius and the ratio of membrane porosity to membrane thickness) and the charge density of NF membranes (Desal-S, NF-40, NTR7450 and G-20) were estimated on the basis of SHP model and the TMS model, respectively. Then, we selected an aqueous solution of different tracer charged solutes (sodium benzenesulfonate, sodium naphthalenesulfonate and sodium tetraphenyl-borate) and a supporting salt (sodium chloride) to verify the ES model. The prediction based on the ES model was in good agreement with the experimental results.     

Asymmetric ion exchange mosaic membranes with unique selectivity

In this paper, we describe the investigation of membranes to concentrate aqueous low molecular weight (<500 Da) organics streams, while removing electrolytes including divalent salts such as sodium sulfate. Such membranes would be useful in many industrial applications as currently used pressure driven process such as nanofiltration (NF) or electrical processes such as electric dialysis (ED) cannot achieve such separations and concentrations. An analysis of ion/water transport in different membranes and, selectivity and flux requirements indicated that ion exchange mosaics in the form of integrally skinned asymmetric structures could achieve the required performance. The relationship between the internal structure of the mosaic membrane elements and the required separation properties was further analyzed as a development guide. It was found that such membranes could be made by casting a homogenous solution of two mutually incompatible polymers in a common solvent, containing non-solvents and additives, followed by a chemical modification. The process of forming such membranes involves phase separation between the two polymers and the phase inversion of each polymer. In this study the membrane consists of a cation exchange asymmetric membrane with a uniform distribution of anion exchange particles in the dense integrally skin layer. The choice of polymeric materials, their molecular weights, solvent combinations and surfactants determined the membranes’ surface morphology, mosaic dimensions and particle density. In this way membranes were formed with ∼1 μm sized anion exchange particles uniformly dispersed in a thin (∼1.0 μm) cation exchange selective layer of an asymmetric membrane. The best performance to date: Fluxes of 500+LMD, 10% rejection to sodium sulfate, 90% to sucrose and >98% rejection to 400 molecular weight organic ions. The membranes also show a mosaic effect of decreasing sulfate rejection with decreasing sulfate concentration. The membranes also show a musaic effect of decreasing sulfate rejection with decreasing sulfate concentration, which is desired to perform effectively the removal of mono and bivalent ions during diafiltration.     

Removal of organic acid salts from simulated fermentation broth containing succinate by nanofiltration

In microbial cultures for the production of sodium succinate, often monovalent salts of sodium formate, sodium acetate and/or sodium lactate are produced as major by-products. In this study, nanofiltration (NF) was employed for the recovery of sodium succinate and the removal of by-products from simulated fermentation broth. In a series of preliminary experiments with synthetic single-salt solutions, five nanofiltration membranes were evaluated, and NF45 and ESNA1 membranes with a relatively low rejection to monovalent anions were selected for the subsequent experiments. The rejection of each salt at various fluxes was measured for single, binary, ternary and quaternary organic acid salts solutions containing succinate, formate, acetate, and/or lactate, simulating a real fermentation broth. Succinate rejection in multi-salt solutions was observed much higher than that in its single-salt solution, which was quite opposite to the cases of the monovalent acid salts involved. This could be well described by the facilitated transport of the monovalent anions due to Donnan effect in the presence of succinate, a divalent anion. Finally, nanofitration of a quaternary salts solution in a diafiltration mode was carried out for 36 h. With time, the rejection of succinate increased and the rejection of the by-products drastically decreased as the concentration ratio of succinate to by-products increased. From the extrapolation using a diafiltration model developed in this study, it was expected that almost complete removal of by-products was possible with no significant loss of succinate.     

Effects of membrane degradation on the removal of pharmaceutically active compounds (PhACs) by NF/RO filtration processes

The impacts of membrane degradation due to chlorine attack on the rejection of pharmaceutically active compounds (PhACs) by nanofiltration and reverse osmosis membranes were investigated in this study. Membrane degradation was simulated by soaking the membranes in a sodium hypochlorite solution of various concentrations over 18 h. Changes in membrane surface properties were characterised by contact angle measurement, atomic force microscopy analysis, and streaming potential measurement. The impacts of hypochlorite exposure to the membrane separation processes were ascertained by comparing the rejection of PhACs by virgin and chlorine-exposed membranes. Overall, the reverse osmosis BW30 membrane and the tight nanofiltration NF90 membrane were much more resilient to chlorine exposure than the larger pore size TFC-SR2 and NF270 nanofiltration membranes. In fact, rejection of all three PhACs selected in this study by the BW30 remained largely unchanged after hypochlorite exposure and further characterisation did not reveal any evidence of compromised separation capability. In contrast, the effects of chlorine exposure to the two loose nanofiltration membranes were quite profound. While chlorine exposure generally resulted in reduced rejection of PhACs, a small increase in rejection was observed when a more dilute hypochlorite solution was used. Changes in the membrane surface morphology as well as observed rejection of inorganic salts and PhACs were found to be consistent with mechanisms of chlorine oxidation of polyamide membranes reported in the literature. Chlorine oxidation consistently resulted in a more negative zeta potential of all four membranes investigated in this study. Conformational alterations of the membrane polyamide active skin layer were also evident as reflected by changes in surface roughness before and after chlorine exposure. Such alterations can either loosen or tighten the effective membrane pore size, leading to either a decrease or an increase in rejection. Both of these phenomena were observed in this study, although the decrease in the rejection of PhACs was overwhelming from exposure to highly concentrated hypochlorite solution.     

High flux nanofiltration membranes based on interfacially polymerized polyamide barrier layer on polyacrylonitrile nanofibrous scaffolds

Electrospun polyacrylonitrile (PAN) nanofibrous scaffold was used as a mid-layer support in a new kind of high flux thin film nanofibrous composite (TFNC) membranes for nanofiltration (NF) applications. The top barrier layer was produced by interfacial polymerization of polyamides containing different ratios of piperazine and bipiperidine. The filtration performance (i.e., permeate flux and rejection) of TFNC membranes based on electrospun PAN nanofibrous scaffold was compared with those of conventional thin film composite (TFC) membranes consisting of (1) a commercial PAN ultrafiltration (UF) support with the same barrier layer coating and (2) two kinds of commercial NF membranes (i.e., NF90 and NF270 from Dow Filmtec). The nanofiltration test was carried out by using a divalent salt solution (MgSO₄, 2000 ppm) and a cross-flow filtration cell. The results indicated that TFNC membranes exhibited over 2.4 times more permeate flux than TFC membranes with the same chemical compositions, while maintaining the same rejection rate (ca. 98%). In addition, the permeate flux of hand-cast TFNC membranes was about 38% higher than commercial NF270 membrane with the similar rejection rate.     

Enhancing the performance of TFC nanofiltration membranes by adding organic acids in polysulfone support layer

Throughout this study, the effect of certain organic acids, methacrylic acid, lactic acid and tartaric acid, doped in polysulfone (PSF) casting solution onto the performance of nanofiltration (NF) membranes was investigated. Different NF membranes have been prepared from m-phenylenediamine and trimesoylchloride onto the top surface of the acid-modified PSF membranes through regulating the concentration and contact time of the conventional interfacial polymerization process. The study of scanning electron microscopy (SEM) was used to investigate the influence of acids on the morphology of membranes and cross-sectional structures. The functional groups, hydroxyl and carboxylic acid, of the acids have resulted in a significant increase in membrane thickness, porosity and hydrophilicity, with a decrease in macrovoid capacity of the PSF layer. The acid-modified PSF/TFC membranes showed higher rejection of salt, with an increment in water flux compared to the neat membrane. Water flux and salt rejection (R_s %) of the control membrane was 7.6 L/m² h and 65.4%, whereas polysulfone/methacrylic acid (PSF/MAAc), polysulfone/tartaric acid (PSF/TAc), and polysulfone/lactic acid (PSF/LAc) were 16.8, 18.5, and 20.2 L/m² h and 88, 88.2 and 94.1%, respectively. Efficiency of prepared NF membranes under various inlet pressures and specific salts was investigated with selectivity and salt rejection. The salt rejection of a mixed salt solution was found to meet the order of R_s % CaSO₄ ≥ R_s % Na₂SO₄ ˃ R_s % MgSO₄ ˃ R_s MgCl₂ ˃ Rs % NaCl.     

Bipolar reverse osmosis membrane for separating mono-and divalent ions

Bipolar reverse osmosis membranes that have both negatively and positively charged layers have been prepared to enhance the selectivity towards mono- and divalent ions in respect of both cations and anions. Positively charged layers are formed on low pressure reverse osmosis membranes having negative charge (NTR-7410 and 7450) by an adsorption method using polyethyleneimine (PEI) or a quaternary ammonium polyelectrolyte (QAP). These layers attach to the membrane's dense layer, which is made of sulfonated polyether sulfone. The selectivity of mono- and divalent ions is proven by experimental results for single electrolytes (NaCl, Na₂SO₄ and MgCl₂). Although negatively charged membranes repulse divalent anions more strongly than cations and monovalent anions, bipolar reverse osmosis membranes reject both divalent cations and divalent anions better than monovalent ions. An optimal preparation method for bipolar membranes showing selectivity towards mono- and divalent ions were developed. The bipolar membranes showed good ion selectivity for artificial sea water.