Influence of membrane structure on fouling layer morphology during apple juice clarification

The flux behavior of 0.2 μm nylon, polysulfone (PS), polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes was examined during dead-end microfiltration of commercial apple juice. On nylon membranes, a 0.1 μm thick surface fouling layer rapidly formed that acted as a secondary membrane. The colloidal particles retained by this surface layer aggregated to form a thick loose gel structure, producing an anisotropic fouling structure. In contrast, the 4 μm thick surface fouling layer of PES was slower to form and had a more open structure with a lower flux resistance per unit thickness. The morphology of the PES surface layer also did not differ dramatically from the loose gel structure that subsequently formed on top of this secondary membrane. The PS surface fouling layer was similar in structure to nylon whereas the PVDF layer more closely resembled that found with PES. The density of the surface fouling layer did not directly correlate to membrane surface hydrophobicity or pure water flux. Atomic force microscopy (AFM) indicated that surface roughness strongly influenced surface fouling layer morphology. The membrane surface appears to act as a template for the fouling process; therefore, smooth membranes (nylon and PS) produce a dense surface fouling layer whereas this same layer on rough membranes (PES and PVDF) is much more open. Consequently, the fluxes of PES and PVDF membranes are less affected by fouling formation.     

Formation of appropriate sites on nanofiltration membrane surface for binding TiO2 photo-catalyst: Performance, characterization and fouling-resistant capability

Titanium dioxide (TiO₂) nanoparticles were assembled on the surface of nanofiltration blend membrane. For settling TiO₂ on the membrane surface, two membrane categories were used: (i) unmodified polyethersulfone (PES)/polyimide (PI) blend membrane, and (ii) –OH functionalized PES/PI blend membrane with different concentrations of diethanolamine (DEA). These membranes were radiated by UV light after TiO₂ depositing with different concentrations. 15 min immersion in colloidal suspension and 15 min UV irradiation with 160 W lamps were used for modification. The modification resulted in the formation of a photo-catalytic property with enhanced membrane hydrophilicity. The self-assembly of TiO₂ nanoparticles was established through coordinance bonds with –OH functional groups on the membrane surface. A comparison between the UV irradiated TiO₂ deposited blend membrane and deposited-functionalized blend membranes showed that –OH groups originate excellent adhesion of TiO₂ nanoparticles on the membrane surface, increase reversible deposition, and diminish irreversible fouling. The membranes were characterized using SEM, FTIR, EDX, contact angle, cross flow filtration, and antifouling measurements. SEM images show that the presence of –OH groups on the DEA-modified membrane surface is the main parameter for extra uniformly settlement of TiO₂ nanoparticles on the membrane surface. This procedure is a superior technique for modification of PES/PI nanofiltration membranes to enhance water flux and minimization membrane fouling.     

Preparation and pervaporation performance of surface crosslinked PVA/PES composite membrane

This study describes the facile preparation of poly(vinyl alcohol) (PVA)/polyethersulfone (PES) composite membranes by interfacial reaction technique, aiming at acquiring the improved structural and operational stability of the resulting membranes. The effect of interfacial crosslinking agent and hydrophilicity of support layer on the interfacial adhesive strength and pervaporation performance of composite membranes were investigated. The optimal recipe for PVA/PES composite membrane preparation was as follows: PES support layer was treated with 0.1 wt.% borax aqueous solution, fully dried and then immersed into 2 wt.% PVA aqueous solution. The resulting PVA active layer was 1–1.5 μm thick after twice dip-coating. The as-prepared PVA/PES composite membrane exhibited high separation factor of over 438, high permeation flux of 427 g m⁻² h⁻¹ for 80 wt.% EG in the feed at 70 °C and desirable structural stability. It could be derived that adoption of interfacial reaction would be an effective method for preparing the composite membranes suitable for large-scale dehydration of ethylene glycol/water mixture.     

pH stimuli-responsive and fouling resistance PES membrane fabricated by using photochromic spiropyran and spironaphthoxazine nanofillers for pesticide removal

New fouling resistance and stimulus–responsive nanofiltration membranes were fabricated by adding photochromic spiropyran (SPO) and spironaphthoxazine (SNO) nanofillers to the polyethersulfone (PES) matrix via the phase inversion method. The effect of SPO and SNO, as novel photoresponsive molecule nanofillers, were evaluated in terms of membrane morphology, porosity, wettability, pure water flux (PWF), antifouling resistance, and stimulus–responsive properties. All the modified membranes indicated better performance compared to the bare PES. The membrane PWF was notably enhanced from 7.7 kg/m²h for the bare PES up to 18.68 and 20.58 kg/m²h for the 0.1 wt.% SPO and SNO blended membranes, respectively. Also, the 0.1 wt.% of SNO-based PES membrane indicated the best flux recovery ratio compared to the other membranes. The photo stimulus–responsive assessment showed a color change for both SPO and SNO photochromic in membranes. In the case of variable effect investigation, the response surface methodology at three levels (pressure: 4, 5, 6 bar and flow rate: 50, 100, and 150 L/h) was applied. A suitable flux (23.39 kg/m² h) and high removal efficiency (more than 90%) was achieved at optimum conditions. Also, the modified membranes by photochromic materials were sensitive to environmental variables such as acidic and alkaline conditions by changing their color.     

Enhanced antifouling performance of ZnS/GO/PVDF hybrid membrane by improving hydrophilicity and photocatalysis

In order to improve the antifouling performance of PVDF membrane, a novel zinc sulfide/graphene oxide/polyvinylidene fluoride (ZnS/GO/PVDF) composite membrane was prepared by immersed phase inversion method. The surface morphology, crystal structure, photocatalytic activity, and antifouling property of the as‐prepared membranes were systematically studied. Results showed that the ZnS/GO/PVDF hybrid membranes were successfully fabricated with uniform surface. The hybrid membrane surface possessed higher hydrophilicity with water contact angle decreasing from 77.1° to 62.2°. The permeability of the hybrid membrane was therefore enhanced from 222.9 to 326.1 L/(m² hour). Moreover, bovine serum albumin (BSA) retention experiment showed that the hybrid membrane separation was also promoted by 7.2%. The blending of ZnS and GO enhanced the hydrophilic and photocatalytic performances of PVDF membrane, which mitigated the membrane fouling effectively. This novel hybrid membrane could accelerate the practical application of photocatalytic technology in membrane separation process.     

Preparation and performance evaluation of carboxylic acid containing polyamide incorporated microporous ultrafiltration PES membranes

Improved ultrafiltration membranes were prepared by the phase inversion technique via immersion precipitation of synthesized carboxylic acid containing polyamide (CPA) and polyethersulfone (PES) in dimethylacetamide. The CPA was synthesized and characterized by Fourier transform infrared (FTIR), nuclear magnetic resonance, thermogravimetric analysis, and differential scanning calorimetry analyses. Next, the influence of CPA adding and its different concentrations on the performances and membrane structure were investigated. The obtained membranes were characterized by means of FTIR in the attenuated total reflection mode, scanning electron microscopy, and contact angle. The membrane performance studies revealed that the presence of CPA in the membrane structure increased water permeability while reducing protein fouling. It turned out that the PES/CPA membranes had better porosity, more hydrophilic surface, and more vertically finger‐like pores in comparison with the bare PES membrane. When the CPA concentration in the blending solution reached 1 wt%, the water permeability increased from 7.3 to 153.6 L/m² h¹. The attenuated total reflection‐FTIR analysis confirmed that CPA was captured in the membrane matrix.     

Non-invasive monitoring of fouling in hollow fiber membrane via UTDR

Fouling is the most critical problem associated with membrane separations in liquid media. But it is difficult to control the inevitable membrane fouling because of its invisibility, especially on the inside surface of hollow fiber membranes. This study describes the extension of ultrasonic time-domain reflectometry (UTDR) for the real-time measurement of particle deposition in a single hollow fiber membrane. A transducer with a frequency of 10 MHz and polyethersulfone hollow fiber membranes with 0.8 mm inside diameter (ID) and 1.2 mm outside diameter (OD) were used in this study. The fouling experiments were carried out with 1.8 g/L kaolin suspension at flow rates 16.7 and 10.0 cm/s. The results show that UTDR technique is able to distinguish and recognize the acoustic response signals generated from the interfaces water/upper outside surface of the hollow fiber, lumen upside surface/water, water/lumen underside surface and lower outside surface/water in the single hollow fiber membrane module in pure water phase. The systemic changes of acoustic responses from the inside surfaces of the hollow fiber in the time- and amplitude-domain with operation time during the fouling experiments were detected by UTDR. It is associated with the deposition and formation of the kaolin layer on the inside surfaces. Further, the acoustic measurement indicates that the deposited fouling layer is denser on the lumen underside surface of the hollow fiber than that on the lumen upside surface as a result of weight. Moreover, it is found that the fouling layer grows faster on the inside surface of the hollow fiber at a flow rate of 10.0 cm/s than that at 16.7 cm/s due to the lower shear stress. The fouling layer formed is thicker at a flow rate of 10.0 cm/s than that at 16.7 cm/s. The flux decline data and SEM analysis corroborate the ultrasonic measurement. Overall, this study confirms that UTDR measurement will provide not only a new protocol for the observation of hollow fiber membrane fouling and cleaning, but also a quantitative approach to the optimization of the membrane bioreactor system.     

A novel method of surface modification on thin-film-composite reverse osmosis membrane by grafting hydantoin derivative

Membrane degradations by biofouling and free chlorine oxidation are the major obstacles for aromatic polyamide thin-film-composite (TFC) reverse osmosis (RO) membranes to realize high performance over a long period of operation. In this work, a hydantoin derivative, 3-monomethylol-5,5-dimethylhydantoin (MDMH), was grafted onto the nascent aromatic polyamide membrane surfaces by the reactions with active groups (e.g., acyl chloride groups) in the surfaces. The grafted MDMH moieties with high reaction activity and free chlorine could play as sacrificial pendant groups when membranes suffer from chlorine attacks, and the chlorination products N-halamines with strong antimicrobial function could sterilize microorganisms on membrane surfaces and then regenerate to MDMH. This was designed as a novel means to improve both chlorine resistances and anti-biofouling properties of the aromatic polyamide TFC RO membranes.Attenuated total reflectance mode Fourier transform infrared spectroscopy (ATR-FTIR) revealed that the MDMH-modified membranes had two characteristic bands at 1772 and 1709 cm⁻¹ corresponding to two carbonyl groups in hydantoin ring. This suggested the successful grafting of MDMH onto the membrane surfaces, which was further confirmed and quantified by X-ray photoelectron spectroscopy (XPS) analysis. After modification with MDMH, the membrane surface hydrophilicity increased obviously as contact angles decreased from 57.7° to 50.4–31.5°. But, there was no obvious change in membrane surface roughness after modification. The MDMH-modified membranes were shown to possess high chlorine resistances with small changes in water fluxes and salt rejections after chlorination with 100–2000 ppm h chlorine at pH 4. The chlorinated MDMH-modified membranes demonstrated obvious sterilization effects on Escherchia coli and substantial preventions against microbial fouling. Therefore, the MDMH-modified membranes offer a potential use as a new type of chlorine resistance and anti-biofouling TFC RO membranes.     

Fabrication and development of interfacial polymerized thin-film composite nanofiltration membrane using different surfactants in organic phase; study of morphology and performance

The effects of addition of cationic cetyltrimethylammonium bromide (CTAB), non-ionic (Triton X-100) and anionic sodium dodecyl sulfate (SDS) surfactants in organic phase for preparing the composite nanofiltration membranes were investigated. The interfacial polymerization technique was employed by applying trimesoyl chloride (TMC) and piperazine (PIP) as the reagents for the preparation of poly(piperazineamide) on a UF support. The obtained thin layer membranes were placed in oven for 2 min at 70 °C. Water permeation performance, salt rejection, membrane surface charge, chemical structure and membrane morphology including top surface and cross-section were investigated for characterization of the prepared membranes using IR-ATR, SEM, filtration and zeta potential measurement. The prepared membranes using SDS showed higher flux compared to the other membranes. SEM surface images demonstrate some defects and cracks on the thin layer surface of the membrane prepared with SDS. For membrane containing CTAB, the salt rejection increased in the order of Na₂SO₄ > NaCl > MgCl₂ with variation around 50–90%.     

Ultrafiltration polyanionic poly (3-sulfopropyl methacrylate) membranes with enhanced antifouling and water flux

Polyethersulfone (PES) membranes are prevalent in the field of water treatment owing to their exceptional separation efficiency, robust mechanical properties, and resistance to chemical degradation. Nevertheless, these membranes are prone to fouling, resulting in a decrease in both flux and ultrafiltration efficiency. In the present study, PES membranes are blended with poly (3-Sulfopropyl Methacrylate) (PSPMA) in various weight percentages (0%–3%) to improve their antifouling and ultrafiltration properties. The physicochemical properties of the blended membranes, including surface morphology, contact angle, hydrophilicity and surface energy are evaluated. The findings indicate that incorporation PSPMA results in an enhancement of the hydrophilic properties and surface charge of the PES membranes, assessed by employing Bovine Serum Albumin (BSA) as a representative protein. Modified blended membranes display greater Flux Recovery Ratio (FRR%) and exhibit superior fouling resistance. Under the same experimental conditions (0.2 MPa applied pressure), a pure water flux of 154.18 L·m⁻²·h⁻¹ for PES/PSPMA membrane found substantially greater than pure PES membrane (103.52 L·m⁻²·h⁻¹) along with Total Fouling Ratio (TFR) of 36% and 64.9% respectively. Exceptional antimicrobial efficacy for modified membranes is revealed against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) using disc diffusion technique rendering them well-suited for water treatment applications.     

Investigating the effect of PEG200 and two-dimensional h-BN on PVDF membrane performance for membrane distillation–crystallization

Although water supplies are prominently dependent on desalination technology, desalination plant facing severe issues of discharged brine concentrate. Membrane distillation crystallization is an emerging synergistic technology that resolves the issue of brine concentrate by recovering clean water and value-added minerals simultaneously. In the present study, properties of polyvinylidene fluoride (PVDF) membrane were modified by incorporation of exfoliated fillers of hexagonal boron nitride and polyethylene glycol. The changes in morphology, surface roughness, hydrophobicity, thermal stability, and chemical composition of the prepared membranes were evaluated by scanning electron microscopy, atomic force microscopy, contact angle, thermogravimetric analysis, Fourier-transform infrared spectroscopy, respectively. Membrane distillation crystallization experiments were conducted to observe the effect of modified membranes on the permeate flux and salts recovery at different feed temperatures. The results showed a significant improvement in the permeate flux with modified membranes compared with pure PVDF membrane. It was found that hexagonal boron nitride/polyethylene glycol200 incorporated PVDF membrane gave the higher permeate flux (3.41 kg/m² h for K₂SO₄ and 2.62 kg/m² h for KNO₃) at a temperature of 80 °C along with higher salts recovery than pure PVDF membranes. A 100 h long run test was conducted on modified membranes, which showed consistency in permeate flux with a marginal increase in conductivity.     

In-situ manipulation of gel layer fouling into gel layer membrane formation on porous supports for water treatment

The inaccessibility of clean water is one of the growing issues of this era. Indeed, cost-effective and sustainable methods for recycling wastewater are essential. Although membrane separation is an efficient technology for the recycling and purification of water, membrane fouling is still a major drawback of this technology. This work is aimed to develop a dynamic method to form gel layer membranes (GLMs) by manipulating the irreversible fouling process itself as a problem-solving approach. A microporous polyvinylidene fluoride (PVDF) support is subjected to gel layer formation by applying a supernatant of industrial aerobic sludge (containing soluble extracellular polymeric substances EPS) as a feed. Retention of polysaccharides and calcium during the filtration and the topographical analysis after the filtration show that EPS uniformly formed a gel layer on the PVDF support. No further decline in permeability is observed (i.e. remained around 27–33 L/m² hr) when the formed GLM is subjected to fouling under similar conditions. Moreover, the percent flux recovery ratio (FRR) of the GLM is also significant (i.e. 90.1 ± 2.71). The retention ability, hydrophilicity, porosity, and water uptake capability of the formed GLM also increased significantly. The optimal performance and stability of GLMs are observed at room temperature (RT) under neutral pH and sub-critical trance membrane pressures (TMP). Based on these results it is suggested that the in-situ manipulation of gel layer fouling is a viable approach for preparing fouling resistant GLMs with high retention efficiency, potentially applicable to wastewater treatment under normal conditions.     

Adsorptive fouling of modified and unmodified commercial polymeric ultrafiltration membranes

The fouling tendency, due to adsorption on the pore walls, of two pairs of modified and unmodified ultrafiltration membranes, with similar observed retentions determined by dextran and gel permeation chromatography, was studied. The membranes investigated were made of modified and unmodified polyaramide (PA) and modified and unmodified polyvinylidene fluoride (PVDF). The PVDF membrane was surface-modified and the PA membrane was made from a modified polymer solution. Membrane modification was used to reduce fouling by adsorption. Octanoic acid was used as the fouling substance, representing a large number of small, hydrophobic compounds. It is demonstrated in this investigation that membrane modification is not always successful. It was determined that at lower concentrations of octanoic acid, the modified PA membrane exhibits a smaller fouling tendency than the unmodified PA membrane, while the result is reversed for concentrations above 60% of the saturation concentration. The fouling tendency of the unmodified PVDF membrane is much lower than that of the modified PVDF membrane at all concentrations. The cross-sections of the membranes were visually examined with scanning electron microscopy, but no difference could be observed between the modified and unmodified membranes. The membranes were also examined with Fourier transform infrared spectroscopy. The spectra of the two PA membranes were different, while no difference was observed for the unmodified and surface-modified PVDF membranes. Remains of octanoic acid were found in the membranes, although they had been thoroughly rinsed with deionized water and the initial pure water flux was recovered.     

Effect of sonication on polymeric membranes

This article studies the effect of 47 kHz ultrasonic (US) waves on polymeric membranes immersed in an aqueous bath. The membranes under study are made from three different polymers: polyethersulfone (PES), polyvinylidenefluoride (PVDF) and polyacrylonitrile (PAN) and present various molecular weight cut-off (MWCO). The evolution of the polymeric structure exposed to US was followed by the measurement of the water permeability and the A_k/Δx parameter which represents the ratio of surface porosity to thickness. Results showed that important variations occurred on certain membranes after irradiation. In addition, microscopic imaging using field emission electron scanning microscopy (FESEM) was performed on irradiated membranes in order to visualize the nature of the degradation. An image analysis method gives the evolution of the pore density, porosity and pore size distribution of a homogeneous area of this membrane before and after irradiation.It has been shown that, over the three materials tested, only the PES is affected by the ultrasonic treatment over all its surface, whereas the others present no significant change in the measured parameters except the PAN (50 kDa) and PVDF (40 kDa) membranes whose edges are affected. In conclusion, in spite of their great efficiency in enhancing filtration processes, ultrasonic waves have to be used with care as the polymeric material itself is sensitive to the ultrasonic waves at the chosen frequency.     

Poly(vinyl alcohol)/cellulose nanocrystal barrier membranes

In this study, barrier membranes were prepared from poly(vinyl alcohol) (PVOH) with different amounts of cellulose nanocrystals (CNXLs) as filler. Poly(acrylic acid) (PAA) was used as a crosslinking agent to provide water resistance to PVOH. The membranes were heat treated at various temperatures to optimize the crosslinking density. Heat treatment at 170 °C for 45 min resulted in membranes with improved water resistance without polymer degradation. Infrared spectroscopy indicated ester bond formation with heat treatment. Mechanical tests showed that membranes with 10% CNXLs/10% PAA/80% PVOH were synergistic and had the highest tensile strength, tensile modulus and toughness of all the membranes studied. Polarized optical microscopy showed agglomeration of CNXLs at filler loadings greater than 10%. Differential thermogravimetric analysis (DTGA) showed a highly synergistic effect with 10% CNXL/10% PAA/80% PVOH and supported the tensile test results.Transport properties were studied, including water vapor transport rate and the transport of trichloroethylene, a representative industrial toxic material. Water vapor transmission indicated that all the membranes allowed moisture to pass. However, moisture transport was reduced by the presence of both CNXLs and PAA crosslinking agent. A standard time lag diffusion test utilizing permeation cups was used to study the chemical barrier properties. The membranes containing ≥10% CNXLs or PAA showed significantly reduced flux compared to the control. The CNXLs were then modified by surface carboxylation in order to better understand the mechanism of transport reduction. While barrier performance improvements were minimal, the chemical modification improved the dispersion of the modified CNXLs which led to improved performance. Of special note was an increase in the initial degradation temperatures of both modified and unmodified systems, with the modified system showing an initial degradation temperature >100 °C higher than the cellulose alone. This may reflect more extensive crosslinking in the modified composite.     

Hydrophilic modification of polyethersulfone membranes by low temperature plasma-induced graft polymerization

A complete and permanent hydrophilic modification of polyethersulfone (PES) membranes is achieved by argon plasma treatment followed by polyacrylic acid (PAA) grafting in vapor phase. Both Ar plasma treatment alone and post-PAA grafting rendered a complete hydrophilicity to the PES membranes. The hydrophilicity of the membranes treated with only the Ar plasmas is not, however, permanent. In contrast, the PES membranes treated with Ar plasma and subsequent acrylic acid (AA) grafting are permanently hydrophilic. High energy resolution X-ray photoelectron spectroscopy (XPS) confirmed the grafting of PAA to all surfaces of the membrane. Furthermore, water bubble point measurements remain unaffected. The pore sizes of the grafted membranes at higher grafting yield are slightly decreased. The modified membranes are less susceptible to protein fouling than the unmodified membranes and the pure water flux for the modified membranes was tremendously increased by plasma treatment. Furthermore, the modified membranes are easier to clean and required little caustic to recover permeation flux.     

Ceramic asymmetric hollow fibre membranes—One step fabrication process

Ceramic hollow fibre membranes which have an asymmetric structure have been prepared in one step, using an immersion induced phase inversion technique. With this method, membranes with a high surface area per unit volume ratio can be produced, while production cost is dramatically reduced. Yttria-stabilised zirconia (YSZ) is selected as a membrane material, as it is relatively inexpensive and has superior mechanical strength as well as oxygen ion conducting properties. Therefore, both the porous and non-porous membranes prepared from the YSZ have potential applications. For example, the porous YSZ membranes can be used for fluid separations in harsh environments where normal polymeric membranes cannot be sustained, while the non-porous YSZ membranes can be applied as a solid electrolyte in electrochemical devices such as solid oxide fuel cells, oxygen pumps and chemical gas sensors.Gas permeation analysis suggests that non-porous YSZ hollow fibre membranes can be prepared at sintering temperature of 1400 °C or greater, below which the membrane contains pores. Pore sizes of the YSZ porous membrane prepared fall into the pore size range of ultrafiltration membranes. However, the surface porosities of the membranes prepared from two-population sized particles at sintering temperatures of 1200 °C and 1400 °C are around 5000 m⁻¹ and 300 m⁻¹, respectively. The former is comparable to polymeric membranes, while the latter is an order of the magnitude smaller.     

Amino acids functionalized graphene oxide for enhanced hydrophilicity and antifouling property of poly(vinylidene fluoride) membranes

Herein, functionalized graphene oxide (GO) was prepared by the covalent functionalization with amino acids (lysine, glycine, glutamic acid and tyrosine) in this study. Zeta potential results demonstrated that covalent functionalization of GO with amino acids was favourable for their homogeneous dispersion in water and organic solvents. Based on the higher absolute value of zeta potential and the better dipersion stability of GO-lysine, the PVDF/GO-lysine hybrid membranes were then prepared via the phase inversion induced by immersion precipitation technique. SEM images showed a better pore diameter and porosity distribution on the PVDF/GO-lysine membrane surface. The zeta potential absolute value of the PVDF/GO-lysine membrane surface was higher than that of the virgin PVDF membrane. Furthermore, the PVDF/GO-lysine membranes surface exhibited good hydrophilicity. The water flux of PVDF/GO-lysine membranes can reach to two times of that of the virgin PVDF membrane. And the BSA adsorbed amount on PVDF/GO-lysine surface was decreased to 0.82 mg/cm² for PVDF/GO-lysine-8% membrane. Filtration experiment results indicated that the fouling resistance was significantly improved for the PVDF/GO-lysine membranes. As a result, lysine functionalized GO will provide a promising method to fabricate graphene oxide based hybrid membranes with effective antifouling property and hydrophilicity.     

Enhancing the antifouling property of polyethersulfone ultrafiltration membranes through surface adsorption-crosslinking of poly(vinyl alcohol)

An adsorption-crosslinking process of poly(vinyl alcohol) (PVA) was introduced to modify the surface of polyethersulfone (PES) ultrafiltration membranes for enhancement of their antifouling property. XPS and water contact angle measurement confirmed the obvious enhancement of surface hydrophilicity. Ultrafiltration results showed that the spreading of PVA chains over the hydrophobic membrane surface caused substantial but acceptable decrease on membrane flux. The fouling type analysis indicated that PVA adsorption effectively improved the antifouling property of PES membranes. With a PVA concentration of 0.5 wt% and three cycles of alternative adsorption-crosslinking, the total and irreversible fouling ratio of modified membranes were 0.38 and 0.22, respectively, much lower than those of control PES membrane (0.61 and 0.47), and the flux recovery ratio was increased accordingly. The long-term ultrafiltration experiment demonstrated the improvement of recycling property and the reliability of adsorption-crosslinking process.     

Adsorption of amylase enzyme on ultrafiltration membranes

A method to measure the static adsorption on membrane surfaces has been developed and described. The static adsorption of amylase-F has been measured on two different ultrafiltration membranes, both with a cutoff value of 10 kDa (a PES membrane and the ETNA10PP membrane, which is a surface-modified PVDF membrane). The adsorption follows the Langmuir adsorption theory. Thus, the static adsorption consists of monolayer coverage and is expressed both as a permeability drop and an adsorption resistance. From the adsorption isotherms, the maximum static permeability drops and the maximum static adsorption resistances are determined. The maximum static permeability drop for the hydrophobic PES membrane is 75%, and the maximum static adsorption resistance is 0.014 m2.h.bar/L. The maximum static permeability drop for the hydrophilic surface-modified PVDF membrane (ETNA10PP) is 23%, and the maximum static adsorption resistance is 0.0046 m2.h.bar/L. The difference in maximum static adsorption, by a factor of around 3, affects the performance during the filtration of a 5 g/L amylase-F solution at 2 bar. The two membranes behave very similarly during filtration with almost equal fluxes and retentions even though the initial water permeability of the PES membrane is around 3 times larger than the initial water permeability of the ETNA10PP membrane. This is mainly attributed to the larger maximum static adsorption of the PES membrane. The permeability drop during filtration exceeds the maximum static permeability drop, indicating that the buildup layer on the membranes during filtration exceeds monolayer coverage, which is also seen by the increase in fouling resistance during filtration. The accumulated layer on the membrane surface can be described as a continually increasing cake-layer thickness, which is independent of the membrane type. At higher concentrations of enzyme, concentration polarization effects cannot be neglected. Therefore, stagnant film theory and the osmotic pressure model can describe the relationship between flux and bulk concentration.