Streaming potential for identification of foulants adsorption on PVDF membrane surface

Synchronously improved permeability, selectivity and fouling resistance of Fe-N-C functionalized ceramic catalytic membrane for effective water treatment: The critical role of Fe

Constructing catalytic membrane simultaneously displaying high permeability, selectivity and antifouling performance in water treatment remains challenging. Herein, the surface and pore channels of the ceramic membrane were co-functionalized with nitrogen doped carbon supported Fe catalyst (CN-F), and the Fe content was varied to investigate its effect on performance of CN-F coupled with peroxymonosulfate (PMS) activation (CN-F/PMS) for water treatment. Results confirmed the introduced Fe (in Fe-N coordination form) greatly enhanced the permeability, selectivity and fouling resistance of CN-F. Optimal CN-F3/PMS achieved 96.5% removal and 52.1% mineralization of sulfamethoxazole in short retention duration (2.7 min), whose performance was 5.4 and 6.7 times higher than that of nitrogen doped carbon functionalized ceramic catalytic membrane (CN/PMS) and CN-F3 filtration alone, respectively. CN-F3/PMS also efficiently inhibited fouling on both surface and pores with 2.8 and 2.4 times lower flux loss than that of CN/PMS and CN-F3 filtration alone, respectively. Moreover, CN-F3/PMS displayed superior performance in long-term treatment of real coking wastewater. The outstanding performance of CN-F was mainly attributed to the dual role of supported Fe, which served as hydrophilic site for enhanced water permeation and major active site for PMS adsorption and reduction into reactive species (mainly high-valent Fe(IV)=O species) towards pollutant elimination.

Preparation of Ni@UiO-66 incorporated polyethersulfone (PES) membrane by magnetic field assisted strategy to improve permeability and photocatalytic self-cleaning ability

Metal-organic frameworks (MOFs) have been considered as promising nanofillers to fabricate mixed matrix membranes for water treatment. However, manipulating distribution of MOFs nanoparticles in the membrane matrix remains a great challenge. In this study, UiO-66 was firstly coated by magnetic Ni via an in-situ reduction reaction, and then incorporated into polyethersulfone (PES) membrane matrix to prepare PES-Ni@UiO-66 membrane. The magnetic Ni allowed to manipulate the distribution of magnetic Ni@UiO-66 in the phase-inversion process by an external magnetic field. The hydrophilic Ni@UiO-66 can be pulled onto membrane surface by the magnetic force, endowing the prepared membrane with rather higher hydrophilicity. The prepared membrane exhibited superior water permeability with a pure water flux of 611.5 ± 19.8 L·m^-2·h^-1 and improved antifouling performance. Moreover, benifiting from photocatalytic activity of the exposed Ni@UiO-66 on membrane surface, the obtained PES-Ni@UiO-66 membrane demonstrated excellent photocatalytic self-cleaning ability with a flux recovery rate (FRR) higher than 95% under UV irradiation. Analyzing by extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory indicated that the improved antifouling performance could be attributed to less attractive or even repulsive interaction between the prepared membrane and pollutants. This work provided valuable guidance for structural regulation and development of high-performance MOFs-based membranes for water treatment.

Influence of Pore-Size/Porosity on Ion Transport and Static BSA Fouling for TiO2-Covered Nanoporous Alumina Membranes

The influence of geometrical parameters (pore radii and porosity) on ion transport through two almost ideal nanoporous alumina membranes (NPAMs) coated with a thin TiO2 layer by the atomic layer deposition technique (Sf-NPAM/TiO2 and Ox-NPAM/TiO2 samples) was analyzed by membrane potential and electrochemical impedance spectroscopy measurements. The results showed the significant effect of pore radii (10 nm for Sf-NPAM/TiO2 and 13 nm for Ox-NPAM/TiO2) when compared with porosity (9% and 6%, respectively). Both electrochemical techniques were also used for estimation of protein (bovine serum albumin or BSA) static fouling, and the results seem to indicate deposition of a BSA layer on the Sf-NPAM/TiO2 fouled membrane surface but pore-wall deposition in the case of the fouled Ox-NPAM/TiO2 sample. Moreover, a typical and simple optical technique such as light transmission/reflection (wavelength ranging between 0 and 2000 nm) was also used for membrane analysis, showing only slight transmittance differences in the visible region when both clean membranes were compared. However, a rather significant transmittance reduction (~18%) was observed for the fouled Sf-NPAM/TiO2 sample compared to the fouled Ox-NPAM/TiO2 sample, and was associated with BSA deposition on the membrane surface, thus supporting the electrochemical analysis results.

Modification of porous polyetherimide hollow fiber membrane by dip-coating of Zonyl® BA for membrane distillation of dyeing wastewater

Wetting and fouling have significantly affected the application of membrane distillation (MD). In this work, a dip-coating method was used for improving surface hydrophobicity of the polyetherimide (PEI) hollow fiber membrane. An air gap membrane distillation (AGMD) process was applied for treatment of the methylene blue (MB) solution. The porous PEI membrane was fabricated by a dry-wet spinning process and the hydrophobic 2-(Perfluoroalkyl) ethanol (Zonyl^® BA) was used as the coating material. From FESEM, the modified PEI-Zonyl membrane showed an open structure with large finger-like cavities. The modified membrane displayed a narrow pore size distribution with mean pore size of 0.028 μm. The outer surface contact angle of the PEI-Zonyl membrane increased from 81.3° to 100.4° due to the formation of an ultra-thin coated layer. The pure water flux of the PEI-Zonyl membrane was slightly reduced compared to the pristine PEI membrane. A permeate flux of 6.5 kg/m² h and MB rejection of 98% were found for the PEI-Zonyl membrane during 76 h of the AGMD operation. Adsorption of MB on the membrane surface was confirmed based on the Langmuir isotherm evaluation, AFM and FESEM analysis. The modified PEI-Zonyl membrane can be a favorable alternative for AGMD of dyeing wastewaters.

One-Step and Low-Cost Designing of Two-Layered Active-Layer Superhydrophobic Silicalite-1/PDMS Membrane for Simultaneously Achieving Superior Bioethanol Pervaporation and Fouling/Biofouling Resistance

Recently, the coupling of biofuel fermentation broths and pervaporation has been receiving increasing attention. Some challenges, such as the destructive effects of constituents of the real fermentation broth on the membrane performances, the lethal effects of the membrane surface chemical modifiers on the microorganisms, and being expensive, are against this concept. For the first time, a continuous study on the one-step and low-cost preparation of superhydrophobic membranes for bioethanol separation is made to address these challenges. In our previous work, spraying as a fast, scalable, and low-cost procedure was applied to fabricate the one-layered active-layer hydrophobic (OALH) silicalite-1/polydimethylsiloxane (PDMS) membrane on the low-cost mullite support. In this work, the spraying method was adopted to fabricate a two-layered active-layer superhydrophobic (TALS) silicalite-1/PDMS membrane, where the novel active layer consisted of two layers with different hydrophobicities and densities. Contact-angle measurements, surface charge determination, scanning electron microscopy, atomic force microscopy, and pervaporation separation using a 5 wt % ethanol solution were used to statically evaluate the fouling/biofouling resistance and pervaporation performances of OALH and TALS membranes in this study. The TALS membrane presented a better resistance and performance. For dynamic experiments, the Box-Behnken design was used to identify the effects of substrates, microorganisms, and nutrient contents as the leading indicators of fermentation broth on the TALS membrane performances for the long-term utilization. The maximum performances of 1.88 kg/m²·h, 32.34, and 59.04 kg/m²·h concerning the permeation flux, separation factor, and pervaporation separation index were obtained, respectively. The dynamic fouling/biofouling resistance of the TALS membrane was also characterized using energy-dispersive X-ray spectroscopy of all the tested membranes. The TALS membrane demonstrated the synergistic resistance of membrane fouling and biofouling. Eventually, the novel TALS membrane was found to have potential for biofuel recovery, especially bioethanol.

Blood Oxygenation Using Fluoropolymer-Based Artificial Lung Membranes

Artificial lung (AL) membranes are used for blood oxygenation for patients undergoing open-heart surgery or acute lung failures. Current AL technology employs polypropylene and polymethylpentene membranes. Although effective, these membranes suffer from low biocompatibility, leading to undesired blood coagulation and hemolysis over a long term. In this work, we propose a new generation of AL membranes based on amphiphobic fluoropolymers. We employed poly(vinylidene-co-hexafluoropropylene), or PVDF-co-HFP, to fabricate macrovoid-free membranes with an optimal pore size range of 30-50 nm. The phase inversion behavior of PVDF-co-HFP was investigated in detail for structural optimization. To improve the wetting stability of the membranes, the fabricated membranes were coated using Hyflon AD60X, a type of fluoropolymer with an extremely low surface energy. Hyflon-coated materials displayed very low protein adsorption and a high contact angle for both water and blood. In the hydrophobic spectrum, the data showed an inverse relationship between the surface free energy and protein adsorption, suggesting an appropriate direction with respect to biocompatibility for AL research. The blood oxygenation performance was assessed using animal sheep blood, and the fabricated fluoropolymer membranes showed competitive performance to that of commercial polyolefin membranes without any detectable hemolysis. The data also confirmed that the bottleneck in the blood oxygenation performance was not the membrane permeance but rather the rate of mass transfer in the blood phase, highlighting the importance of efficient module design.

Pre-ozonation for the mitigation of reverse osmosis (RO) membrane fouling by biopolymer: The roles of Ca2+ and Mg2

Despite plenty of literatures focused on the application of pre-ozonation prior to membrane, it was still unclear about the role of divalent cations (Ca²⁺ and Mg²⁺) in reverse osmosis (RO) membrane fouling mitigation. In this study, ozone pre-treatment (0.10, 0.25 and 0.50 mg O₃/mg DOC (dissolved organic carbon)) was employed to oxidize model biopolymer, which was represented by bovine serum albumin (BSA) and sodium alginate (SA) in the presence of Ca²⁺ and Mg²⁺ (0.5, 1.0 and 2.0 mM). Cross-flow filtration was conducted to investigate RO membrane fouling by concentration mode. The results showed that at appropriate ozone dose there were measurable changes in physicochemical properties of BSA and SA, including increases in particle size, hydrophilicity, density of negative charge and carboxylic groups. Pre-ozonation markedly alleviated RO fouling by BSA at ozone dose of 0.25 mg O₃/mg DOC when Ca²⁺ and Mg²⁺ concentrations raised from 0.5 to 2.0 mM since the increase in electrostatic (EL) repulsion and decrease in hydrophobic (HP) interaction compensated the increase in divalent cation bridging. Similar results were obtained for SA fouling in the presence of Mg²⁺. In contrast, the effect of pre-ozonation on SA fouling strongly depended on the concentration of Ca²⁺. In brief, it mitigated SA fouling at 0.5 mM Ca²⁺, whereas accelerated irreversible fouling at higher Ca²⁺ concentration (1.0 and 2.0 mM) due to the overwhelming effect of divalent cation bridging compared to EL and HP interactions, as revealed by adsorption experiments (in-situ streaming potential measurement). Pre-ozonation shifted the fouling layer from compact to porous and weakened the adhesion forces between foulants and membrane (foulants) except for SA containing 1.0 and 2.0 mM Ca²⁺. This study may provide the guidance for the application of pre-ozonation prior to RO filtration.

Poly (3, 4-ethylenedioxythiophene) modified polyvinylidene fluoride membrane for visible photoelectrocatalysis and filtration

A polyvinylidene fluoride (PVDF) organic conductive membrane with photoelectric activity was successfully developed via printing of oxidant (FeCl₃·6H₂O) layer by layer and then chemical vapor deposition (CVD) of Poly (3, 4-ethylenedioxythiophene) (PEDOT). Four kinds of membranes were prepared by changing the number of oxidant coatings layers. The structure and photoelectric properties of four membranes were well characterized. The photocurrent density of 3.7 × 10^-4 A indicated that the four-oxidant coating layers membrane achieved the best performance in photo-electricity activity. A comprehensive study of degradation efficiency under different photoelectric conditions was carried out. Results showed that the photoelectrocatalytic removal of tetracycline hydrochloride was 1.6 and 7.9 times higher than that of photocatalysis and photolysis, respectively, under a voltage of 3 V assisted with visible light irradiation. The anti-interference and stability tests in continuous filtration process demonstrated that the dissolved organic matters (DOMs) can result in a 30% fluctuation on removal rate. The streaming potential tests of DOMs adsorption on membrane surface indicated that the more obvious the adsorption phenomenon was, the degradation of tetracycline hydrochloride was weaker. The degradation intermediates were identified and pathways were proposed in this work. The photoelectrocatalysis of PEDOT modified PVDF membrane provided a new potential for water purification.