纳滤膜在功能性低聚糖分离纯化中的应用研究进展

功能性低聚糖具有抗肿瘤、抗放射、抗凝血、消炎和调节免疫力等医疗保健作用,广泛应用于食品科学和生物医药等领域。纳滤作为一种高效的膜分离技术,在功能性低聚糖的分离与纯化中的应用得到越来越多的关注。本文分析了纳滤膜对功能性低聚糖的分离机理,综述了纳滤膜在功能性多糖分离纯化中的应用进展,讨论了纳滤分离过程的影响因素,主要包括功能性多糖料液的性质、膜过程的操作参数以及膜材料本身的性质等。其中,料液的性质主要体现在组成、浓度、黏度等方面;操作参数主要体现在压力、温度、膜面流速和pH等方面;而膜材料的性质主要体现在微结构和表面性质两个方面。最后,进一步指出纳滤膜技术用于功能性多糖分离纯化时在设备成本、膜材料及膜污染等方面存在的问题,并对未来纳滤膜技术在低成本专用膜材料及系统开发和膜污染控制方面的研究进行了展望。     

Fouling effects of humic and alginic acids in nanofiltration and influence of solution composition

The objectives of this research were to investigate the combined and individual influence of hydrophobic and hydrophilic fractions of NOM on the fouling of thin-film composite nanofiltration (NF) membranes, and also the roles of solution chemistry on the permeate flux and fouling. Combined fouling is compared to the individual fouling behaviors (i.e., alginate or humic acid alone).Experiments were conducted using a “cross-flow” pilot-scale membrane unit with a full circulation mode. Fouling experiments were performed with individual and combined humic acid and alginate.The results demonstrated that increasing organic concentration increased greatly the rate and extent of flux reduction. Individual alginate fouling was more detrimental than individual humic acid fouling, and alginate exhibited greater flux decline than humic acid fouling alone at the same conditions. A higher flux decline was observed with increasing proportions of aliginate in combined fouling. In other word, there are antagonistic effects during combined fouling because the charge functional groups of two above foulants are negative and increase electrostatic repulsion between two foulants and also foulant-membrane. The flux reduction increased with increasing ionic strength, foulant concentrations, and with lower pH. This observation implies the importance of interaction between various foulants for deeper understanding of fouling phenomena. The membrane fouling was largely dependent on organic properties and fractions.     

The role of hydrodynamic conditions and solution chemistry on protein fouling during ultrafiltration

This study investigates the effect of hydrodynamic conditions and solution chemistry on protein fouling during ultrafiltration. Drastic flux reduction was observed at high initial flux and/or low cross-flow velocity. A limiting flux existed during BSA filtration, beyond which membrane flux cannot be sustained. Further increase in pressure over the limiting value did not enhance the stable flux. The rate and extent of BSA fouling were also strongly dependent on the feedwater composition, such as BSA concentration, pH, and ionic strength. Foulant concentration had no effect on the stable flux, although the rate approaching to the stable flux increased proportionally with increasing foulant concentration. Fouling was most severe at the isoelectric point of BSA (pH 4.7), where the electrostatic repulsion between foulant molecules is negligible. Membrane fouling became less severe at pHs away from the isoelectric point. Increasing ionic strength at pH 3.0 promoted severe fouling likely due to electric double layer (EDL) compression. On the other hand, the flux behavior was insensitive to salt concentration at pH 4.7 due to the lack of electrostatic interaction. At a solution pH of 5.8, effect of ionic strength on long-term flux behavior was directly opposite to that on the transient behavior. While the long-term flux was lower at higher ionic strength due to EDL compression, the transient behavior was also affected by the BSA retention of the membrane.     

Membrane fouling and chemical cleaning in water recycling applications

Fouling and subsequent chemical cleaning are two important issues for sustainable operation of nanofiltration (NF) membranes in water treatment and reuse applications. Fouling strongly depends on the feed water quality, especially the nature of the foulants and ionic composition of the feed water. Consequently, appropriate selection of the chemical cleaning solutions can be seen as a critical factor for effective fouling control. In this study, membrane fouling and chemical cleaning under condition typical to that in water recycling applications were investigated. Fouling conditions were achieved over approximately 18 h with foulant cocktails containing five model foulants namely humic acids, bovine serum albumin, sodium alginate, and two silica colloids in a background electrolyte solution. These model foulants were selected to represent four distinctive modes of fouling: humic acid, protein, polysaccharide, and colloidal fouling. Three chemical cleaning solutions (alkaline solution at pH 11, sodium dodecyl sulphate (SDS), and a combination of both) were evaluated for permeate flux recovery efficiency. The results indicated that with the same mass of foulant, organic fouling was considerably more severe as compared to colloidal fouling. While organic fouling caused a considerable increase in the membrane surface hydrophobicity as indicated by contact angle measurement, hydrophobicity of silica colloidal fouled membrane remained almost the same. Furthermore, a mechanistic correlation amongst cleaning efficiency, characteristics of the model foulants, and the cleaning reagents could be established. Chemical cleaning of all organically fouled membranes by a 10 mM SDS solution particularly at pH 11 resulted in good flux recovery. However, notable flux decline after SDS cleaning of organically fouled membranes was observed indicating that SDS was effective at breaking the organic foulant—Ca²⁺ complex but was not able to effectively dissolve and completely remove these organic foulants. Although a lower permeate flux recovery was obtained with a caustic solution (pH 11) in the absence of SDS, the permeate flux after cleaning was stable. In contrast, the chemical cleaning solutions used in this study showed low effectiveness against colloidal fouling. It is also interesting to note that membrane fouling and chemical cleaning could permanently alter the hydrophobicity of the membrane surface.     

Fouling studies of a polyamide nanofiltration membrane by selected natural organic matter: an analytical approach

The present study compares the fouling properties of two selected and well-characterized hydrophobic (denoted HPOA) and hydrophilic (denoted TPIA) natural organic matters (NOM) fractions with a commercial polyamide (PA) nanofiltration (NF) membrane (denoted NF-55). Analytical tools such as elemental analyses using microanalyses, specific UV absorbance, solid-state cross-polarization magic angle spinning (CPMAS) ¹³C-NMR, high pressure size exclusion (HPSEC) chromatography, HPLC system and acid/base titration for characterizing NOM fractions and contact angle, hydraulic permeability, streaming potential (SP) and observed rejection of a NaCl aqueous solution to evaluate the affinity of the PA material with both selected NOM fractions isolated from the Blavet river (Britany region, France). The PA material was found to be more sensitive to hydrophobic NOM adsorption leading to irreversible fouling with drastic modifications of the initial physico-chemical properties of the membrane: (a) increase of its hydrophobicity; (b) decrease of its hydraulic permeability associated with a decrease in its pore size and consequently (c) increase the observed rejection of salty solutions. The higher decrease in the performances of this PA NF membrane is observed for the more hydrophobic foulant, HPOA. At the same time a displacement of the isolectric point (IEP) of the membrane material was observed from 4.5 for the clean membrane (KCl 10⁻⁴ mol.L⁻¹) to 3.4 after HPOA sorption. At a lower pH range than the IEP, the effects of cations and H⁺ on the charge properties of the membranes increases near the shear plane, yielding more positive SP values. For the hydrophilic TPIA foulant no displacement of the IEP was observed. Then the results of SP experiments conducted through the membrane with a homemade apparatus has indicated that HPOA is more retained inside the pores as compared to the TPIA that was mainly sorbed at the surface of the membrane. Furthermore the membranes acidic-basis properties were amplified after foulants deposit in comparison to the cleaned membrane where a dominant specific sorption of monovalents and divalents ions occurred.     

腐殖酸聚集体对膜蒸馏过程膜污染的作用机理

膜污染是膜蒸馏过程应用于工业水处理中遇到的主要问题之一。选取水体中具有代表性的有机物（腐殖酸）、微溶无机盐（碳酸钙）作为典型污染物,研究有机腐殖酸聚集体对于膜蒸馏过程膜污染进程的影响规律,探讨天然有机物-无机微溶盐混合水体中腐殖酸聚集体对于无机盐结晶过程的控制机理。结果表明：膜蒸馏通量的衰减大致可分为由滤饼层的形成造成的不可恢复部分以及由浓差极化、膜孔“半润湿”而造成的可部分恢复的通量降低。Ca²⁺通过加速腐殖酸分子的聚集过程,使表面负电性降低的腐殖酸聚集体率先吸附在聚偏氟乙烯中空纤维膜内表面,形成有机基质污染层;碳酸钙在有机腐殖酸聚集体的诱导下在膜内表面异相成核,最终成长为稳定的晶体。腐殖酸聚集体为无机盐晶体在疏水性膜内表面的生长提供了异相成核的基础。可通过控制污染水体中有机物的含量控制微溶碳酸钙在膜内表面成核及生长,实现控制其在膜内表面附着进而诱发疏水膜发生亲水化的过程。     

PROTEIN ADSORPTION AND FOULING OF CERAMIC MEMBRANES AS MEASURED BY SCANNING ELECTRON MICROSCOPY WITH DIGITAL X-RAY MAPPING

A technique for studying fouling in ceramic membranes using the energy dispersive x-ray spectroscopy capability of an electron microscope is described. The location and amount of foulant within the membrane are presented on a digital x-ray map showing elements constituent to or stained on the foulant.

Fouling of alumina membranes during filtration of the protein hemoglobin has been studied as a function of filtration time, pH, and membrane pore size. After each filtration run, the protein within a piece of the membrane was stained with phosphotungstic acid and located on a digital map of either phosphorus or tungsten.

For a 0.2 μm pore size membrane, time dependent fouling was observed consistent with an observed flux decline within the first few minutes of filtration. A pH dependence was also observed indicating much greater fouling at pH 6.9 near the protein isoelectric point than at pH 8.5. This observation is consistent with pH dependent adsorption, flux, and rejection studies. No internal fouling was observed for a 40 Å pore size membrane, which is consistent with the size of hemoglobin in solution being larger than the 40 Å pores and with the fact that the 40 Å membrane can be more easily cleaned after use than can the 0.2 μm membrane.     

Implications of enhancing critical flux of particulates by AC fields in RO desalination and reclamation

This paper discusses fouling in reverse osmosis in terms of the critical flux of foulants and the fouling mechanisms based on hydraulic resistance and loss of driving force due to cake-enhanced osmotic pressure (CEOP). In many cases CEOP is the dominant effect and this is exacerbated by the use of constant flux processing. The implications of increasing critical flux are described and the potential benefit of using AC field gradients to do this is illustrated for a model foulant.     

Synergistic behavior between silica and alginate: Novel approach for removing silica scale from RO membranes

The formation of mineral scale deposits on membranes is a pervasive and expensive problem for the water treatment industry. A series of experiments run on a laboratory-scale reverse osmosis membrane system examined the fouling of membranes when the feed water was spiked with organic and inorganic foulants. Alginic acid was used as the organic foulant and silica was used as the inorganic foulant. Studies involving interactions of these two foulants have not previously been reported in literature. Experiments were run with each foulant individually to characterize fouling at different velocities and pressures. Experiments were then run using both foulants together to characterize the synergistic effects on membrane fouling. One set of experiments with both foulants demonstrated that alginic acid inhibits silica fouling on reverse osmosis membranes. Further experiments indicated that alginic acid added after silica fouling had already occurred was able to remove silica scale from the membrane and restore permeate flux.     

Separation of monoclonal IgM antibodies using tangential flow ultrafiltration

Experimental results are presented for the separation of monoclonal IgM antibodies from hybridoma cell cultures using tangential flow ultrafiltration with total recycle of the retentate. IgM antibodies are pentameric immunoglobulin molecules with a molar mass of 900 kDa and a tip-to-tip distance of 38 nm. The major impurity (foulant) in the supernatant sample was albumin, whose molar mass and diameter are 67 kDa and 7 nm, respectively. The antibody (product) recovery rate, variations in the permeation velocity and the time for a 90%-reduction in feed volume were investigated using 100 and 300 kDa NMWCO membranes at three transmembrane pressures and two tangential velocities. A model is also presented, in which the ultrafiltration process is divided into two regimes: the surface fouling regime (characterized by the adsorption of antibody molecules on the membrane surface) and the internal fouling regime (characterized by pore-blockage due to deposition of foulant protein molecules). Approximately 16% of the effective membrane area was blocked in the surface fouling regime. The model predictions are in satisfactory agreement with the experimental results.     

Determination of an optimum frequency of square wave power for fouling mitigation in desalting electrodialysis in the presence of humate

Membrane fouling in electrodialysis (ED) can be minimized by using optimized pulsed power instead of direct current (DC) power. For the determination of an optimum frequency, effects of the square wave power with various frequencies on fouling mitigation were studied in the presence of humate as a foulant in a NaCl solution. In desalting of a NaCl solution with humate an optimum frequency of the square wave power was found to be 30 Hz, determined by the membrane fouling index for Neosepta AMX and CMX membranes. AMX membrane has a lower optimum frequency than that of the relatively loose AM-1, which was near 100 Hz in a previous study. Molecular weight distribution of humate transported to the concentrate compartment through AMX and AM-1 membranes indicated the structural difference between the two membranes. Through cell tests and desalting ED, a higher performance was achieved with the optimized square wave power than the DC power in terms of conductivity and cell resistance.     

Nanofiltration flux, fouling and retention in filtering dilute model waters

Fouling of membranes decreases the applicability of the nanofiltration (NF) process, and thus a deeper understanding of membrane fouling is needed. Fouling and retention of different NF membranes by model feed waters was investigated in a laboratory-scale filtration unit. The model waters were composed so as to imitate the characteristics of chemically pre-treated surface water. No differences were seen in membrane flux declines when filtering feed waters containing the studied organic compounds of different characteristics. However, organic matter containing feed waters resulted in remarkably lower flux than the metal ions containing feed waters. An additional decrease in flux was seen when both organic matter and metal ions,especially silica, were present in the feed water. An increased feed water organics concentration increased the retention of organic matter, but the addition of metal ions to organics containing feed water caused a decrease in the retention of both organic matter and conductivity. The different behaviour is most probably caused by the differences in the fouling layers and foulant characteristics as well as by the electroneutrality effect. Generally, it is difficult to change feed water composition to non-fouling, but the operating pH can be used to some extent in optimising membrane performance.     

An assessment of the Silt Density Index based on RO membrane colloidal fouling experiments with iron oxide particles

RO membrane colloidal fouling experiments were performed in the laboratory under well controlled and realistic conditions. Iron oxide was selected as a typical inorganic colloidal foulant, due to its importance, as evidenced from well known manufacturer recommendations on iron concentrations in feed waters and from frequently encountered problems in membrane installations. A range of iron concentrations was identified where a linear relationship existed between flux reduction rate and concentration. The performance of the Silt Density Index (SDI) was tested on the basis of the RO fouling data obtained. The range of iron concentrations where measurable and meaningful SDI values could be obtained was remarkably close to membrane manufacturer recommendations. A notable sensitivity of the SDI was also observed with particles for which retention is negligible. However, on the basis of the RO fouling data obtained, it appears that the SDI is not conservative enough. Furthermore, since the SDI cannot predict fouling rates, it cannot discriminate between different types of membranes.     

Fouling limitations of osmotic pressure-driven processes and its remedial strategies: A review

The scarcity of freshwater is one of the major challenges to the civilized community around the World. So, the various engineered processes have been developed for the remediation of this problem such as osmotic pressure-driven processes (OPDPs). OPDPs includes forward osmosis and pressure retarded osmosis, which have gained enormous attention and becoming a flexible membrane-based technology for desalination and wastewater treatment. The most significant components of the OPDPs to be efficient are the membranes and the draw solution. Although numerous studies highlighted the membrane fouling and concentration polarization are one of the critical parameters to assess the system efficiency in the OPDPs and other pressure driven membrane processes. This study delivers an up-to-date review of the OPDPs in terms of concentration polarization, membrane fouling (organic, inorganic, and biofouling), foulant interactions, remedial strategies, monitoring and characterization techniques, and future directions of research. Numerous foulant interactions are also elaborated for the OPDPs in the field of the membrane separation technique. State of the art in antifouling membrane fabrication methods are critically reviewed along with different remedial strategies for fouled membranes in OPDPs.     

Concentration polarization and fouling

Concentration polarization is often the reason for the serious limitation of the membrane processes due to its negative influence on the transmembrane flux. Many theoretical studies on this polarization phenomena have resulted in mathematical models for concentration polarization. In most of them, solutions are sought for the coupled nonlinear system of the equations of continuity and motion. Each of these solutions makes use of some assumptions in order to simplify the equations which represent the phenomenon.Different kinds of flow systems have been constructed in order to reduce the concentration polarization. The aim of these flow systems has mainly been to improve the mass transport from the membrane surface back to the bulk solution.Fouling often is a result of concentration polarization, but can also have other reasons.The attention that was drawn to fouling ten to fifteen years ago mostly consisted of recognizing the fact as such and to roughly identify the foulant. Later, efforts to avoid the negative effects have basically followed the paths of altering the composition of the feed solution by a pretreatment to change or get rid of the foulant, to change the hydrodynamics of the membrane module or to alter the membrane itself.However, unveiling the basic mechanisms of fouling attracted little attention until the late seventies when for example fouling of an RO sea water desalination membrane could be subdivided into four consecutive steps.Also, studies of the chemistry and physics of fouling have being performed on whey lately and revealed in closer detail some of the responsible phenomena.     

Modeling of RO Flux Decline in Textile Wastewater Reclamation Plants Using Variable Fouling Index

Abstract

This research proposes the model for estimation of flux decline using the concept of “variable fouling index.” The change in fouling index is induced by the accumulative foulant on the membrane and by the adhesion of the foulant on the membrane. The analysis of operating data of textile wastewater reclamation plants using the variable fouling index concept has shown that this model which was derived from short-term data of bench scale achieves a promising accuracy for the estimation of flux decline and increase of net driving pressure in a long-term operation of pilot-scale and full-scale system.     

Performances of RO and NF processes for wastewater reuse: Tertiary treatment after a conventional activated sludge or a membrane bioreactor

Wastewater reclamation requires processes and technologies having the ability to reduce the presence of micropollutants which are not wholly treated in conventional WWTP. Due to the complexity of membrane-solute interactions and the diversity of secondary treatment effluent (STE) matrices, deeper investigations are required to identify the major foulant species and more specifically their behaviour at high concentration in real waters. This study investigates the rejection and fouling potential of nanofiltration (NF) and low-pressure reverse osmosis (RO) membranes with two STEs sampled from i) a conventional activated sludge process coupled with ultrafiltration (CAS-UF) and from ii) a membrane bioreactor MBR (AquaRM^®, SAUR (France)). Whatever the origin of the effluent, RO seems to be the best solution to prevent pollution of tertiary effluents (expected result) but also to obtain low fouling levels. The different composition and molecular weight distribution of MBR and CAS-UF effluents can explain the different fouling behaviours that were observed.     

Interactions controlling biopolymer fouling of reverse osmosis membranes

Laboratory experiments and model calculations were performed to elucidate the fundamental interactions that control organic fouling in reverse osmosis (RO) processes. Bovine serum albumin and alginic acid were selected as model aquatic organic macromolecules (organic foulants). An extended Derjaguin-Landau-Verwey-Overbeek (DLVO) characterization analysis was used to elucidate mechanisms of organic matter fouling on a commercial, polyamide composite RO membrane. Surface tension parameters derived from contact angle analyses are used to demonstrate that the apparent thermodynamic stability of macromolecules determines and adhesive free energy between membranes and macromolecules explained the observed differences in flux decline. Further, foulant–membrane and foulant–foulant interfacial forces helped explain why hydrophilic macromolecules formed polarization layers causing minimal flux decline, while hydrophobic macromolecules formed gel (or cake) layers that led to severe flux decline.     

直接接触式膜蒸馏过程的膜曝气强化

针对直接接触式膜蒸馏（direct contact membrane distillation,DCMD）过程存在的膜通量小及膜污染问题,设计了一种新型结构的膜蒸馏组件。以蔗糖溶液为处理液,考察了膜组件装填密度Φ、膜曝气量q、蔗糖浓度c与温度T₀对DCMD过程的影响。结果表明：随着Φ、q的增加,DCMD过程的膜通量先增大,后逐渐降低,Φ、q均存在最优值;随着c的增加,膜通量逐渐降低;随着T₀的增加,膜通量增大;对c为30%（mass）的蔗糖溶液进行DCMD法处理330 min时,膜曝气可使DCMD的初始膜通量J_initial提升24.7%、膜通量衰减率ΔJ降低55.0%,维持高膜通量的连续运行时间t₀延长4倍。主要原因是膜曝气强化了DCMD过程的传热传质,进而强化过程的分离性能;有效控制了DCMD过程的浓差极化,进而延缓过程的膜污染进程。研究结果有利于推进DCMD的工程化应用。     

Fouling behavior of poly(vinylidene fluoride) (PVDF) ultrafiltration membrane by polyvinyl alcohol (PVA) and chemical cleaning method

Severe fouling to poly(vinylidene fluoride) (PVDF) membrane is usually caused as filtrating the papermaking wastewater in the ultrafiltration (UF) process. In the paper, fouling behavior and mechanism were investigated, and the low-concentration polyvinyl alcohol (PVA) contained in the sedimentation tank wastewater was found as the main foulant. Consequently, the corresponding cleaning approach was proposed. The experiment and modeling results elaborated that the fouling mode developed from pore blockage to cake layer along with filtration time. Chemical cleaning conditions including the composition and concentration of reagents, cleaning duration and trans-membrane pressure were investigated for their effect on cleaning efficiency. Pure water flux was recovered by over 95% after cleaning the PVDF membrane using the optimal conditions 0.5 wt% NaClO (as oxidant) and 0.1 wt% sodium dodecyl benzene sulfonate (SDBS, as surfactant) at 0.04 MPa for 100 min. In the chemical cleaning method, hypochlorite (ClO^-) could first chain-scissor PVA macromolecules to small molecules and SDBS could wrap the fragments in micelles, so that the foulants were removed from the pores and surface of membrane. After eight cycling tests, pure water flux recovery maintained above 95% and the reused membrane was found intact without defects.