Ceramic Ultrafiltration Membrane Research Articles

Development of composite ultrafiltration membrane from fly ash microspheres and alumina nanofibers for efficient dye removal from aqueous solutions

In this work, a novel type of ultrafiltration ceramic membranes with the support based on fine fly ash microspheres and selective layer based on the alumina nanofibers with an aluminosilicate binder is proposed. The average pore sizes of the support and selective layer are 0.46 μm and 29 nm, respectively. The membrane is characterized by the compressive strength of 96 MPa and water permeability of 207 L m−2 h−1 bar−1. It is shown that the binder provides structural stability of selective layer and adhesion to the support. With increasing the binder content, the water permeability increases, reaches maximum, and then slightly decreases. The developed membranes are used for ultrafiltration of Blue Dextran dyes aqueous solutions with molecular weights of 70 kDa and 500 kDa and concentrations of 50 and 100 mg/L. The dyes rejection varies in the range 97–99 %, while the permeate flux is 100–140 L m−2 h−1 at the transmembrane pressure of 4 bars. The dye retention occurs via adsorption at the initial stage, which leads to the narrowing of pore size. Further, the dye filtration proceeds mainly due to size effects. The proposed membranes can be employed for dye removal from wastewater, and also allow chemical modification by carbon coating to be employed in electrochemically assisted ultrafiltration. The developed methodology promotes the recycling of thermal energy waste and introduces novel approaches to combine waste and synthesized raw materials in the production of low-cost ceramic membranes.

Purification of Liquid Fraction of Digestates from Different Origins-Comparison of Polymeric and Ceramic Ultrafiltration Membranes Used for This Purpose.

Circular economy, clean technologies, and renewable energy are key to climate protection and modern environmental technology. Recovering water and valuable minerals from the liquid fraction of digestate is in line with this strategy. Digestate, a byproduct of anaerobic methane fermentation in biogas plants, is a potential source of water, minerals for fertilizers, and energy rather than waste. This study examined digestate from municipal and agricultural biogas plants and highlights the need for research on both due to their differences. The use of membrane techniques for water recovery from liquid digestate offers an innovative alternative to conventional methods. This study used standalone membrane filtration and an integrated system to produce water suitable for agricultural use. Ceramic membranes with cut-offs of 1, 5, 15, and 50 kDa and polymeric membranes of polyethersulfone and regenerated cellulose with cut-offs of 10 and 30 kDa were tested. The results showed that the membrane material significantly affects the transport and separation properties. Higher cut-off values increased permeate flux across all membranes. Ceramic membranes were more susceptible to fouling in standalone ultrafiltration, but were more effective in purifying digestate than polymeric membranes. The best results were obtained with a ceramic membrane with a 1 kDa cut-off (for example, for the integrated process and the municipal digestate, the retention rates of COD, BOD5 and DOC were 69%, 62%, and 75%, respectively).

A Novel Modeling Optimization Approach for a Seven-Channel Titania Ceramic Membrane in an Oily Wastewater Filtration System Based on Experimentation, Full Factorial Design, and Machine Learning.

This comprehensive study looks at how operational conditions affect the performance of a novel seven-channel titania ceramic ultrafiltration membrane for the treatment of produced water. A full factorial design experiment (23) was conducted to study the effect of the cross-flow operating factors on the membrane permeate flux decline and the overall permeate volume. Eleven experimental runs were performed for three important process operating variables: transmembrane pressure (TMP), crossflow velocity (CFV), and filtration time (FT). Steady final membrane fluxes and permeate volumes were recorded for each experimental run. Under the optimized conditions (1.5 bar, 1 m/s, and 2 h), the membrane performance index demonstrated an oil rejection rate of 99%, a flux of 297 L/m2·h (LMH), a 38% overall initial flux decline, and a total permeate volume of 8.14 L. The regression models used for the steady-state membrane permeate flux decline and overall permeate volume led to the highest goodness of fit to the experimental data with a correlation coefficient of 0.999. A Multiple Linear Regression method and an Artificial Neural Network approach were also employed to model the experimental membrane permeate flux decline and analyze the impact of the operating conditions on membrane performance. The predictions of the Gaussian regression and the Levenberg-Marquardt backpropagation method were validated with a determination coefficient of 99% and a Mean Square Error of 0.07.

Removal of heavy metals from mine water using a hybrid electrocoagulation-ceramic membrane filtration process

Mining operations must account for increasingly stringent wastewater discharge limits for various contaminants, such as heavy metals and suspended solids. In the treatment of metal-bearing wastes, electrochemical processes, including electrocoagulation (EC), are gaining popularity. At large scales, EC processes can be hindered by considerable amounts of flocculants and by sedimentation tank sizes required to properly operate. This work sought to address these concerns by evaluating EC in mine water treatment using a pilot system coupled with ceramic membrane microfiltration (MF) and ultrafiltration (UF). The use of MF and UF after EC allows for the removal of suspended solids without needing a flocculation-sedimentation process. EC evaluation was performed using field samples of mine water from a North American concentrator. Under optimal conditions, EC removed over 95 % of Cr, Co, Cu, Fe, Mn, Ni, Pb, Ti and Zn from the mine water. Effluent from EC was used as feed in MF and UF processes with ceramic membranes. The tested membranes rejected 100 % of suspended solids from the EC effluent. Permeate from the membranes was free of solids and possessed turbidities of 0.05–0.15 NTU. Filtrate from the hybrid process satisfied environmental regulations for heavy metal concentrations, offering new opportunities for reuse or discharge. The hybrid process applied in this work can thus be used in mining operations to safely discharge their water, and/or to increase their water recirculation due to environmental concerns and water shortage.

Combination of ultrafiltration, activated carbon and disinfection as tertiary treatment of urban wastewater for reuse in agriculture

In the present study, three ceramic ultrafiltration (UF) membranes UF1 (20 nm), UF2 (50 nm) and UF3 (100 nm) were optimized using the Box-Behnken design (BBD) of response surface methodology (RSM) to determine the most adaptable membrane for reuse. The effectiveness of membranes was based on the retention capacity of chemical oxygen demand (COD), biochemical oxygen demand at 5 days (BOD5) and total suspended solids (TSS) as a function of transmembrane pressure (TMP), circulation velocity (CV) and volume concentration factor (VCF). In addition, the study investigated membrane fouling based on the variation in permeate flux under different operational conditions. Among the tested membranes, UF1 demonstrated superior retention efficiency compared to UF2 and UF3. Using granular activated carbon (GAC) and powdered activated carbon (PAC) with UF1 significantly improved water quality. Moreover, the combination of UF1 separately with GAC and PAC reduced amoxicillin residues by 60 % and 69 %, respectively. Furthermore, the UF1 membrane alone showed significant removal of bacterial loads, exceeding 90 % removal of total coliforms, fecal coliforms and Escherichia coli, as well as complete (100 %) elimination of helminth eggs. The treatment process ended with disinfection by sodium hypochlorite (NaOCl), providing total (100 %) elimination of all remaining bacteria in the water.

Catalytic Ozonation of Pharmaceuticals Using CeO2-CeTiOx-Doped Crossflow Ultrafiltration Ceramic Membranes.

The removal of persistent organic micropollutants (OMPs) from secondary effluent in wastewater treatment plants is critical for meeting water reuse standards. Traditional treatment methods often fail to adequately degrade these contaminants. This study explored the efficacy of a hybrid ozonation membrane filtration (HOMF) process using CeO2 and CeTiOx-doped ceramic crossflow ultrafiltration ceramic membranes for the degradation of OMPs. Hollow ceramic membranes (CM) with a 300 kDa molecular weight cut-off (MWCO) were modified to serve as substrates for catalytic nanosized metal oxides in a crossflow and inside-out operational configuration. Three types of depositions were tested: a single layer of CeO2, a single layer of CeTiOx, and a combined layer of CeO2 + CeTiOx. These catalytic nanoparticles were distributed uniformly using a solution-based method supported by vacuum infiltration to ensure high-throughput deposition. The results demonstrated successful infiltration of the metal oxides, although the yield permeability and transmembrane flow varied, following this order: pristine > CeTiOx > CeO2 > CeO2 + CeTiOx. Four OMPs were examined: two easily degraded by ozone (carbamazepine and diclofenac) and two recalcitrant (ibuprofen and pCBA). The highest OMP degradation was observed in demineralized water, particularly with the CeO2 + CeTiOx modification, suggesting O3 decomposition to hydroxyl radicals. The increased resistance in the modified membranes contributed to the adsorption phenomena. The degradation efficiency decreased in secondary effluent due to competition with the organic and inorganic load, highlighting the challenges in complex water matrices.

Evaluation of the potential ultrafiltration to improve the quality of secondary effluents in tertiary treatment and reuse

Unsatisfactory effluent purification, limited to secondary treatment, considerably restricts the reuse of treated water. In this study, the performance of three ceramic ultrafiltration (UF) membranes UF1 (0.02 μm), UF2 (0.05 μm), and UF3 (0.1 μm) was evaluated for the recovery of secondary effluent (SE) from a wastewater treatment plant (WWTP) in Kenitra. The effects of different parameters on treated water quality were assessed, including transmembrane pressure (TMP), circulation velocity (CV), and membrane pore size. The results indicated that the UF1 membrane was more effective than UF2 and UF3, achieving maximum removal rates for chemical oxygen demand (COD), biochemical oxygen demand at 5 days (DBO5), and total suspended solids (TSS) of 75%, 72%, and 96%, respectively, under optimal operating conditions of TMP at 1 bar and CV at 1 m/s. UF2 achieved removal rates of 70%, 71%, and 94% for COD, BOD5, and TSS, respectively, under the same conditions. For UF3, the highest removal rates were 64%, 62%, and 92% for COD, BOD5, and TSS, respectively, at 2 bar and 1 m/s. Moreover, the experiments also demonstrated that an increase in TMP results in increased permeate flux and fouling resistance, while CV has also an impact on the permeate flux. Additionally, a very satisfactory retention rate for microorganisms was observed, exceeding 90%, with no presence of helminth eggs (100% removal) detected in the treated water. Nutrient elimination was also notable, with reductions of 14% for total Kjeldahl nitrogen (TKN) and 50% for total phosphorus (TP) using the UF1 membrane, which was selected based on its optimal performance.

β-galactosidase immobilization on ceramic ultrafiltration membrane for simultaneous lactose hydrolysis and protein separation

Protein and lactose of cheese whey can be separated using ultrafiltration membranes and lactose can be hydrolyzed by β-galactosidase. This study investigated the integration of ultrafiltration and enzymatic hydrolysis for simultaneous protein concentration and lactose hydrolysis. β-galactosidase was immobilized on ceramic membrane via surface activation with gelatin and subsequent enzyme cross-linking with glutaraldehyde. Membrane geometries, buffer types, gelatin and enzyme concentrations were optimized. The optimal results yielded a 63 % enzyme immobilization efficiency and a 93 % lactose hydrolysis rate by using disc membrane, sodium acetate buffer (pH 4.8), sodium phosphate buffer (pH 7.5), 0.1 g/L gelatin concentration, and 5.0 g/L enzyme concentration. The optimized enzymatic membrane was evaluated with synthetic and real whey. In synthetic whey, 85 % lactose hydrolysis and 89 % protein rejection were achieved. While in real whey, 72 % lactose hydrolysis and 83 % protein recovery were obtained. This approach offers a single-step, efficient, and scalable method for whey valorization with potential for industrial applications.

Mo-BiVO4 Photocatalytically Modified Ceramic Ultrafiltration Membranes for Enhanced Water Treatment Efficiency.

This study highlights the effectiveness of photocatalytically modified ceramic ultrafiltration (UF) membranes in alleviating two major drawbacks of membrane filtration technologies. These are the generation of a highly concentrated retentate effluent as a waste stream and the gradual degradation of the water flux through the membrane due to the accumulation of organic pollutants on its surface. The development of two types of novel tubular membranes, featuring photocatalytic Mo-BiVO4 inverse opal coatings, demonstrated a negligible impact on water permeance, ensuring consistent filtration and photocatalytic efficiency and suggesting the potential for maintaining membrane integrity and avoiding the formation of highly concentrated retentate effluents. Morphological analysis revealed well-defined coatings with ordered domains and interconnected macropores, confirming successful synthesis of Mo-BiVO4. Raman spectroscopy and optical studies further elucidated the composition and light absorption properties of the coatings, particularly within the visible region, which is vital for photocatalysis driven by vis-light. Evaluation of the tetracycline removal efficiency presented efficient adsorption onto membrane surfaces with enhanced photocatalytic activity observed under both UV and vis-light. Additionally, vis-light irradiation facilitated significant degradation, showcasing the versatility of the membranes. Total Organic Carbon (TOC) analysis corroborated complete solute elimination or photocatalytic degradation without the production of intermediates, highlighting the potential for complete pollutant removal. Overall, these findings emphasize the promising applications of Mo-BiVO4 photocatalytic membranes in sustainable water treatment and wastewater remediation processes, laying the groundwork for further optimization and scalability in practical water treatment systems.

Investigating effective strategies for sustainable operation of a submerged anaerobic membrane bioreactor (SAnMBR) coupled with ceramic ultrafiltration membrane

This study presents a mechanistic approach to explore the key operational parameters governing the flux performance of an SAnMBR system coupled with a ceramic flat-sheet ultrafiltration membrane. Experiments were conducted at mixed liquor suspended solid (MLSS) concentrations of 12, 18, and 24 g/L using biogas sparging, backwashing, and a combination of both to investigate the most effective fouling mitigation strategy by exploring the fouling mechanism at a critical flux regime. To validate the experimental findings, a mathematical model was used to simulate the time-based variation of membrane effective pore radius (m), decrease in membrane porosity (%), thickness of cake layer (m), and membrane resistance (1/m). The study found that at a low biomass concentration, there was a sharp decline in the membrane pore radius leading to an increase in membrane resistance in the absence of any fouling mitigation strategies. However, by implementing these strategies individually and in combination, they maintained a higher pore radius and controlled the increase in membrane resistance even at high biomass concentrations. It was observed that biogas sparging outperformed the conventional backwashing strategy and the SAnMBR system exhibited superior performance attaining higher flux rates for prolonged duration. The advanced spectroscopic analysis confirmed the presence of a higher concentration of polysaccharides responsible for cake layer biofouling, and significant pore blocking due to inorganic foulants at high biomass concentrations. This suggests that the SAnMBR system must be operated at optimised biomass levels below the critical flux for sustained operation. Additionally, the key operational parameters identified using the mathematical model provide a precise assessment of SAnMBR performance, to improve its design efficiency for field applications.

Rapid thermal process for fabricating α-alumina tight ultrafiltration membrane with narrow pore size distribution

The development of tight ceramic ultrafiltration membranes for water purification facilitates is challenging because of high fabrication costs and low permeate flux. In this study, a low-energy rapid thermal process (RTP) was comprehensively investigated for fabricating of high-precision, permeable α-alumina tight ultrafiltration membranes. Compared to the conventional thermal process (CTP), RTP offers an ultra-fast heating rate and shorter holding time, effectively mitigating grain growth and pore aggregation. Moreover, the decomposition of organic matter is delayed during RTP, providing a lower interfacial energy that enables direct formation of α-alumina at a low temperature of 750 °C. During a secondary subsequent RTP, the residual phase is fully converted to α-alumina by the in-situ seeding effect. By utilizing RTP, we successfully prepared α-alumina ultrafiltration membranes within 30 min by calcination, with an average pore size of approximately 6 nm, a narrow pore distribution coefficient of 1.12, and high permeance of 219 L−1·m−2·h−1·bar−1.

Fractionation of Glycans from Hot‐Water Extracts with Ceramic Ultrafiltration Membranes

AbstractExtraction of hemicelluloses from biomass with pressurized hot‐water employs the mechanism of autohydrolysis for molecular weight reduction. In a pilot‐scale extraction of depectinized apple pomace, molecular weight was 30 kDa with a broad mass distribution including monomers and their degradation products. Ultrafiltration with sequential use of ceramic membranes with decreasing cut‐off yielded a suitable fraction with molecular weight of 9 kDa representing 10 % of the extracted hemicelluloses. Further autohydrolytic treatment of the extracted hemicellulose decreases molecular weight while increasing the mass of degradation products. For a series of hydrolysates sequential filtration through ceramic membranes was further tested for fractionation and purification. The efficiency of this fractionation was analyzed by size exclusion chromatography and a total mass balance of the ultrafiltration process is given as an evaluation for suitability of purifying and fraction glycans.

Pretreatment of Glucose-Fructose Syrup with Ceramic Membrane Ultrafiltration Coupled with Activated Carbon.

Ceramic membranes are applied to remove non-sugar impurities, including proteins, colloids and starch, from glucose-fructose syrup that is dissolved from raw sugar using acid. The performance of ceramic membranes with 0.05 μm pores in clarifying high-fructose syrup was investigated under various operating conditions. The flux decreased rapidly at the start of the experiment and then tended to stabilize at a temperature of 90 °C, a transmembrane pressure of 2.5 bar, and cross-flow velocity of 5 m/s under total reflux operation. Moreover, the steady-state flux was measured at 181.65 Lm-2 h-1, and the turbidity of glucose-fructose syrup was reduced from 92.15 NTU to 0.70 NTU. Although membrane fouling is inevitable, it can be effectively controlled by developing a practical approach to regenerating membranes. Mathematical model predictions, scanning electron microscopy, energy dispersive X-ray spectroscopy, and Fourier-transform infrared spectroscopy revealed that foulants primarily responsible for fouling are composed of polysaccharides, proteins, sucrose, phenols, and some metal elements, such as calcium, aluminum, and potassium. Due to the removal of suspended colloidal solids, the membrane-filtered glucose-fructose syrup was decolorized using activated carbon; the filtration rate was effectively improved. A linear relationship between volume increase in syrup and time was observed. A decolorization rate of 90% can be obtained by adding 0.6 (w/w) % of activated carbon. The pretreatment of glucose-fructose syrup using a ceramic membrane coupled with activated carbon results in low turbidity and color value. This information is essential for advancing glucose-fructose syrup and crystalline fructose production technology.

Treatment of highly saline effluents laden with organic pollutants using ceramic ultrafiltration membranes and application to leachate treatment

The aim of this study is to assess the efficacy of three ultrafiltration ceramic membranes with distinct porosities (20, 50, and 100 nm) in treating highly charged effluents and their potential application in leachate treatment. In the initial phase, Response Surface Methodology (RSM) was employed using a solution containing NaCl (TDS = 20 g/L) and glucose (mass concentration (Cm) = 10 g/L). The investigation focused on operational parameters, specifically Circulation velocity (CV) and volumetric concentration factor (VCF). The responses considered encompass permeate flux (PF) and retention (R). In the subsequent phase, as part of a practical application, leachate was subjected to treatment, and the fouling mechanisms specific to each membrane were identified. The Hermia model was employed, distinguishing between four fouling modes: cake formation, intermediate blocking, pore constriction, and complete blocking. The results demonstrate that, during the treatment of the synthetic solution, VCF has a more significant influence on both responses compared to CV. Furthermore, CV impacts the critical flux of the three membranes, without affecting the retention of suspended matter (SS). When treating the leachate, removal efficiencies of 55%, 51%, and 47% for COD and 36%, 33%, and 29% for SS were achieved for membranes with porosities of 20 nm, 50 nm, and 100 nm, respectively. Modeling the flux over time showed that fouling of these three UF membranes is superficial, and the main fouling mechanism is cake formation.

A CuO-modified ultrafiltration ceramic membrane inducing micro-nano bubble collapse to generate hydroxyl radicals for enhancing pollutant removal

It is desirable to develop ceramic membranes with functional performances for wastewater treatment. This work developed a CuO-modified Al2O3 ceramic membrane (CuO/Al2O3 CM) via a sol-gel method. The coated CuO particles provided hydrophobicity and a high zero point of charge for CM, which could accumulate H+ ions and form a liquid depletion layer on the membrane surface. When coupled with micro-nano bubbles (MNBs) in a neutral aqueous solution, the modified CM led to MNBs collapse through the accumulation of H+ and uneven expansion. This collapse generated high temperature sites that activated the CuO particles, leading to the production of reactive oxygen species (ROSs) through the interaction of CuO and surface OH–. Therefore, the modified CM could remove model pollutants (methylene blue) in a stable and high efficiency (88.91 %) through inducing MNBs collapse. Besides, the tests on the pure water flux, reusability and leaching revealed the modified CM had good fouling resistance, stability, and environmental friendliness. Mechanism analysis showed the ROSs, mainly hydroxyl radicals (·OH), were responsible for the methylene blue degradation. This work provides valuable insights into the development of functionalized membranes that can induce MNB collapse to enhance pollutant removal in wastewater treatment.

The pre-coated photocatalytic ceramic membrane with Bi2O3–TiO2/powdered activated carbon nanoparticles to alleviate ultrafiltration fouling caused by intracellular organic matter of Microcystis aeruginosa

Algal intracellular organic matter (IOM), released during algae's normal growth phase and water treatment processes, has long been a source of significant membrane fouling, considerably limiting the effective use of ultrafiltration(UF) technology. In this study, a Bi-doped TiO2 composite with powdered activated carbon (Bi2O3–TiO2/PAC) was loaded on ceramic UF membranes by vacuum filtration, and the prepared photocatalytic modified ceramic membranes (MCMs) were investigated for the removal of IOM and the mitigation of membrane fouling at different loading levels and under three operating conditions: Dark, Dark-UV, and UV. The results showed that the photocatalytic MCMs were able to remove the protein and humic acid fluorescent components of IOM and decompose or mineralize organic matter with molecular weights of 10–50 kDa. Under the Dark, Dark-UV, and UV conditions, the final normalized flux of MCM-2.88 was significantly increased by 22.6 %, 82.1 %, and 108.0 %, respectively. The hydrophilicity of MCMs not only transformed the interactions between IOM and the membrane into repulsive forces, but also the composite material on the membrane surface transforms the membrane fouling mechanism from intermediate blocking model to cake layer formation. Furthermore, the application of photocatalytic oxidation under the Dark-UV and UV conditions yielded a reduction in both reversible and irreversible fouling. This action led to an increased hydrophilicity and elevated free energy of adhesion with IOM during the filtration process. Moreover, the main fouling mechanism changed into complete or intermediate blocking. These outcomes collectively underscore the capability of MCMs to alleviate membrane fouling induced by IOM. This advancement holds substantial promise for the advancement and widespread implementation of photocatalytic ceramic membrane filtration techniques.

Synthesis and application of modified membranes for efficient cationic and anionic dye elimination

This study focuses on developing ceramic ultrafiltration membranes using locally sourced kaolin clay and feldspar. An active layer composed of graphene and alkoxide is added to enhance the performance of these clay supports. This layer is thin, porous, and can vary in thickness. The support and filter layer's porous properties are characterized using SEM, DRX, ATG/ATD, XPS, and FTIR techniques. Filtration tests are conducted on modified porous tubular supports using cationic and anionic dye solutions. The filtration is performed tangentially at a pressure of 3.5 bar and a processing time of 120 minutes. The retention rates of the colored solutions are measured using a UV-visible spectrophotometer. The results show a 100% retention rate for supranol yellow and orange II at a concentration of 10-4 M, while for crystal violet and malachite green, the retention rates are 92.03% and 95.30%, respectively.

Investigating the impact of stormwater fouling on polysulfone ultrafiltration membranes modified with deep eutectic solvents

In this study, we evaluate the performance of modified polysulfone (PSF) ultrafiltration (UF) membranes, which incorporate deep eutectic solvents (DES), in treating stormwater laden with natural organic matter e.g. chemical oxygen demand (COD) and total suspended solids (TSS). We also aim to understand how these organic substances, e.g. COD, TSS, from the water source contribute to the fouling of the synthesized membranes. PSF membranes were synthesized using a non-solvent induced phase separation technique and integrated with varying concentrations of ChCl:FR (Choline Chloride: D-(−)-Fructose) 1:1 DES. The surface and porous structures of the membranes were characterized through Fourier-transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), contact angle measurements, scanning electron microscopy (SEM), and mechanical testing. The UF performance of these membranes was assessed and compared with different commercially available UF flat sheet membranes in terms of pure water permeability and antifouling behavior against collected stormwater from a sedimentation pond. Furthermore, the study evaluated the quality of the permeate based on parameters such as COD, turbidity, TSS, pH, and conductivity and compared the permeate quality of a pilot-scale ceramic UF membrane unit. The findings indicate that the inclusion of DES in the polysulfone membrane structure enhances the membranes' antifouling properties and permeability. This research offers valuable insights into the role of DES in the formation of polysulfone UF membranes and their potential for practical use e.g. sedimented stormwater.

Membrane fouling monitoring by 3ω sensing

Membrane fouling significantly reduces membrane permeability, leading to higher operational expenses. In situ monitoring of membrane fouling can potentially be used to reduce operation cost by optimizing operational parameters and cleaning conditions. In this study, a platinum wire with a diameter of 20 µm was attached to the surface of a ceramic ultrafiltration membrane, and by measuring the voltage across the wire while applying an AC current, the amplitude of the third harmonic wave, the so-called 3ω signal, was obtained. Results showed increasing 3ω signals during formation of fouling layers, which correlates directly to the hydraulic resistance of the formed fouling layer in semi-dead end filtration of polymeric core shell particles and crossflow filtration of diluted milk. This is explained by the insulating effect of the fouling layers which reduces heat convection by crossflow and the different thermal conductivity in the fouling layer compared with the feed. After membrane cleaning, the permeability and the magnitude of the 3ω signal were partly restored, showing that the 3ω method can be used to monitor the effect of cleaning. The frequency of the AC current was varied so it was possible to measure the heat conductivity in the fouling layer (high frequency) and heat convection due to cross-flow (low frequency). This may potentially be used to get information of the type of fouling (heat conductivity) and thickness of the fouling layer (AC frequency where heat conductivity becomes dominating).

Hybrid ceramic nanofiltration membranes prepared by impregnation and solid-state grafting of organo-phosphonic acids

Grafting of oligomers into the mesopores of ceramic membranes (e.g. γ-alumina) represents an attractive strategy for deploying durable, resistant, and efficient hybrid membranes for the industrial treatment of wastewater containing organic solvents and divers solutes. In general, to control the membrane/solute/solvent interactions, homogeneous surface modification and high grafting yields are required. However, reaching this goal is rather challenging with the current grafting approaches where the active diffusion of the oligomers to the membrane pore surface may be hampered. To mitigate such diffusion limitation, we present here a novel synthesis method consisting of an infiltration of concentrated PEG organophosphonic acid oligomer solution in ceramic ultrafiltration membranes before initiating the solid-state grafting reaction to form a PEG-brush-grafted nanofiltration ceramic membrane. The infiltration step was found to be decisive for the formation of weak bonds between the oligomer linking functions and the ceramic membrane ensuring the final synthesis of robust PEG-based membranes by a grafting reaction. The influence of the solvent polarity (water vs. cyclohexane) on the conformation of the grafted PEG brush inside the γ-alumina mesopores was observed as a key parameter in the analysis of the pore size and permeability results. In addition to this, it was studied here for the first time by molecular dynamic simulations. The as-prepared PEG-brush/ceramic membranes were found very efficient in the separation of small organic dyes such as Rhodamine B (479 g mol−1) up to 85%. As a result, the present synthesis method represents a sustainable and simple alternative way for the preparation of hybrid nanofiltration membranes with controlled surface properties and attractive separation performance. Moreover, the vacuum impregnation and solid-state grafting approach requires a minimal amount of functional oligomers and solvent (0.5 mmol for 2–3 mL of solvent) and can be applied to other porous materials used in separation technologies.