Optimization of photocatalytic ceramic membrane filtration by response surface methodology: Effects of hydrodynamic conditions on organic fouling and removal efficiency

The effects of the operating conditions, including the applied pressure, feed organic concentration, and recirculation flowrate along the TiO2-coated ceramic membrane, on the normalized membrane permeability and organic removal efficiency were systematically investigated by operating a photocatalytic membrane reactor (PMR). Response surface methodology (RSM) was conducted to better understand the interactive effect of operational conditions as well as their individual and combined effects to control membrane performance. Our results showed that the applied pressure and feed organic concentration, as single parameter, affected the normalized membrane permeability and organic removal efficiency more dominantly than the recirculation flowrate. The polynomial performance equations generated by RSM successfully predicted the membrane performance of the PMR. The responses to the normalized membrane permeability and organic removal efficiency with respect to the operational conditions were less sensitive to any combination of operational conditions than to their individual impacts. The combined effects of the operating conditions were less pronounced in promoting the catalytic performance of organic contaminants on the TiO2 surface. Our RSM analysis based on experimental observations designed by Box-Behnken Design (BBD) suggested that 1.3 bar of applied pressure, 44 mg/L of feed organic dye concentration and 0.8 L/min as recirculation flowrate as optimum conditions achieved more than 98% of organic removal efficiency and less than 5% of decline in normalized membrane permeability. This research shows that the RSM provides effective tool to optimize operational conditions to determine fouling rate and organic removal in PMR.

Preparation of mixed matrix self-cleaning membrane incorporated by Z-scheme heterostructure via robust engineering in terms of dimension for decreasing cake fouling in a cross-flow reactor

Reducing irreversible fouling in polymer membranes by integrating photocatalytic and membrane processes as the self-cleaning photocatalytic membrane is a promising candidate for improving membrane filtration performance. In this study, mixed matrix photocatalytic membranes were prepared from the combination of different morphologies ZnO-g-C3N4 heterostructure in the polymer matrix by the phase-separation method. To investigate the self-cleaning and performance properties of mixed matrix photocatalytic membranes prepared from different morphologies heterostructures, the photocatalytic membrane reactor with a visible-light source was applied. Nanoflower/nanosheet (NF/NS) ZnO-g-C3N4 photocatalytic membrane showed good self-cleaning performance owing to the high photocatalytic performance of NF/NS ZnO-g-C3N4 heterostructure by the reduction of irreversible membrane fouling, thus improving the antifouling and filtration performance of the membrane. Also, the morphology and the uniform distribution of the NF/NS ZnO-g-C3N4 heterostructure in the membrane matrix caused good hydrophilic properties, high porosity, and a more symmetrical structure in the (NF/NS) ZnO-g-C3N4 photocatalytic membrane (F4). For the F4 membrane, the permeability and rejection values increased from 40.35 L m-2 h-1 and 90.9% in the dark environment to 84.37 L m-2 h-1 and 97.4% under visible-light for dye pollutants. Accordingly, F4 had the best filtration and self-cleaning performance, which can be used as a promising visible-light photocatalytic membrane in wastewater treatment processes.

Performance evaluation of PdO/ CuO TiO2 photocatalytic membrane on ceramic support for removing pharmaceutical compounds from water

This study investigated photocatalytic degradation of pharmaceutical compound using CuO or PdO-TiO2 membrane. The synthesized membranes were characterized by some techniques including X-ray powder diffraction (XRD), dynamic light scattering (DLS), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FT-IR). The structural properties confirmed that the photocatalytic membranes were successfully prepared on ceramic supports. The PdO-TiO2 and CuO-TiO2 membranes were employed as photocatalytic membranes to degrade metronidazole (MNZ) and diphenhydramine (DPH) in aqueous solutions, respectively. Some parameters affecting the photocatalytic reaction such as pH, initial concentration, and light source were also investigated. The maximum degradation for both pharmaceutical compounds was obtained at basic pH (pH = 10), low initial concentration (C0 = 10 ppm) under UV irradiation. At high transmembrane pressure (ΔP = 3 bar), the flow rate across the membrane increased up 0.0078 and 0.0082 cc/s.cm2 for CuO-TiO2 and PdO-TiO2 photocatalytic membrane respectively while not affected on degradation efficiency (DE). At the same condition operation (C0 = 10 ppm, pH = 10, ΔP = 2 bar under UV irradiation), the MNZ and DPH degradation of the PdO-TiO2 membrane was 94 and 95% respectively that relatively higher than the CuO-TiO2 membrane. It is probably due to the lower energy band gap of PdO-TiO2 (2.5 eV) than CuO-TiO2 (2.7 eV). The membrane stability tests confirmed the high performance of the prepared membranes.

UV-LED silicon carbide composite photocatalytic membrane reactor for the degradation of organic contaminants

The utilization of membranes in the water industry has been growing rapidly; however, the technology still experiences problems with fouling. A potential solution is to immobilize photocatalyst particles on the surface of the membranes to encourage in situ degradation of the organic contaminants contributing to the fouling. In this study, we developed a photocatalytic membrane (PM) by coating a silicon carbide membrane with Zr/TiO₂ sol. The performance of the PM in degrading different concentrations of humic acid was evaluated comparatively under UV irradiation of two wavelengths, 275 and 365 nm. The results indicated that (i) the PM achieved high levels of humic acid degradation, (ii) the photocatalytic activity of the PM reduced the formation of fouling and hence the loss of permeability, (iii) the formation of fouling was reversible; no trace of fouling was observed after cleaning, and (iv) the PM showed high durability during multiple rounds of operation.

Ultrathin g-C3N4 composite Bi2WO6 embedded in PVDF UF membrane with enhanced permeability, anti-fouling performance and durability for efficient removal of atrazine

A novel Bi₂WO₆-g-C₃N₄/polyvinylidene fluoride (PVDF) composite ultrafiltration (UF) membrane (BWO-CN/PVDF) was prepared by microwave hydrothermal and immersion precipitation phase transformation method. The BWO-CN/PVDF-0.10 exhibited an outstanding photocatalytic removal rate of atrazine (ATZ) (97.65 %) under the simulated sunlight and enhanced permeate flux (1356.09 L·m^-2·h^-1). The multiple optical and electrochemical detection confirmed that combining ultrathin g-C₃N₄ and Bi₂WO₆ can increase carrier separation rate and prolong its lifetime. The quenching test revealed that h⁺ and ¹O₂ were the prominent reactive species. Additionally, after a 10-cycle photocatalytic process, the BWO-CN/PVDF membrane presented remarkable reusability and durability. And it showed excellent anti-fouling performance by filtering BSA, HA, SA, and Songhua River under simulated solar irradiation. The molecular dynamic (MD) simulation showed that the combination of g-C₃N₄ and Bi₂WO₆ can enhance the interaction between BWO-CN and PVDF. This work opens up a new idea for designing and constructing a highly efficient photocatalytic membrane for water treatment.

Visible-light-driven photocatalytic dual-layer hollow fibre membrane ameliorates the changes of bisphenol A exposure in gastrointestinal tract

Various treatments of choice are available to overcome contamination of bisphenol A (BPA) in the environment including membrane technologies; however, the treatment still releases contaminants that threaten the human being. Therefore, the present study is conducted to investigate the degradation of BPA by recently developed visible-light-driven photocatalytic nitrogen-doping titanium dioxide (N-doped TiO₂) dual-layer hollow fibre (DLHF) membrane and its efficiency in reducing the level of BPA in contaminated water. Fabricated with suitable polymer/photocatalyst (15/7.5 wt.%) via co-extrusion spinning method, the DLHF was characterized morphologically, evaluated for BPA degradation by using submerged photocatalytic membrane reactor under visible light irradiations followed by the investigation of intermediates formed. BPA exposure effects were accessed by immunohistochemistry staining of gastrointestinal sample obtained from animal model. BPA has been successfully degraded up to 72.5% with 2 intermediate products, B1 and B2, being identified followed by total degradation of BPA. BPA exposure leads to the high-intensity IHC staining of Claudin family which indicated the disruption of small intestinal barrier (SIB) integrity. Low IHC staining intensity of Claudin family in treated BPA group demonstrated that reducing the level of BPA by N-doped TiO₂ DLHF is capable of protecting the important component of SIB. Altogether, the fabricated photocatalytic DLHF membrane is expected to have an outstanding potential in removing BPA and its health effect for household water treatment to fulfil the public focus on the safety of their household water and their need to consume clean water.

Comprehensive review on advanced reusability of g-C3N4 based photocatalysts for the removal of organic pollutants

Graphitic carbon nitride (g-C₃N₄) has attained significant research attention in energy and environmental remediation due to its excellent electronic structure, greater physical and chemical properties, and abundance. However, graphitic carbon nitride faces severe problems because of its high recombination rate and higher mass loss of the catalyst during recovery operations. This review emphasizes the methods to overcome the difficulties associated with recovery and reusability of the g-C₃N₄ based photocatalyst towards the redemption of pollutants present in wastewater. Different strategies like magnetic g-C₃N₄ based photocatalysts, immobilized photocatalytic systems, and photocatalytic membranes and their usage in photocatalytic applications are well described. Different preparation strategies of the graphic carbon nitride-based composites are elucidated. The key challenges and future perspectives of adopting these methods for photocatalytic applications are also mentioned.

Photocatalytic membranes: a new perspective for persistent organic pollutants removal

The presence of conventional and emerging pollutants infiltrating into our water bodies is a course of concern as they have seriously threatened water security. Established techniques such as photocatalysis and membrane technology have proven to be promising in removing various persistent organic pollutants (POP) from wastewaters. The emergence of hybrid photocatalytic membrane which incorporates both photocatalysis and membrane technology has shown greater potential in treating POP laden wastewater based on their synergistic effects. This article provides an in-depth review on the roles of both photocatalysis and membrane technology in hybrid photocatalytic membranes for the treatment of POP containing wastewaters. A concise introduction on POP's in terms of examples, their origins and their effect on a multitude of organisms are critically reviewed. The fundamentals of photocatalytic mechanism, current directions in photocatalyst design and their employment to treat POP's are also discussed. Finally, the challenges and future direction in this field are presented.

Surface-constructing of visible-light Bi2WO6/CeO2 nanophotocatalyst grafted PVDF membrane for degradation of tetracycline and humic acid

The synthesis of Bi₂WO₆ and CeO₂ photocatalytic nanomaterials exhibit a great ability to photodegrade the antibiotics and shown excellent oxidation of various organic pollutants. Heterostructure 1:1 & 2:1 Bi₂WO₆/CeO₂ nanocomposite was successfully synthesized via the facile sono-dispersion method and exquisite photocatalytic activity. The 0.5 wt% of nanocomposites were well-grafted on PVDF membrane surface via an in-situ polymerization method using polyacrylic acid. The fourier transform infrared (FTIR) spectra demonstrated that the network formation in PVDF induced by the -COOH functional group in acrylic acid. The grafted membrane morphology and strong binding ability over the membranes were validated by scanning electron microscope with energy dispersion (SEM-EDS) and X-ray photoelectron spectroscopy (XPS), respectively. The permeate flux of 49.2 L.m^-2 h^-1 and 41.65 L.m^-2 h were observed for tetracycline and the humic acid solution respectively for 1 wt% of PVP and 0.5 wt% of photocatalytic nanomaterials in PVDF membrane. The tetracycline and humic acid photodegradation rate of 82% and 78% and total resistance of 1.43 × 10¹⁰ m^-1 and 1.64 × 10¹⁰ m^-1, 83.5% and 77% flux recovery ratio were observed with N5 membrane. The 2:1 Bi₂WO₆/CeO₂ nanocomposite grafted membrane showed a high permeate flux and better photodegradation ability of organic pollutants in the wastewater.

Reduced graphene oxide/TiO2(B) immobilized on nylon membrane with enhanced photocatalytic performance

Taking advantage of the unique properties of reduced graphene oxide (rGO) and monoclinic crystalline titanium dioxide (TiO₂(B)) nanomaterials, a novel rGO-TiO₂(B) composite membrane (MrGO-TiO₂(B)) was constructed by UV-light-assisted self-assembly of rGO and TiO₂ on a nylon membrane. The structure of MrGO-TiO₂(B) was characterized by scanning electron microscopy, transmission electron microscopy, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction analysis. Through 2D/2D self-assembly, rGO and TiO₂(B) were more tightly combined, and then MrGO-TiO₂(B) exhibited outstanding photocatalytic activity and an excellent methylene blue (MB) removal rate. MB was completely removed in 60 min at a constant rate of 0.042 min^-1 by the MrGO-TiO₂(B)/H₂O₂/MB system upon solar simulating Xe lamp irradiation. The synergistic effect of rGO and TiO₂(B) facilitated the photocatalytic degradation of MB. TiO₂(B) was excited and generated electrons and holes upon irradiation. Some electrons migrated to the surface of TiO₂(B) to react with H₂O₂ to produce hydroxyl radicals (OH), while the other electrons migrated to the surface of rGO to react with H₂O₂, producing OH. In addition, a number of superoxide radicals (O₂^-) was detected. The holes in the valence band of TiO₂(B) directly oxidized MB. The catalytic activity of MrGO-TiO₂(B) toward MB degradation remained stable after four rounds of reuse. Therefore, the surface modification of a nylon membrane with TiO₂(B) and rGO can serve as a promising route to fabricate photocatalytic membranes for use in the water treatment industry.

Fabrication of self-assembled 0D-2D Bi2MoO6-g-C3N4 photocatalytic composite membrane based on PDA intermediate coating with visible light self-cleaning performance

A Self-cleaning surface can efficaciously solve the problem of irreversible contamination buildup on filtration membranes. Photocatalytic membranes were fabricated via vacuum assisted layer-by-layer (LBL) self-assembly of 0D-2D Bi₂MoO₆-g-C₃N₄ on a PDA coated thin-film composite PVDF substrate by Schiff base reaction. The rejection rate of the simulated polysaccharide was more than 90%, and that of the simulated protein was more than 80%. The combination of the membrane and the photocatalyst promoted the degradation of tetracycline hydrochloride by the composite membrane to 67.85% when original membranes had minor effect. Under visible light, reversible radiation pollutants (R_r) gradually replaced irreversible pollutants (R_ir) as the main pollutants. The flux recovery ratio (FRR) of 0D-2D Bi₂MoO₆-g-C₃N₄/PVDF membrane was 85% after being irradiated with visible light for 30 min. The flux recovery rate of contaminated photocatalytic membrane remained 75%, and the rejection was maintained in a stable range after four cycles of the cleaning operation under visible light. The results indicated that the excellent photocatalytic performance of 0D-2D Bi₂MoO₆-g-C₃N₄ photocatalysis material and the increase of multi-dimensional functional layer morphology on pollutant contact area improved the mechanical stability, interception performance and self-cleaning performance of the composite membrane. This work not only builds a new type of composite coating membranes, but also help us to further understand the relationship between the dimensions of photocatalytic materials and the improvement of photocatalytic membrane performance.

Photocatalytic study of nanocomposite membrane modified by CeF3 catalyst for pharmaceutical wastewater treatment

Cerium fluoride (CeF₃) nanoparticles (NPs) were synthesized and applied in polysulfone (PS) membrane fabricated by phase inversion method. The produced nanocomposite membranes (PS/CeF₃) with different contents of CeF3 NPS (0.25%, 0.5%, 0.75% and 1% w/w) were used to treat pharmaceutical wastewaters. The membranes were characterized by FESEM, EDX, XRD, FTIR, porosity, and water contact angle analyses. Evaluation of the characteristics and performance of the nanocomposite membranes confirmed that utilizing photocatalytic CeF₃ NPs in membrane structure could effectively decompose organic contaminants in pharmaceutical wastewaters. It also improves the hydrophilicity and antifouling ability of membrane during filtration especially, in the presence of UV irradiation. The permeate flux of the PS membrane increased from 35.1 to 63.77 l/m²h by embedding 0.75% of CeF₃ NPs in membrane structure due to the porosity enhancement from 71.36-78.42% and the decrease in contact angle from 62.9º to 53.73º. Moreover, the flux decline of PS/CeF₃-0.75% membrane under UV irradiation was from 63.6 to 46.1 l/m²h that considerably lower than that of the neat PS membrane (from 34.7 to 4.9). On the other hand, the degradation efficiency of PS/CeF₃-0.75% membrane was more than 97%, and COD removed was more than 65% while they were 75% and 31%, respectively for the nascent PS membrane. Therefore, applying the appropriate amount of CeF₃ NPs in PS membranes not only greatly increased the permeate flux but also significantly enhanced the degradation efficiency and COD removal. This indicates that nanocomposite membranes can be confidently applied for pharmaceutical wastewater treatment UV irradiation.

A Sm-doped Egeria-densa-like ZnO nanowires@PVDF nanofiber membrane for high-efficiency water clean

A biomimetic Egeria-densa-like hybrid composite nanofiber membrane was fabricated to degrade organic pollutants in water, with PVDF nanofibers as stems to provide support, and ZnO nanowires as leaves to provide active sites. The Sm-doped ZnO nanowires@PVDF nanofiber membranes were characterized by FE-SEM, X-ray photoelectron spectroscopy, Fourier transform infrared, X-ray diffraction, and UV-vis diffuse reflectance spectrometer. Compared with the pure ZnO nanowires@PVDF nanofiber membrane, the Sm-doped membrane showed higher photocatalytic performance. The excellent photocatalytic activity was attributed to the increased specific surface area and the decreased bandgap of ZnO nanowires after Sm doping, which inhibited the recombination rate of electrons and holes and improved the absorption of visible light. We found that the superoxide free radicals (O₂^-) played a critical role in photocatalytic degradation. The Sm-doped ZnO nanowires@PVDF nanofiber membrane exhibited good stability after 5 cycles of RhB degradation. We believe such Sm-doped hybrid membrane can work as an effective photocatalyst for wastewater treatment.

Stable LBL self-assembly coating porous membrane with 3D heterostructure for enhanced water treatment under visible light irradiation

The development of visible light-responsive photocatalytic membranes (vis-PMs) has opened a promising direction in water purification field. Herein, supramolecular aggregates from cyanuric acid (C), melamine (M), and urea (U) in dimethyl sulfoxide (DMSO) were used to prepare the porous carbon nitride nanosheet (MCU-C₃N₄) with excellent photocatalytic performance. A sort of 3D heterostructure PMs consisting of MCU-C₃N₄ and carbon nanotube (CNTs) interposed into graphene oxide (GO) on the PVDF membrane was firstly fabricated by the layer-by-layer (LbL) assembly method, in which CNTs/MCU-C₃N₄/GO material was immobilized on the polyelectrolytes (PE) modified PVDF based on their electrostatic attractions. Such PMs with abundant nano-channels had excellent mechanical strength, satisfactory water permeability (14.35 L m^-2 h^-1 bar^-1) and synergetic removal efficiency of rhodamine B (RhB, 98.31%) in long -term operation, relative to the pristine GO membrane and MCU-C₃N₄/GO membrane fabricated by the same method. In addition, such PMs also exhibited the satisfactory tetracycline hydrochloride (TC) removal rate (84.81%) under visible light irradiation. Construction and performance of such carbon-based PMs might provide guidance for development of vis-PMs in terms of bonding strength, multidimensional morphology and water purification application.

An overview of photocatalytic degradation: photocatalysts, mechanisms, and development of photocatalytic membrane

Photocatalysis is an ecofriendly technique that emerged as a promising alternative for the degradation of many organic pollutants. The weaknesses of the present photocatalytic system which limit their industrial applications include low-usage of visible light, fast charge recombination, and low migration ability of the photo-generated electrons and holes. Therefore, various elements such as noble metals and transition metals as well as non-metals and metalloids (i.e., graphene, carbon nanotube, and carbon quantum dots) are doped into the photocatalyst as co-catalysts to enhance the photodegradation performance. The incorporation of the co-catalyst which alters the photocatalytic mechanism was discussed in detail. The application of photocatalysts in treating persistent organic pollutants such as pesticide, pharmaceutical compounds, oil and grease and textile in real wastewater was also discussed. Besides, a few photocatalytic reactors in pilot scale had been designed for the effort of commercializing the system. In addition, hybrid photocatalytic system integrating with membrane filtration together with their membrane fabrication methods had also been reviewed. This review outlined various types of heterogeneous photocatalysts, mechanism, synthesis methods of biomass supported photocatalyst, photocatalytic degradation of organic substances in real wastewater, and photocatalytic reactor designs and their operating parameters as well as the latest development of photocatalyst incorporated membrane.

Photocatalytic membrane combined with biodegradation for toluene oxidation

Photocatalytic membrane coupled to biodegradation offers potential for degrading volatile organic compounds (VOCs) in photocatalytic membrane biofilm reactor. An intimately coupled photocatalysis and biodegradation reactor was operated in continuous operation for 500 days to treat simulated waste gas containing toluene. Toluene removal efficiency obtained 99%, with the elimination capacity of 550 g m^-3·h^-1. Membrane photocatalysis coupled to biodegradation was created to improve toluene removal from 11 to 20%. The dominant genera were Lysinibacillus, Hydrogenophaga, Pseudomonas at 30 d, Rudaea, Dongia, Litorilinea at 230 d xyl, Tod, Tcb, Bed, Tmo, Tbu, Tou, Dmp, Cat were functional genes of toluene metabolism, as shown by16S rDNA and metagenomic sequencing. Photocatalysis destroyed part of the toluene into biodegradable intermediates that were immediately mineralized by microorganisms in biofilm, some toluene was directly degraded by toluene degrading bacterial community into carbon dioxide and water. The novel hybrid photocatalytic membrane biofilm reactor is a cost-effective and robust alternative to VOCs treatment.

Evaluation of self-cleaning performance of the modified g-C3N4 and GO based PVDF membrane toward oil-in-water separation under visible-light

Photocatalytic membranes (PMs), coupling of membrane filtration and photocatalysis, have exhibited the potential for application in the wastewater treatment. In this study, we firstly adopted the supramolecular aggregates of melamine (M), cyanuric acid (C), and urea (U) in specific dimethyl sulfoxide (DMSO) as precursors to prepare carbon nitride MCU-C₃N₄ with high photocatalytic performance, and a kind of novel-designed photocatalytic membrane was prepared via filtrating the mixture of graphene oxide (GO) nanosheets and MCU-C₃N₄ on PVDF membrane supports, and then crosslinked using glutaraldehyde (GA) to construct a steady coating on the GO/MCU-C₃N₄/PVDF membrane. GO/MCU-C₃N₄/PVDF composite membrane exhibited higher permeation flux than that of GO/PVDF membrane and exhibited excellent separation performance for oil-in-water emulsion. A visible light-driven self-cleaning four-stage filtration by a self-built dead-end filtration system was carried out to evaluate membrane antifouling property, and GO/MCU-C₃N₄/PVDF membrane (M2) possessed higher flux recovery ratio (FRR) (∼92.36%) and lower irreversible fouling resistance (R_ir) ratio (∼8%) under 30min visible-light irradiation, maintaining relatively higher FRR (>72%) during 4 cycling of four-stage filtrating experiments. GO/MCU-C₃N₄/PVDF PMs are equipped with high permeation flux, separation performance, anti-fouling property and stability, indicating potential application in water treatment.