A novel paradigm of photocatalytic cleaning for membrane fouling removal

Critical review on emerging photocatalytic membranes for pollutant removal: From preparation to application

Due to synergistically enhanced separation and degradation performances, photocatalytic membranes offer an environmentally friendly and energy-sustainable method for water purification. However, a comprehensive review on preparation and application of photocatalytic membranes is still lacking. Systematically comparing different photocatalytic membrane fabrication methods and revealing the underlying mechanisms of their respective applications are of particular interest. In this review, we first discuss the common preparation methods for photocatalytic membranes in detail, focusing on the main approaches to improve their photocatalytic performance. We elucidate the mechanisms of photocatalytic membrane-based degradation processes, and describe some representative applications of photocatalytic membranes in water treatment. At the same time, the influencing factors that are critical for achieving high removal efficiency are also proposed. In the end, the practical applications and the perspectives for future studies and implementation of photocatalytic membranes are evaluated. This review will serve as a summary to advance researchers' understanding of the advantages of photocatalytic membranes, with the ultimate goal of achieving large-scale relevant applications of photocatalytic membranes in water treatment.

Heparin-modified polyether ether ketone hollow fiber membrane with improved hemocompatibility and air permeability used for extracorporeal membrane oxygenation

To expand the selection of raw material for fabricating extracorporeal membrane oxygenation (ECMO) and promote its application in lung disease therapy, polyether ether ketone hollow fiber membrane (PEEK-HFM) with designable pore characteristics, desired mechanical performances, and excellent biocompatibility was selected as the potential substitute for existing poly (4-methyl-1-pentene) hollow fiber membrane (PMP-HFM). To address the platelet adhesion and plasma leakage issues with PEEK-HFM, a natural anticoagulant heparin was grafted onto the surface using ultraviolet irradiation. Additionally, to explore the substitutability of the heparin layer while considering cost and scalability, a heparin-like layer composed of copolymers of acrylic acid and sodium p-styrenesulfonate was also constructed on the surface of PEEK-HFM Even though the successful grafting of heparin and heparin-like layers on the PEEK-HFM surface reduced the pore parameters, improvements in surface hydrophilicity also prevented the platelet-adhesion phenomenon and improved the anticoagulant behaviour, making it a viable alternative for commercial PMP-HFMs in ECMO production. Furthermore heparin-modified and heparin-like modified PEEK-HFMs demonstrated similar performance, indicating that synthetic layers can effectively replace natural heparin. This study holds practical and instructive significance for future research and the application of membranes in the development of oxygenators.

Photocatalytic and antifouling performance of titania-coated alumina membranes produced using a facile sol-gel dip-coating approach

Photocatalytic membranes (PM) have been investigated as an antifouling strategy for membrane separation processes. Coating ceramic membranes with photocatalytic layers can provide a highly active surface capable of degrading foulants and smaller molecules improving the membrane's performance when the surface is irradiated by a suitable light. Nevertheless, the coating process often leads to pore blockage due to the formation or deposition of thick layers of photocatalyst on membrane surfaces, which modifies the original membranes' average pore size and reduces membrane permeability. A facile sol-gel dip coating process was used to produce PM without modifying the original surface morphology of alumina microfiltration membranes. A 3.7-fold increase in permeate volume after 90 min of permeation of an acetaminophen solution in continuous filtration mode under UV light (λ = 365 nm LED, 10W) using titania as photocatalyst compared to the bare alumina membrane without irradiation. Furthermore, fouling modelling proved a reduction in the fouling constant, while fouling mechanisms were not modified. Raman analysis showed 100% anatase formed on the membrane surface. Although membranes could remove up to 87% TOC for oily wastewater filtration, antifouling capabilities for this type of effluent were not observed for the photocatalytic membranes mainly due to fouling inside the pores and light attenuation due to the thick fouling layer on the membrane surface.

Durable Nano-Flower Structured Foam Coupled with Electrically-Driven in Situ Aeration Enable High-Flux Oil/Water Emulsion Separation with Dynamic Antifouling Ability

The conventional membranes used for separating oil/water emulsions are typically limited by the properties of the membrane materials and the impact of membrane fouling, making continuous long-term usage unachievable. In this study, a filtering electrode with synchronous self-cleaning functionality is devised, exhibiting notable antifouling ability and an extended operational lifespan, suitable for the continuous separation of oil/water emulsions. Compared with the original Ti foam, the in situ growth of NiTi-LDH (Layered double hydroxide) nano-flowers endows the modified Ti foam (NiTi-LDH/TF) with exceptional superhydrophilicity and underwater superoleophobicity. Driven by gravity, a rejection rate of over 99% is achieved for various emulsions containing oil content ranging from 1% to 50%, as well as oil/seawater emulsions. The flux recovery rate exceeds 90% after one hundred cycles and a 4-h filtration period. The enhanced separation performance is realized through the "gas bridge" effect during in situ aeration and electrochemical anodic oxidation. The internal aeration within the membrane pores contributes to the removal of oil foulants. This study underscores the potential of coupling foam metal filtration materials with electrochemical technology, providing a paradigm for the exploration of novel oil/water separation membranes.

Reduction of Superoxide Radical Intermediate by Polydopamine for Efficient Hydrogen Peroxide Photosynthesis

The synthesis of hydrogen peroxide through artificial photosynthesis is a green and promising technology with advantages in sustainability, economy and safety. However, superoxide radical (⋅O2 -), an important intermediate in photocatalytic oxygen reduction to H2O2 production, has strong oxidizing properties that potentially destabilize the catalyst. Therefore, avoiding the accumulation of ⋅O2 - for its rapid conversion to H2O2 is of paramount significance in improving catalyst stability and H2O2 yield. In this work, a strategy was developed to utilize protonated groups for the rapid depletion of converted ⋅O2 -, thereby the efficiency of photocatalytic synthesis of H2O2 from CN was successfully enhanced by 47-fold. The experimental findings demonstrated that polydopamine not only improved carrier separation efficiency, and more importantly, provided the adsorption reduction active site for ⋅O2 - for efficient H2O2 production. This work offers a versatile approach for synthesizing efficient and stable photocatalysts.

Photocatalytic hydrogel film assisted forward osmosis (PFO) for water treatment: Sustainable performance and contaminant control

The integration of catalytic oxidation with forward osmosis (FO) holds promising potential to address two crucial challenges encountered by FO: fouling and unsustainable performance, but suitable approaches are still rare. Herein, we have successfully developed a photocatalysis-assisted forward osmosis (PFO) system. In the PFO, a self-made porous carbon nitride doped functional carbon nanotube photocatalytic hydrogel film (PCN@CNTM) was engaged in the FO process in an inventive way by simply sticking to the commercial FO membrane surface, preventing damage to the membrane from the catalyst's direct insertion and delaying the assault from the oxidation groups. PFO allowed organic pollutants to decompose in the feed solution (90%) and on the membrane surface, regulating the water chemical potential and giving the FO membrane antifouling properties. This resulted in sustainable water flux (11.8 LMH) with no significant membrane fouling in PFO, whereas in FO alone there was a significant fouling and flux drop (from 12.73 to 7.23 LMH in 4 h). Moreover, the expensive FO membrane was protected while the hydrogel film can be replaced on demand. The PFO exemplifies the concept of synergistic technology integration, presenting a new perspective on harnessing the strengths of distinct technologies in a mutually beneficial manner.

Research on the Antibacterial Properties of MXene-Based 2D-2D Composite Materials Membrane

Novel MXene-based two-dimensional (2D) membranes are widely used for water purification due to their highly controllable structure and antibacterial properties. However, in the process of membrane separation, the problems of membrane fouling, especially biological fouling, limits the further application of MXene-based membranes. In this study, in order to improve the antibacterial and separation properties of membranes, three kinds of MXene-based 2D-2D composite membranes (M2~M4) were prepared using polyethersulfone (PES) as the substrate, which were GO@MXene, O-g-C₃N₄@MXene and BiOCl@MXene composite membranes respectively. The results showed that the antibacterial activity of M2~M4 against Escherichia coli and Staphylococcus aureus was further improved, especially the antibacterial ratio of M4 against Escherichia coli and Staphylococcus aureus was up to 50% and 82.4%, respectively. By comparing the surface morphology of MXene membrane and modified membrane treated bacteria through scanning electron microscopy (SEM), it was found that the cell density on modified membrane was significantly lower than that of pure MXene membrane.

Nanomaterials Aspects for Photocatalysis as Potential for the Inactivation of COVID-19 Virus

Coronavirus disease-2019 is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and is the most difficult recent global outbreak. Semiconducting materials can be used as effective photocatalysts in photoactive technology by generating various reactive oxidative species (ROS), including superoxide (•O2−) and hydroxyl (•OH) radicals, either by degradation of proteins, DNA, and RNA or by inhibition of cell development through terminating the cellular membrane. This review emphasizes the capability of photocatalysis as a reliable, economical, and fast-preferred method with high chemical and thermal stability for the deactivation and degradation of SARS-CoV-2. The light-generated holes present in the valence band (VB) have strong oxidizing properties, which result in the oxidation of surface proteins and their inactivation under light illumination. In addition, this review discusses the most recent photocatalytic systems, including metals, metal oxides, carbonaceous nanomaterials, and 2-dimensional advanced structures, for efficient SARS-CoV-2 inactivation using different photocatalytic experimental parameters. Finally, this review article summarizes the limitations of these photocatalytic approaches and provides recommendations for preserving the antiviral properties of photocatalysts, large-scale treatment, green sustainable treatment, and reducing the overall expenditure for applications.

Existing Filtration Treatment on Drinking Water Process and Concerns Issues

Water is one of the main sources of life's survival. It is mandatory to have good-quality water, especially for drinking. Many types of available filtration treatment can produce high-quality drinking water. As a result, it is intriguing to determine which treatment is the best. This paper provides a review of available filtration technology specifically for drinking water treatment, including both conventional and advanced treatments, while focusing on membrane filtration treatment. This review covers the concerns that usually exist in membrane filtration treatment, namely membrane fouling. Here, the parameters that influence fouling are identified. This paper also discusses the different ways to handle fouling, either based on prevention, prediction, or control automation. According to the findings, the most common treatment for fouling was prevention. However, this treatment required the use of chemical agents, which will eventually affect human health. The prediction process was usually used to circumvent the process of fouling development. Based on our reviews up to now, there are a limited number of researchers who study membrane fouling control based on automation. Frequently, the treatment method and control strategy are determined individually.

Application of BiVO4/TiO2/CNT Composite Photocatalysts for Membrane Fouling Control and Photocatalytic Membrane Regeneration during Dairy Wastewater Treatment

This study aimed to investigate the performance of composite photocatalytic membranes fabricated by incorporating multiple nanoparticles (TiO2, carbon nanotubes, BiVO4) into polyvinylidene fluoride membrane material for real dairy wastewater treatment. The composite photocatalytic membranes exhibited superior antifouling behavior, lower filtration resistance, better flux, and higher flux recovery ratio than the pristine membrane. Salinity, pH, and lactose concentration are determinant factors that affect filtration resistance and rejection performance during the ultrafiltration of dairy wastewater. Generally, higher irreversible and total resistances and slightly lower chemical oxygen demand (COD) rejections were found at higher salinity (expressed by electric conductivity values of >4 mS/cm) than lower salinity (<4 mS/cm) levels. The presence of lactose in dairy wastewater increased irreversible resistance and severely reduced COD rejection during ultrafiltration due to the ability of lactose to pass through the membranes. It was ascertained that membranes require further treatment after filtrating such wastewater. Lower resistances and slightly better COD rejections were observed at pH 7.5 and pH 9.5 compared to those observed at pH 4. Photocatalytic membranes fouled during the ultrafiltration of real dairy wastewater were regenerated by visible light irradiation. The membrane containing all constituents (i.e., TiO2, carbon nanotubes, and BiVO4) showed the best regeneration performance, exceeding that of the pristine membrane by 30%.

Recent Advances in Responsive Membrane Functionalization Approaches and Applications

In recent years, significant advances have been made in the field of functionalized membranes. With the functionalization using various materials, such as polymers and enzymes, membranes can exhibit property changes in response to an environmental stimulation, such as heat, light, ionic strength, or pH. The resulting responsive nature allows for an increased breadth of membrane uses, due to the developed functionalization properties, such as smart-gating filtration for size-selective water contaminant removal, self-cleaning antifouling surfaces, increased scalability options, and highly sensitive molecular detection. In this review, new advances in both fabrication and applications of functionalized membranes are reported and summarized, including temperature-responsive, pH-responsive, light-responsive, enzyme-functionalized, and two-dimensional material-functionalized membranes. Specific emphasis was given to the most recent technological improvements, current limitations, advances in characterization techniques, and future directions for the field of functionalized membranes.

Interdependence of Kinetics and Fluid Dynamics in the Design of Photocatalytic Membrane Reactors

Photocatalytic membrane reactors (PMRs) are a promising technology for wastewater reclamation. The principles of PMRs are based on photocatalytic degradation and membrane rejection, the different processes occurring simultaneously. Coupled photocatalysis and membrane filtration has made PMRs suitable for application in the removal of emerging contaminants (ECs), such as diclofenac, carbamazepine, ibuprofen, lincomycin, diphenhydramine, rhodamine, and tamoxifen, from wastewater, while reducing the likelihood of byproducts being present in the permeate stream. The viability of PMRs depends on the hypotheses used during design and the kinetic properties of the systems. The choice of design models and the assumptions made in their application can have an impact on reactor design outcomes. A design’s resilience is due to the development of a mathematical model that links material and mass balances to various sub-models, including the fluid dynamic model, the radiation emission model, the radiation absorption model, and the kinetic model. Hence, this review addresses the discrepancies with traditional kinetic models, fluid flow dynamics, and radiation emission and absorption, all of which have an impact on upscaling and reactor design. Computational and analytical descriptions of how to develop a PMR system with high throughput, performance, and energy efficiency are provided. The potential solutions are classified according to the catalyst, fluid dynamics, thickness, geometry, and light source used. Two main PMR types are comprehensively described, and a discussion of various influential factors relating to PMRs was used as a premise for developing an ideal reactor. The aim of this work was to resolve potential divergences that occur during PMRs design as most real reactors do not conform to the idealized fluid dynamics. Lastly, the application of PMRs is evaluated, not only in relation to the removal of endocrine-disrupting compounds (EDCs) from wastewater, but also in dye, oil, heavy metals, and pesticide removal.

Chemoenzymatic Cascade Reaction for Green Cleaning of Polyamide Nanofiltration Membrane

Chemical cleaning is indispensable for the sustainable operation of nanofiltration (NF) in wastewater treatment. However, the common chemical cleaning methods are plagued by low cleaning efficiency, high chemical consumption, and separation performance deterioration. In this work, a chemoenzymatic cascade reaction is proposed for pollutant degradation and polyamide NF membrane cleaning. Glucose oxidase (GOD) enzymatic reaction in this cascade system produces hydrogen peroxide (H₂O₂) and gluconic acid to trigger the oxidation of foulants by Fe₃O₄-catalyzed Fenton reaction. By virtue of the microenvironment (pH and H₂O₂ concentration) engineering and substrate enrichments, this chemoenzymatic cascade reaction (GOD-Fe₃O₄) exhibits a favorable degradation efficiency for bisphenol A and methyl blue (MB). Thanks to the strong oxidizing degradation, the water flux of the NF10 membrane fouled by MB is almost completely recovered (∼95.8%) after a 3-cycle fouling/cleaning experiment. Meanwhile, the chemoenzymatic cascade reaction improves the applicability of the Fenton reaction in polyamide NF membrane cleaning because it prevents the membrane from damaging by high concentration of H₂O₂ and inhibits the secondary fouling caused by ferric hydroxide precipitates. By immobilizing GOD on the aminated Fe₃O₄ nanoparticles, a reusable cleaning agent is prepared for highly efficient membrane cleaning. This chemoenzymatic cascade reaction without the addition of an acid/base/oxidant provides a promising candidate for sustainable and cost-effective cleaning for the polyamide NF membrane.

Effects of electroflocculation/oxidation pretreatment on the fouling characteristics of ultrafiltration membranes

In order to reduce the membrane pollution of ultrafiltration caused by natural organic matter and improve the treatment efficiency, electroflocculation/oxidation is used as the premembrane treatment method. The membrane specific flux attenuation characteristics was compared and analyzed under the conditions of direct ultrafiltration and electroflocculation/oxidation-ultrafiltration. Combined with the analysis of the reversibility of membrane fouling, the mechanism of electroflocculation/oxidation pretreatment to alleviate ultrafiltration membrane fouling was evaluated, and the membrane pore clogging model was used to fit the fouling law. The results show that, in the continuously fed filtration experiment, the electroflocculation/oxidation process involved in the pretreatment and the direct ultrafiltration membrane filtration decreased the ultrafiltration membrane flux to 79.1% and 28.5%, respectively. The reversible resistance generated by ultrafiltration and electroflocculation/oxidation-ultrafiltration processes accounted for 37.70% and 62.26% of their total pollution resistance, whereas the irreversible resistance generated accounted for 47.30% and 12.40%, respectively. Meanwhile, the direct correlation between the the flux dropped and complete clogging became less than that of the ultrafiltration process. The pretreatment significantly strengthened irreversible fouling resistance of the membrane pores. The membrane permeation flux was significantly increased after the electroflocculation/oxidation pretreatment.