工业废水电渗析过程中膜污染研究进展

综述了工业废水电渗析体系膜污染类型、性质、影响因素等的研究进展. 无机污染主要是Ca2+, Mg2+或其他高价离子在离子膜表面或内部形成的结垢现象,原理是极化或溶液过饱和形成沉淀. 有机污染是由蛋白、腐植酸、表面活性剂及大分子有机物在离子膜表面沉积或渗透到膜内部而形成,原因主要是带负电荷的有机物与阴膜荷正电基团的静电作用及带苯环有机物与基膜的亲和作用,其次是有机物分子大小与膜的网络结构的作用. 膜污染形成及其性质与施加电流、共存组分、温度、pH值和运行时间等密切相关,可造成离子膜导电性、离子交换容量、水含量和极限电流密度等减小,且有机污染对电渗析性能的影响比无机污染更严重. 由有机物凝胶层形成的膜污染可用电渗析膜污染指数定量描述.     

Characterization of anion exchange membranes with natural organic matter (NOM) during electrodialysis

Natural organic matter (NOM) is thought to be a major source of fouling during membrane filtration of natural waters. The organic matter present in surface waters was characterized in terms of its molecular weight distribution, acidity and electrokinetic properties. The fouling potentials of anion exchange membranes were predicted by the characterization. Changes in the physicochemical properties of anion exchange membranes were also examined during electrodialysis (ED) process of solutions containing NOM. The ED performances were evaluated for the three anion exchange membranes (AMX, AM-1 and ACM) in the presence of NOM. Fouling phenomena in terms of current efficiency and NaCl flux were in good agreement with the fouling potentials predicted by the characterization results. Observations of the molecular weight distribution and the constituents of NOM revealed that the hydrophobic NOM fraction with high molecular weights deposited mainly on the membrane surface, providing fouling effects on the anion exchange membrane.     

Polymer adsorption on multicomponent surfaces with relevance to membrane fouling

Block copolymer thin films that include low surface energy domains are analyzed as a possible way to overcome the problem of membrane fouling by proteins and other natural organic matter. A model is presented that accounts for both fouling due to chemical interactions between the solute and surface and due to convective deposition. Guidelines for the formation of novel membranes with improved fouling-resistant properties are suggested based on comparison of the relative permeate flux decline due to fouling on different model copolymer membranes. In general, it is observed that copolymer films having small and dispersed polymer blocks that interact unfavorably with the fouling species show an overall decrease in fouling and increase in permeate flux compared with the homopolymer films.     

Structure–property relationships of anionic exchange membranes for Fe/Cr redox storage batteries

Membranes with lower area resistivity, higher selectivity, and reduced susceptibility toward fouling were required to improve the efficiency and lifetime of Fe/Cr redox storage batteries. The relationship of membrane structure to these electrochemical properties was not well understood. To gain a better understanding of structure–property relationships, a series of model membranes was synthesized in which the degree of crosslinking, ion exchange capacity, and porosity could be varied independently. This permitted the completion of several factorial studies in which the effect of each structural variable and interaction between variables could be defined and quantified. It was found that increasing the ion exchange capacity had a beneficial effect on all three electrochemical properties. Increasing the porosity reduced the resistivity and fouling but resulted in poorer selectivity. The degree of crosslinking had no effect on resistivity but improved selectivity and reduced fouling was observed when this parameter was increased. These relationships are consistent with theoretical models that have been proposed to account for the behavior of ion exchange membranes in general. From this data, empirical models were constructed which could be used to predict the area resistivity and water content of other anionic exchange membranes.     

阴离子交换膜改性及抗污染性能研究进展

电渗析技术应用于工业废水脱盐时,废水中有机物及其它杂质组分等会造成膜污染,进而影响脱盐性能。电渗析膜污染防治对促进电渗析在工业废水处理中的应用有重要意义。相比于阳离子交换膜,阴离子交换膜更易形成有机污染,且更严重。阴离子交换膜污染主要由腐殖酸、牛血清蛋白、阴离子表面活性剂等有机物造成,污染过程主要受静电作用、亲和作用和几何因素的影响。膜改性提高阴离子交换膜的抗污染性能是电渗析膜污染防治的有效方法,目前已有许多有关膜改性提高阴离子交换膜抗污染性能的报道。膜改性方法主要有化学改性法、等离子体改性法、表面涂覆改性法、电沉积改性法、自聚合改性法及改进基膜结构法等。本工作对阴离子交换膜改性及抗污染性能的研究进展进行了综述,对不同改性方法的优缺点进行了分析和评价。这些改性方法能提高阴膜表面的负电荷密度和亲水性、降低膜表面粗糙度和基膜含水率等,因此可以改善阴离子交换膜的抗污染性能。然而,目前研究获得的改性阴离子交换膜仍存在修饰层不稳定、抗污染性能不理想和性能测试不系统等缺点,需进一步优化改性方法、改性工艺、组分修饰及性能测试等,以获得抗污染性能稳定且效果良好的改性阴离子交换膜。     

Mathematical Modelling of Cation Contamination in a Proton‐exchange Membrane

Transport phenomena in an ion‐exchange membrane containing both H⁺ and K⁺ are described using multicomponent diffusion equations (Stefan–Maxwell). A model is developed for transport through a Nafion 112 membrane in a hydrogen‐pump setup. The model results are analysed to quantify the impact of cation contamination on cell potential. It is shown that limiting current densities can result due to a decrease in proton concentration caused by the build‐up of contaminant ions. An average cation concentration of 30 to 40% is required for appreciable effects to be noticed under typical steady‐state operating conditions.     

Characterization of a sulfonated pentablock copolymer for desalination applications

Water and salt transport properties were determined in a family of sulfonated pentablock copolymers to characterize their potential as chlorine-tolerant desalination membrane materials. The degree of sulfonation, block molecular weight, and casting conditions can be independently varied to tune the transport properties of these materials. Data for water uptake, water permeability, salt permeability, and apparent surface charge are presented. Apparent diffusion coefficients of water in these materials were calculated using the solution-diffusion theory. Generally speaking, water sorption, water diffusivity, water permeability, and salt permeability increase with increasing degree of sulfonation. As ion exchange capacity increases from 0.4 to 2.0 meq/g (dry polymer), water uptake values vary between 21% and 118%, and water permeability values, in units of cm² s⁻¹, vary over 4 orders of magnitude. Salt permeability depends on both the upstream sodium chloride concentration, between 0.01 and 1.0 mol L⁻¹, and the degree of sulfonation. Both water permeability and salt permeability are sensitive to the conditions used when casting the polymer films. Apparent surface charge, as characterized by zeta potential, has been shown to be related to the fouling tendency of several membrane materials. In these materials, zeta potential is most negative in samples with low levels of sulfonation and is near neutral in samples with the highest level of sulfonation.     

焦化生化出水电渗析脱盐研究

采用电渗析技术对焦化生化出水如曝气生物滤池出水及反渗透浓水进行脱盐,考察不同废水中的离子迁移、废水脱盐及离子交换膜污染情况。结果表明:2种焦化废水采用电渗析处理具有较好的脱盐效果,其中不同离子的迁移脱除与其浓度、离子半径等密切相关。膜电阻测试表明,不同焦化废水电渗析体系中不同离子交换膜的污染存在差别。扫描电镜和红外分析表明,曝气生物滤池出水主要由有机物造成阴离子交换膜污染,而反渗透浓水主要在电渗析浓室侧的膜表面形成颗粒状的无机污染,且阳膜浓室侧比阴膜浓室侧更显著。     

Irreversible Fouling in MF/UF Membranes Caused by Natural Organic Matters (NOMs) Isolated from Different Origins

Abstract

For more efficient use of membrane technology in water treatment, it is essential to understand more about the fouling that requires chemical cleaning to be eliminated (i.e., irreversible fouling). In this study, five different MF/UF membranes and four types of organic matter collected from different origins were examined in terms of the degree of irreversible membrane fouling. Experimental results demonstrated that the extent of irreversible fouling differed significantly depending on the properties of both the membrane and organic matter. Among the tested membranes, UF membranes made of polyacrylonitrile (PAN) exhibited the best performance in terms of prevention of irreversible fouling. In contrast, MF membranes, especially one made of polyvinylidenefluoride (PVDF), suffered significant irreversible fouling. Conventional methods for characterization of organic matter such as specific ultraviolet absorption (SUVA), XAD fractionation, and excitation‐emission matrix (EEM) were found to be inadequate for prediction of the degree of irreversible fouling. This is because these analytical methods represent an average property of bulk organic matter, while the fouling was actually caused by some specific fractions. It was revealed that hydrophilic fraction of the organic matter was responsible for the irreversible fouling regardless of the type of membranes or organic matter.     

Determination of transport properties and limiting current densities of ion exchange membrane stacks

For the economy of electrodialysis desalination plants it is important to know the permselectivities and permeabilities of the ion exchange membranes used. In particular, when dealing with high salt contents the permeabilities should be small. The relationship between ion flux density and current density was measured with a membrane stack. The experimental data are reasonably accurate, however only as obtained for a combination of an anion exchange membrane with a cation exchange membrane.The permselectivities are compared with data of the apparent permselectivities as obtained from measurements of the membrane potential. In addition, permeabilities were calculated from data of tracer experiments as well as from electrochemical membrane data.Economic assessments also require a knowledge of the limiting current density of the combination of a cation- and an anion exchange membrane, for at higher current densities there will be a loss of both energy and product water.The limiting current density was determined for various membrane combinations as a function of liquid flow.     

Reducing fly ash deposition by pretreatment of brown coal: Effect of aluminium on ash character

The effect of pretreatment on the structure and the related fouling/slagging behaviour of fly ash from brown coal has been investigated. Condensable sodium salts and low melting sodium silicates are known as fouling and slagging materials. Experimental proof is given that less of such components were formed after treatment of Loy Yang brown coal with water and, particularly, with Al compounds.For the range of temperatures applied (1000–1400°C) in a drop tube furnace, the quantity of condensable salts generated was found to depend only on the amount of sodium in Loy Yang coal. Aluminium had the effect of reducing the sodium level by ion exchange and thus the formation of fouling ash components. The stickiness of sodium silicates from this coal was also shown to be reduced by aluminium.     

An experimental study of UF membrane fouling by humic acid and sodium alginate solutions: the effect of backwashing on flux recovery

One of the critical issues for the successful application of ultrafitration in water treatment is membrane fouling due to dissolved organic matter, which negatively affects productivity, product quality and process cost. The aim of the present study is to contribute to the understanding of fouling phenomena by organic matter and the efficiency of the backwashing technique, which is applied in practice to restore membrane flux. In this experimental study commercial humic acid and sodium alginate have been used as model substances representative of natural organic matter and extracellular organic matter, respectively. All fouling experiments were carried out in a special single fiber apparatus. An important parameter considered in the study of both model substances is the concentration of calcium ions, which promote their aggregation and influence the rate of flux decline, the reversibility of fouling and rejection. Membrane fouling by humic acid appears to be the outcome of simultaneous action of several fouling mechanisms. Initially, a relatively rapid irreversible fouling takes place due to internal pore adsorption; however, progressively pore blocking becomes important and a fouling layer develops on the membrane. Sodium alginate fouling on the other hand is apparently due to two consecutive mechanisms; i.e. a rapid irreversible fouling due to internal pore constriction, followed by cake development on the membrane surface which becomes the dominant mechanism. Comparing fouling in both cases it can be inferred that even though sodium alginate fouling is more severe than the one caused by humic acids, it is to a large extent reversible by backwashing. On the contrary, fouling caused by humic acid is characterized by greater and increasing with calcium addition irreversibility, which is not remedied by the periodic backwashing. The different fouling propensity of the two types of macromolecules is apparently due to differences of their physical–chemical characteristics.     

The influence of current density on the electrochemical properties of anion-exchange membranes in electrodialysis of phenylalanine solution

Electrochemical properties of heterogeneous anion-exchange membranes can be indicators of their fouling during electrodialysis of phenylalanine solutions. The current–voltage curves of the membrane MA-40 make it possible to reveal the difference in the limiting current values for fresh and fouled membranes. Increase of membrane voltage with time characterizes the development of fouling in the electromembrane system. Membrane conductivity decreases significantly in solutions containing phenylalanine. Overlimiting current conditions lead to “washing out” of amino acid from membrane phase and therefore help to avoid fouling.     

Initial fouling of nontoxic coatings in fresh,brackish, and sea water

Objectives of this project were to evaluate benefits — over the short term — that might derive from control of the surface properties of materials used in energy transfer devices; and to identify preferred ranges of surface parameters that might be specified to minimize deposits of biological fouling known to deteriorate energy exchange efficiencies in seawater, brackish water, and freshwater systems. By modifying the surface chemistry and surface energy of test plates with very thin coatings (nontoxic) and examining the acquired films at various time intervals, the earliest events of biofouling caused by macromolecules and microbial organisms were defined. Overall, the results were remarkably similar for all systems tested in showing that deliberate modification of the initial material surface qualities can significantly reduce the retention of biofouling layers.     

A study on fouling mitigation using pulsing electric fields in electrodialysis of lactate containing BSA

Fouling tendency in electrodialysis was investigated using the electrochemical and physical properties of the foulants and ion exchange membranes. It was found that bovine serum albumin (BSA), a large molecular weight protein, fouled the AMX membrane irreversibly by deposition on the membrane surface. Electrodialysis experiments of lactate with 1.0 wt% of BSA were performed using the square wave powers at different frequencies to examine the pulsing power influences as a fouling mitigation method, and the results were compared with the data obtained using the DC power. Reduced fouling potentials were observed when the square wave powers were used in the electrodialysis of lactate and confirmed the membrane fouling index for electrodialysis (EDMFI). The pulsing electric fields enhanced the mobility of the charged particles in the fouling layer and decreased the electric resistance of the electrodialysis cell. It was clearly observed that the pulsing electric fields with different frequencies reduced the fouling potentials, and consequently the power consumption was reduced significantly as a fouling mitigation method.     

Removal of hardness in fermentation broth by electrodialysis

Lysine fermentation broth was desalinated by electrodialysis to reduce hardness concentrations as a pretreatment procedure for the purification and recovery of amino acids. Electrodialysis performance was investigated in terms of the rate of reduction in conductivity in dilute solutions and electrodialysis cell resistance for different ion exchange membranes at a constant current density. Among the membranes investigated in this study, membranes with high water content showed the better performance for hardness removal. Fouling experiments revealed that organics gave rise to fouling effects on the anion exchange membrane during demineralization of the lysine fermentation broth. The pulsed electric field with the half‐wave power enhanced the electrodialysis performances by mitigating membrane fouling in desalination of the lysine fermentation broth. This study successfully demonstrated the potential use of pulse power as an effective cleaning‐in‐place (CIP) method during electrodialysis of fermentation broth. © 2002 Society of Chemical Industry     

Polarization curves in the ohmic controlled electrodeposition of metals

The conditions under which ohmic controlled metal electrodeposition occurs are discussed using a simple mathematical model. It is shown that ohmic controlled electrodeposition can be operative if the value of the exchange current density for the electrodeposition process is more than 10 times larger than the corresponding value of the limiting diffusion current density. In this case, a linear dependence of the current density on overpotential up to the value of the limiting diffusion current density can be observed. On the other hand, the initiation of dendrite growth under these circumstances is possible, even at very low values of overpotential, at the moment when the limiting diffusion current density is attained in potentiostatic electrodeposition. In this way, instead of a limiting diffusion current density plateau, an inflection point on the polarization curve can be observed, since dendritic growth is followed by an increase in the deposition current density. At the same time, it is shown that the ensemble of tips of dendrites can behave as an ensemble of microelectrodes working independently under mixed or activation control due to the absence of a common diffusion layer. This was confirmed by deposition of copper on a copper dendritic electrode and by silver electrodeposition from a silver nitrate solution onto a graphite substrate.     

Multicomponent ion exchange column dynamics

Simple model equations which consider different rate control mechanisms are formulated for fixed bed multicomponent ion exchange processes. Efficient and accurate numerical methods are developed for solving these equations for liquid phase, solid phase or combined phase control. The algorithms are applicable to both ion exchange and liquid adsorption and are shown to be extendable to a general form of isotherms. The accuracy of the numerical algorithm is evidenced by the fact that the asymptotic limiting equilibrium solutions are closely approached as the dimensionless length parameter is increased, regardless of the rate control mechanism. Numerical examples for ion exchange applications are presented. These examples included multicomponent elution and purification problems. The effect of mass transfer resistance was also examined. Other examples examine the validity of a model reduction assumption and the comparison of equilibrium theory with the results obtained using the present finite mass transfer rate model.     

Membrane Partition and Mass Transfer in Ultrafiltration

Abstract

Through ultrafiltration experiments with a macromolecular solution of dextran and a suspension of bentonite, it is shown how the understanding of mass transfer phenomena involved in ultrafiltration can be improved if several limiting mechanisms working simultaneously over different parts of a same membrane are taken into account. This partition is determined according to the operating conditions as well as the characteristics of the treated fluid. This approach is convenient to describe an entire range of experimental variations in solvent and solute transfer through hollow fiber membranes induced either by macromolecular concentration polarization or by particles fouling. It is shown that reversible fouling can have effects as dramatic as irreversible fouling (pore plugging) in some circumstances.     

Surface modification of ion‐exchange membranes: Methods,characteristics, and performance

Considerable effort has been made to improve ion‐exchange membrane (IEM) properties in order to achieve better performance of IEM‐based processes in various applications. Surface modification is one of the effective ways to improve IEM properties. Various methods have been used to modify IEM surfaces, for example, plasma treatment, polymerization, solution casting, electrodeposition, and ion implantation. These methods are able to produce a thin and fine distributed layer and also to modify the chemical structure of the surface. The new layer can be adsorbed, deposited, or chemically bonded on a membrane surface. By using these methods, IEM properties are improved, and the desired or specific characteristics such as high monovalent ion permselectivity, low fuel crossover, and anti‐organic‐fouling property can be obtained. In this paper, methods for surface modification of IEMs are reviewed. Moreover, the effects of modification on IEM properties and performance are discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45540.