Recent progress of photocatalytic membrane reactors in water treatment and in synthesis of organic compounds. A review

Mechanisms and Strategies of Advanced Oxidation Processes for Membrane Fouling Control in MBRs: Membrane-Foulant Removal versus Mixed-Liquor Improvement

Membrane bioreactors (MBRs) are well-established and widely utilized technologies with substantial large-scale plants around the world for municipal and industrial wastewater treatment. Despite their widespread adoption, membrane fouling presents a significant impediment to the broader application of MBRs, necessitating ongoing research and development of effective antifouling strategies. As highly promising, efficient, and environmentally friendly chemical methods for water and wastewater treatment, advanced oxidation processes (AOPs) have demonstrated exceptional competence in the degradation of pollutants and inactivation of bacteria in aqueous environments, exhibiting considerable potential in controlling membrane fouling in MBRs through direct membrane foulant removal (MFR) and indirect mixed-liquor improvement (MLI). Recent proliferation of research on AOPs-based antifouling technologies has catalyzed revolutionary advancements in traditional antifouling methods in MBRs, shedding new light on antifouling mechanisms. To keep pace with the rapid evolution of MBRs, there is an urgent need for a comprehensive summary and discussion of the antifouling advances of AOPs in MBRs, particularly with a focus on understanding the realizing pathways of MFR and MLI. In this critical review, we emphasize the superiority and feasibility of implementing AOPs-based antifouling technologies in MBRs. Moreover, we systematically overview antifouling mechanisms and strategies, such as membrane modification and cleaning for MFR, as well as pretreatment and in-situ treatment for MLI, based on specific AOPs including electrochemical oxidation, photocatalysis, Fenton, and ozonation. Furthermore, we provide recommendations for selecting antifouling strategies (MFR or MLI) in MBRs, along with proposed regulatory measures for specific AOPs-based technologies according to the operational conditions and energy consumption of MBRs. Finally, we highlight future research prospects rooted in the existing application challenges of AOPs in MBRs, including low antifouling efficiency, elevated additional costs, production of metal sludge, and potential damage to polymeric membranes. The fundamental insights presented in this review aim to elevate research interest and ignite innovative thinking regarding the design, improvement, and deployment of AOPs-based antifouling approaches in MBRs, thereby advancing the extensive utilization of membrane-separation technology in the field of wastewater treatment.

Recent advances in visible light-activated photocatalysts for degradation of dyes: A comprehensive review

The rapid development in industrialization and urbanization coupled with an ever-increasing world population has caused a tremendous increase in contamination of water resources globally. Synthetic dyes have emerged as a major contributor to environmental pollution due to their release in large quantities into the environment, especially owing to their high demand in textile, cosmetics, clothing, food, paper, rubber, printing, and plastic industries. Photocatalytic treatment technology has gained immense research attention for dye contaminated wastewater treatment due to its environment-friendliness, ability to completely degrade dye molecules using light irradiation, high efficiency, and no generation of secondary waste. Photocatalytic technology is evolving rapidly, and the foremost goal is to synthesize highly efficient photocatalysts with solar energy harvesting abilities. The current review provides a comprehensive overview of the most recent advances in highly efficient visible light-activated photocatalysts for dye degradation, including methods of synthesis, strategies for improving photocatalytic activity, regeneration and their performance in real industrial effluent. The influence of various operational parameters on photocatalytic activity are critically evaluated in this article. Finally, this review briefly discusses the current challenges and prospects of visible-light driven photocatalysts. This review serves as a convenient and comprehensive resource for comparing and studying the fundamentals and recent advancements in visible light photocatalysts and will facilitate further research in this direction.

Self-cleaning and photodegradle PVDF separation membranes modified with self-assembled TiO2-g-CS/CNTs particle

This work obtained separation membranes with UV-cleaning performance by adding TiO2-g-CS/CNTs photocatalyst to the PVDF. The positively charged chitosan (CS) and negatively charged carboxylic carbon nanotube (CNTs-COOH) can be self-assembled into the bilayer structure on the surface of TiO2 particles through electrostatic attraction. The presence of many hydrophilic groups in CS and CNTs-COOH significantly improves the hydrophilicity of TiO2-g-CS/CNTs-PVDF membrane, and helps TiO2 to be uniformly dispersed on the upper surface. TiO2-g-CS/CNTs promote the change of pore structure and expand the flux of the modified membrane to 4.5 times that of pure PVDF. Zeta potential demonstrates that the TiO2-g-CS particles successfully attracted CNTs in the PVDF matrix, and the membrane surface was still positively charged. Thus, the combined effect of the positively charged TiO2-g-CS and the highly adsorbed CNTs enhanced the retention of the contaminants. More importantly, there is a charge transfer between the grafted CS and TiO2 interface to obtain a broader light absorption band. The excitation carriers provided by CNTs significantly contribute to the photocatalytic performance after transfer between TiO2 and CS; thus, TiO2-g-CS/CNTs-PVDF produces higher photocatalytic activity for dye molecules (degradation rate > 97 %).

Two dimensional (2D) materials and biomaterials for water desalination; structure, properties, and recent advances

BACKGROUND

An efficient solution to the global freshwater dilemma is desalination. MXene, Molybdenum Disulfide (MoS₂), Graphene Oxide, Hexagonal Boron Nitride, and Phosphorene are just a few examples of two-dimensional (2D) materials that have shown considerable promise in the development of 2D materials for water desalination. However, other promising materials for desalinating water are biomaterials. The benefits of bio-materials are their wide distribution, lack of toxicity, and superior capacity for water desalination.

METHODS

For the rational use of water and the advancement of sustainable development, it is of the utmost importance to research 2D-dimensional materials and biomaterials that are effective for water desalination. The scientific community has concentrated on wastewater remediation using bio-derived materials, such as nanocellulose, chitosan, bio-char, bark, and activated charcoal generated from plant sources, among the various endeavors to enhance access to clean water. Moreover, the 2D-materials and biomaterials may have ushered in a new age in the production of desalination materials and created a promising future.

RESULTS

The present review article focuses on and reviews the progress of 2D materials and biomaterials for water desalination. Their properties, surface, and structure, combined with water desalination applications, are highlighted. Further, the practicability and potential future directions of 2D materials and biomaterials are proposed. Thus, the current work provides information and discernments for developing novel 2D materials and biomaterials for wastewater desalination. Moreover, it aims to promote the contribution and advancement of materials for water desalination, fabrication, and industrial production.

Individual and combined effect of ions species and organic matter on the removal of microcontaminants by Fe3+-EDDS/solar-light activated persulfate

This work is focused on improving the understanding of the complex water matrix interactions occurring during the removal of a microcontaminants mixture (acetamiprid, carbamazepine and caffeine) by solar/Fe³⁺-EDDS/persulfate process. The individual and combined effects of sulfates (100-500 mg/L), nitrates (20-160 mg/L), bicarbonates (77-770 mg/L) and chlorides (300-1500 mg/L) were assessed by comparing the outcomes obtained in different synthetic and actual water matrices. In general, the results showed negligible effects of the different anions on Fe³⁺-EDDS concentration and PS consumption profiles, while the combination of bicarbonates and chlorides seemed to be the key for the MC removal efficiency decrease found when working with complex matrixes. Finally, the influence of dissolved organic matter on process performance was evaluated. It was concluded that there is neither any influence of this variable on Fe³⁺-EDDS concentration and PS consumption profiles. In contrast, there was a general negative effect on MC removal efficiency, which strongly depended on both the concentration and composition of the dissolved organic matter.

Volatile organic compounds (VOCs) removal by photocatalysts: A review

Amplified anthropogenic release of volatile organic compounds (VOCs) gets worse air quality and human health. Photocatalytic degradation of VOCs is the practical strategy due to its low cost, simplicity, high efficiency, and environmental sustainability. Different types of photocatalyst activated by UV and visible lights are applied for VOC degradation. This review tries to investigate the state-of-art of recently published papers on this subject with a focus on the high-efficiency photocatalyst. The novel photocatalysts are introduced and enhancing photocatalytic activity strategies such as the hybrid of two/three photocatalyst, impurity doping, and heterojunctions with narrow bandgap semiconductors have been explained. The procedures of visible light activation of the photocatalysts are discussed with attention to current problems and future challenges. In addition, effective operational parameters in the photocatalytic degradation of VOCs have been reviewed with their advantages and drawbacks. A series of strategies are developed for the efficient utilization of visible light photocatalysts and improving new materials or design structures to degrade produced toxic intermediates/by-products during photocatalytic degradation of VOCs. This review shows that there are significant challenges in the applications of photocatalysts in the selective removal of VOCs. Several approaches should be combined to produce synergistic effects, which may lead to much higher photocatalytic performance than individual strategies. Another challenge is to develop efficient photocatalysts to meet real problems on an industrial scale.

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.

Effect of Polymer Concentration on the Photocatalytic Membrane Performance of PAN/TiO2/CNT Nanofiber for Methylene Blue Removal through Cross-Flow Membrane Reactor

A photocatalytic membrane combining photocatalyst and membrane technology based on polyacrylonitrile (PAN) and TiO2/CNT has been developed. Such combination is to overcome fouling formation on the membrane, thus prolonging the membrane lifetime and enhancing the efficiency on the waste treatment. PAN nanofiber was prepared by electrospinning method. The precursor solution was dissolved PAN and dispersed TiO2/CNT in N,N-Dimethylformamide (DMF). PAN concentration in the precursor solution was varied at 4.5, 5.5, 6.5, 7.5, and 8.5%. The effect of PAN concentration on the fiber morphology and pore size was discussed. The performance of the resulted membrane on methylene blue (MB) removal was also investigated on a cross-flow system. SEM images of the resulted membrane identified that PAN nanofiber was successfully fabricated with random orientation. The PAN 6.5% showed the highest diffraction intensity of the anatase crystalline phase of TiO2. The additions of CNT and TiO2 lead to the formation of a cluster of beads as confirmed by TEM. Increasing the concentration of PAN increased the fiber diameter from 206 to 506 nm, slightly decreased the surface area and pore size, respectively, from 32.739 to 21.077 m2.g−1 and from 6.38 to 4.75 nm. The PAN/TiO2/CNT nanofibers show type IV of the adsorption-desorption N2 isotherms with the H1 hysteresis loops. Membrane PAN/TiO2/CNT at PAN concentration of 6.5% shows the optimum performance on the MB color removal by maintaining the percentage of rejection (%R) at 90% for 240 min and permeability of 750 LMH. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Light-Activated Hydroxyapatite Photocatalysts: New Environmentally-Friendly Materials to Mitigate Pollutants

This review focuses on a reasoned search for articles to treat contaminated water using hydroxyapatite (HAp)-based compounds. In addition, the fundamentals of heterogeneous photocatalysis were considered, combined with parameters that affect the pollutants’ degradation using hydroxyapatite-based photocatalyst design and strategies of this photocatalyst, and the challenges of and perspectives on the development of these materials. Many critical applications have been analyzed to degrade dyes, drugs, and pesticides using HAp-based photocatalysts. This systematic review highlights the recent state-of-the-art advances that enable new paths and good-quality preparations of HAp-derived photocatalysts for photocatalysis.

Hybrid Organic–Inorganic Membranes for Photocatalytic Water Remediation

Mismanagement, pollution and excessive use have depleted the world’s water resources, producing a shortage that in some territories is extreme. In this context, the need for potable water prompts the development of new and more efficient wastewater treatment systems to overcome shortages by recovering and reusing contaminated water. Among the water treatment methods, membrane technology is considered one of the most promising. Besides, photocatalytic degradation has become an attractive and efficient technology for water and wastewater treatment. However, the use of unsupported catalysts has as its main impediment their separation from the water once treated. With this, providing the membranes with this photocatalyzed degradation capacity can improve the application of photocatalysts, since in many cases their application improves their recovery and reuse. This review describes the general photocatalytic processes of the main inorganic nanoparticles used as fillers in hybrid polymeric membranes. In addition, the most recent hybrid organic–inorganic membranes are reviewed. Finally, the membranes formed by metal–organic frameworks that can be considered one of the newest and most versatile developments are described.

Statistical Analysis of Synthesis Parameters to Fabricate PVDF/PVP/TiO2 Membranes via Phase-Inversion with Enhanced Filtration Performance and Photocatalytic Properties

Non-solvent induced phase-inversion is one of the most used methods to fabricate membranes. However, there are only a few studies supported by statistical analysis on how the different fabrication conditions affect the formation and performance of membranes. In this paper, a central composite design was employed to analyze how different fabrication conditions affect the pure water flux, pore size, and photocatalytic activity of polyvinylidene fluoride (PVDF) membranes. Polyvinylpyrrolidone (PVP) was used to form pores, and titanium dioxide (TiO₂) to ensure the photocatalytic activity of the membranes. The studied bath temperatures (15 to 25 °C) and evaporation times (0 to 60 s) did not significantly affect the pore size and pure water flux of the membranes. The concentration of PVDF (12.5 to 17.5%) affected the viscosity, formation capability, and pore sizes. PVDF at high concentrations resulted in membranes with small pore sizes. PVP affected the pore size and should be used to a limited extent to avoid possible hole formation. TiO₂ contents were responsible for the decolorization of a methyl orange solution (10^-5 M) up to 90% over the period studied (30 h). A higher content of TiO₂ did not increase the decolorization rate. Acidic conditions increased the photocatalytic activity of the TiO₂-membranes.

Photocatalytic Nanofiber Membranes for the Degradation of Micropollutants and Their Antimicrobial Activity: Recent Advances and Future Prospects

This review paper systematically evaluates current progress on the development and performance of photocatalytic nanofiber membranes often used in the removal of micropollutants from water systems. It is demonstrated that nanofiber membranes serve as excellent support materials for photocatalytic nanoparticles, leading to nanofiber membranes with enhanced optical properties, as well as improved recovery, recyclability, and reusability. The tremendous performance of photocatalytic membranes is attributed to the photogenerated reactive oxygen species such as hydroxyl radicals, singlet oxygen, and superoxide anion radicals introduced by catalytic nanoparticles such as TiO₂ and ZnO upon light irradiation. Hydroxyl radicals are the most reactive species responsible for most of the photodegradation processes of these unwanted pollutants. The review also demonstrates that self-cleaning and antimicrobial nanofiber membranes are useful in the removal of microbial species in water. These unique materials are also applicable in other fields such as wound dressing since the membrane allows for oxygen flow in wounds to heal while antimicrobial agents protect wounds against infections. It is demonstrated that antimicrobial activities against bacteria and photocatalytic degradation of micropollutants significantly reduce membrane fouling. Therefore, the review demonstrates that electrospun photocatalytic nanofiber membranes with antimicrobial activity form efficient cost-effective multifunctional composite materials for the removal of unwanted species in water and for use in various other applications such as filtration, adsorption and electrocatalysis.

The Evolution of Photocatalytic Membrane Reactors over the Last 20 Years: A State of the Art Perspective

The research on photocatalytic membrane reactors (PMRs) started around the year 2000 with the study of wastewater treatment by degradation reactions of recalcitrant organic pollutants, and since then the evolution of our scientific knowledge has increased significantly, broadening interest in reactions such as the synthesis of organic chemicals. In this paper, we focus on some initial problems and how they have been solved/reduced over time to improve the performance of processes in PMRs. Some know-how gained during these last two decades of research concerns decreasing/avoiding the degradation of the polymeric membranes, improving photocatalyst reuse, decreasing membrane fouling, enhancing visible light photocatalysts, and improving selectivity towards the reaction product(s) in synthesis reactions (partial oxidation and reduction). All these aspects are discussed in detail in this review. This technology seems quite mature in the case of water and wastewater treatment using submerged photocatalytic membrane reactors (SPMRs), while for applications concerning synthesis reactions, additional knowledge is required.

Current advances in microalgae-based bioremediation and other technologies for emerging contaminants treatment

Emerging contaminants (EC) have been detected in effluents and drinking water in concentrations that can harm to a variety of organisms. Therefore, several technologies are developed to treat these compounds, either for their complete removal or degradation in less toxic by-products. Some technologies applied to the treatment of EC, such as adsorption, advanced oxidative processes, membrane separation processes, and bioremediation through microalgal metabolism, were identified by thematic maps. In this review, we used a bibliometric software from >1000 articles. These manuscripts, in general, present removals from 0% to 100% for different ECs. This efficiency varies between treatment technologies and the contaminants' physical-chemical properties and their concentration and operational parameters. This review explored the bioremediation of EC through microalgae with greater emphasis. The main mechanisms of action of microalgae in the bioremediation of ECs are biodegradation bioadsorption, and bioaccumulation. Also, physicochemical properties and removal efficiencies of >50 emerging contaminants are presented. Although there are challenges related to the generation of more toxic by-products and economic and environmental viability, these can be minimized with advances in the development of treatment technologies and even through the integration of different techniques to make the treatment of contaminants emerging from environmental media more sustainable.

A review of clay based photocatalysts: Role of phyllosilicate mineral in interfacial assembly, microstructure control and performance regulation

Over the past decades, inspired by the outstanding properties of clay minerals such as abundance, low-cost, environmental benignity, high stability, and regularly arranged silica-alumina framework, researchers put much efforts on the interface assembly and surface modification of natural minerals with bare photocatalysts, i.e. TiO₂, g-C₃N₄, ZnO, MoS₂, etc. The clay-based hybrid photocatalysts have resulted in a rich database for their tailor-designed microstructures, characterizations, and environmental-related applications. Therefore, in this study, we took a brief introduction of three representative minerals, i.e. kaolinite, montmorillonite and rectorite, and discussed their basic merits in photocatalysis applications. After that, we summarized the recent advances in construction of stable visible-light driven photocatalysts based on these minerals. The structure-activity relationships between the properties of clay types, pore structure, distribution/dispersion and light absorption, carrier separation efficiency as well as redox performance were illustrated in detail. Such representative information would provide theoretical basis and scientific support for the application of clay based photocatalysts. Finally, we pointed out the major challenges and future directions at the end of this review. Undoubtedly, control and preparation of novel photocatalysts based on clays will continue to witness many breakthroughs in the arena of solar-driven technologies.

Self-Organized Implanting of Micro/Nanofiltration Membranes in Advanced Flow μ-Reactors

A low-cost, simple, and one-step synthesis of cellulose acetate nanoparticles (CANPs) has been invented using a continuous-flow advanced microfluidic reactor. For this purpose, the CANPs are self-organized inside a cross-junction microchannel by flowing cellulose acetate (CA) dissolved in N,N-dimethylformamide (DMF) through the axial inlet and the antisolvent water through the pair of side inlets. The preferential solubility (insolubility) of DMF (CA) to antisolvent water stimulates the in situ synthesis of CANPs at the DMF/water miscible interface following a phase-inversion process. Subsequently, nanofiltration, ultrafiltration, and microfiltration membranes of different porosities and permeabilities have been prepared from freshly synthesized CANPs. The porosity, thickness, transparency, and wettability of the membranes are tuned by varying the thickness of the membranes, size of the nanoparticles, and the porosity of the membranes. The as-synthesized CANPs show enhanced bactericidal properties with and without loading an external drug, curcumin, which has been validated against the Gram-negative Pseudomonas aeruginosa species. Importantly, enabling a pulsatile flow during the synthesis, the CANPs are embedded as nanofiltration membranes inside the microfluidic channel. Such microfluidic devices have been used to separate a corrosive dye from water. Concisely, the proposed in situ synthesis of CANPs in the continuous-flow microfluidic reactors, their usage for fabricating membranes with tunable wettability and transparency, and their subsequent integration into the microfluidic channel show the potential of the invention for a host of applications related to health care and environmental remediation.

Reversible filtration redox of methylene blue in dimethylsulfoxide by manganese oxide loaded carbonaceous nanofibrous membrane through Fenton-like oxidation

The reversible redox of methylene blue in organic solvents was highly attractive, yet was rarely reported. In this study, we realized the continuous filtration redox of methylene blue (MB) in dimethylsulfoxide (DMSO) through Fenton-like oxidization by using MnO₂ loaded carbonaceous nanofibrous membrane (cPAN-MnO₂). The carbonaceous nanofibrous membrane (cPAN) was fabricated through electrospun of polyacrylonitrile and subsequent carbonization. The obtained cPAN nanofibrous membrane showed excellent stability in polar DMSO. MnO₂ can be readily coated on cPAN nanofibers through an in situ redox reaction between cPAN and potassium permanganate. The fabricated cPAN-MnO₂ membrane exhibited instantaneous reduction property towards MB in DMSO during a gravity-driven continuous filtration process. Interestingly, MB reduction was initiated by a typical Fenton-like oxidization, where hydroxyl radicals were firstly generated from hydrogen peroxide catalyzed by MnO₂ in DMSO. Then hydroxyl radicals attacked DMSO to further produce methyl radicals, which resulted in the reduction of MB. In addition, MB reduction process in DMSO was reversible. Our study provides a novel strategy for continuous redox of MB in polar organic solvent and might give new ideas for MB applications.

Drawing on Membrane Photocatalysis for Fouling Mitigation

Photocatalysis is an effective and environmentally friendly approach for degrading organic pollutants, particularly in scenarios where sunlight can be utilized as the energy source. Opportunities are emerging to apply materials and methods from photocatalytic pollutant degradation to address the challenge of fouling. Membrane fouling, attributed to organic foulants, is a prevalent problem for all membrane-based technologies and represents a major deleterious impact on membrane performance. Integration of tactics developed in photocatalysis more broadly to membranes reveals new strategies for membrane fouling control-an approach taken by an increasing number of researchers. This review summarizes key developments in photocatalytic materials and methods in water treatment and presents recent progress in the development of processes for photocatalytic alleviation of membrane fouling, including photocatalyst design and modification strategies aimed at enhancing photocatalytic efficiency, as well as different configurations of photocatalysis-membrane systems (PMS). Perspectives on future research and development opportunities for photocatalytic membrane fouling control are also provided.

Degradation pathway of triazole fungicides and synchronous removal of transformation products via photo-electrocatalytic oxidation tandem MoS2 adsorption

A simple and effective tandem process of photo-electrocatalytic oxidation (PECO)-MoS₂ adsorption was developed for the synchronous removal of triazole fungicides (TFs) and toxicological transformation products (TPs). In order to accurately identify trace TPs and evaluate degradation pathway during water treatment, a sensitive analytical method was developed on the basis of the stir bar sorptive extraction (SBSE) pretreatment tandem LC-MS/MS technique. Firstly, the typical TFs (PRO, TET, and DIN, C₀ = 1.0 mg/L) in actual water samples were treated under the optimal process (bias voltage 1.8 V, pH 4, irradiation intensity 50 mW/cm², 0.05 g MoS₂/100 mL, 350 rpm, adsorption of 5 min). The result indicated that the residues of PRO, TET, and DIN in secondary effluent were 0.0973, 0.0617, and 0.0012 mg/L, respectively, with the removal rates of 90.3%, 93.8%, and 99.9%, respectively, undergoing 30-min photo-electrocatalysis and 5-min adsorption. The alkaline medium was favorable for the adsorption of MoS₂ to TFs. The assessment results of potential cancer risk indicated that the residues of TFs in secondary effluent were safe for drinking water consumption. Besides, the major TPs were identified via the SBSE-HRLC-MS/MS technique, and one possible transformation pathway of TFs was proposed. TFs mainly underwent dehydrochlorination, cyclization, hydroxylation, etc. to produce a series of nitrogenous heterocyclic compounds that possess higher polarity than parents, hinting that TPs might pose potential aquatic toxicity. However, TPs can be removed synchronously by this tandem technique. The current study can provide a theoretical basis for the harmless treatment of TFs in the water environment.

Photocatalytic Nanocomposite Polymer-TiO2 Membranes for Pollutant Removal from Wastewater

Photocatalytic TiO2-PVDF/PMMA nano-composites flat sheet membranes were fabricated by phase inversion and then employed in a crossflow filtration pilot to remove model pollutants of various sizes and charge from aqueous solution. The dope solution contained a mixture of PVDF and PMMA as polymers, polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) as additives, triethyl phosphate (TEP) as green solvent and TiO2 as immobilized photo catalyst. After undergoing characterization tests such as SEM morphology thickness, porosity, contact angle and water permeability, the membranes were used to eliminate the model pollutants from synthetic aqueous solution. The impact of the operating conditions (i.e., pH, pressure and initial pollutant concentration) and composition of the doping solution on the performance and photocatalytic and antifouling activity of the membranes was investigated. The results showed that Congo Red and Tartrazine despite their small size were rejected at 99% and 81%, respectively, because of their negative charge, while Ciprofloxacin, which is larger than Tartrazine but of neutral charge, crossed the membrane. The permeability did not decrease with a decline in pollutant concentration but diminished when the pressure increased and was reduced by more than half for wastewater.

Exploitation of Lignocellulose Fiber-Based Biotemplates to Improve the Performance of an Immobilized TiO2 Photocatalyst

The performance of an immobilized photocatalyst has been successfully improved by colloidal processing of a heterostructure composed by TiO2 nanoparticles and lignocellulose nanofibers (LCNFs) obtained from biomass residues. The incorporation of 4 wt.% of biotemplate to the formulation increased the degradation rate and reduced the operating time to remove the 100% of methyl orange of a liquid solution. The reaction rate constant (k = 0.29–0.45 h−1) of the prepared photocatalytic coatings (using commercial particles and templates obtained from natural-derived resources) are competitive with other pure TiO2 materials (no composites), which were prepared through more complex methodologies. The optimization stages of deposition and sintering processes allowed us to obtain homogeneous and crack-free microstructures with controlled thickness and mass values ranging from 3 to 12 µm and 0.9 to 5.6 mg, respectively. The variation of the microstructures was achieved by varying the amount of LCNF in the formulated suspensions. The versatility of the proposed methodology would allow for implementation over the internal surface of photocatalytic reactors or as a photocatalytic layer of their membranes. In addition, the processing strategy could be applied to immobilize other synthetized semiconductors with higher intrinsic photocatalysis properties.

Advanced Materials with Special Wettability toward Intelligent Oily Wastewater Remediation

Clean water resources are essential to our human society. Oil leakage has caused water contamination, which leads to serious shortage of clean water, environmental deterioration, and even increasing number of deaths. It is of great urgency to solve the oil-polluted water problems worldwide. Efficient oil/water separation, especially emulsified oil/water mixture separation, is widely used to mitigate water pollution issues. Recently, advanced materials with special wettability have been employed for oily wastewater remediation. Moreover, by endowing them with various intelligent functions, smart materials can effectively separate complex oil/water mixtures including extremely stable emulsions. In this review, oil/water separation mechanisms and various fabrication methods of special wettability separation materials are summarized. We highlight the special wettable materials with intelligent functions, including photocatalytic, self-healing, and switchable oil/water separation materials, which can achieve self-cleaning, self-healing, and efficient oily wastewater treatment. In each section, the acting mechanisms, fabricating technologies, representative studies, and separation efficiency are briefly introduced. Lastly, the challenges and outlook for oil/water separation based on the special wettability materials are discussed.

Photocatalytic materials and light-driven continuous processes to remove emerging pharmaceutical pollutants from water and selectively close the carbon cycle

Past and present technologies for wastewater purification and future research directions are critically discussed in this review.

Immobilization techniques of a photocatalyst into and onto a polymer membrane for photocatalytic activity

This article reviews the various techniques of immobilizing a photocatalyst into and onto the polymer membrane for pollutant removal and as a problem solver in handling suspended photocatalyst issues from the previous literature. A particular focus is given to the preparation of mixed matrix membranes and deposition techniques for photocatalytic degradation in applications for wastewater treatment. Advantages and disadvantages in this application are evaluated. Various operating conditions during the process are presented. About 90 recently published studies (2008–2020) are reviewed. From the literature, it was found that TiO₂ is the most favoured photocatalyst that is frequently used in photocatalytic water treatment. Dry–wet co-spinning and sputtering techniques emerged as the promising technique for immobilizing a uniformly distributed photocatalyst within the polymeric membrane, and exhibited excellence pollutant removal. In general, the technical applicability is the key factor in selecting the best photocatalyst immobilizing technique for water treatment. Finally, the scope of various techniques that have been reviewed may provide potential for future photocatalytic study.

This article reviews the various techniques of immobilizing a photocatalyst into and onto the polymer membrane for pollutant removal and as a problem solver in handling suspended photocatalyst issues from the previous literature.

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.

Visible-Light Photocatalysts and Their Perspectives for Building Photocatalytic Membrane Reactors for Various Liquid Phase Chemical Conversions

Photocatalytic organic synthesis/conversions and water treatment under visible light are a challenging task to use renewable energy in chemical transformations. In this review a brief overview on the mainly employed visible light photocatalysts and a discussion on the problems and advantages of Vis-light versus UV-light irradiation is reported. Visible light photocatalysts in the photocatalytic conversion of CO2, conversion of acetophenone to phenylethanol, hydrogenation of nitro compounds, oxidation of cyclohexane, synthesis of vanillin and phenol, as well as hydrogen production and water treatment are discussed. Some applications of these photocatalysts in photocatalytic membrane reactors (PMRs) for carrying out organic synthesis, conversion and/or degradation of organic pollutants are reported. The described cases show that PMRs represent a promising green technology that could shift on applications of industrial interest using visible light (from Sun) active photocatalysts.

A novel tubular up-flow magnetic film photocatalytic system optimized by main factors control for efficient removal of chlorophenols wastewater

Chlorophenols (CPs) are still used as raw material or intermediate in some industries. Photocatalytic oxidation is free from secondary pollution, but the efficiency is restricted by some main factors. In this study, a novel high efficiency tubular up-flow magnetic film (TUMF) photocatalytic system was investigated based on the magnetic lanthanum doping core-shell Fe₃O₄@SiO₂@TiO₂ (La-FST) nanoparticles. When the dosage of La-FST was 0.4 g/L, the flow velocity was 94.2 mL/min, and the circulated irradiation of 15 W maintained 40 min, the average removal rate of 2,4-dichlorophenol (2,4-DCP) was reduced significantly from 10 mg/L to 0.0803 mg/L by TUMF system, meeting the limits of the particular items (0.093 mg/L) from national environmental quality standards for surface water, avoiding the problem of photocatalyst separation and loss. The photoinduced holes (h⁺) was the key active radical to oxidize 2,4-DCP, and the main factors of TUMF system could be well controlled to achieve satisfactory effluent quality. A prediction method of photocatalytic reaction time in a multistage series TUMF system was established to remove 2,4-DCP from 100 mg/L to 0.5 mg/L, saving 86 min. The novel high-efficiency TUMF system provides a technical selection for the photocatalytic degradation of CPs and other refractory organics.

2D Nanocomposite Membranes: Water Purification and Fouling Mitigation

In this study, the characteristics of different types of nanosheet membranes were reviewed in order to determine which possessed the optimum propensity for antifouling during water purification. Despite the tremendous amount of attention that nanosheets have received in recent years, their use to render membranes that are resistant to fouling has seldom been investigated. This work is the first to summarize the abilities of nanosheet membranes to alleviate the effect of organic and inorganic foulants during water treatment. In contrast to other publications, single nanosheets, or in combination with other nanomaterials, were considered to be nanostructures. Herein, a broad range of materials beyond graphene-based nanomaterials is discussed. The types of nanohybrid membranes considered in the present work include conventional mixed matrix membranes, stacked membranes, and thin-film nanocomposite membranes. These membranes combine the benefits of both inorganic and organic materials, and their respective drawbacks are addressed herein. The antifouling strategies of nanohybrid membranes were divided into passive and active categories. Nanosheets were employed in order to induce fouling resistance via increased hydrophilicity and photocatalysis. The antifouling properties that are displayed by two-dimensional (2D) nanocomposite membranes also are examined.

Factors affecting the degradation of remazol turquoise blue (RTB) dye by titanium dioxide (TiO2) entrapped photocatalytic membrane

This study investigated the photocatalytic degradation of remazol turquoise blue (RTB) dye using titanium dioxide (TiO₂) entrapped polyvinylidene fluoride (PVDF) membrane synthesized by non-solvent induced phase separation method. Numerous experiments were conducted to investigate the effects of catalyst loading in membranes, the concentration of dye, feed temperature, pH of the solution, and the addition of H₂O₂ to the dye solution on the removal of dye. Results were compared with the performance of virgin PVDF and modified PVDF/TiO₂ membranes. The experimental results indicated that the optimum TiO₂ loading in the membrane was 2 wt%, which enhanced the membrane morphology, permeability, and dye removal performance. The rate of photocatalytic degradation dropped with the increase in dye concentration. The photocatalytic efficiency of the membrane depends on the pH and the temperature of the solution.

Application of Hybrid Membrane Processes Coupling Separation and Biological or Chemical Reaction in Advanced Wastewater Treatment

The rapid urbanization and water shortage impose an urgent need in improving sustainable water management without compromising the socioeconomic development all around the world. In this context, reclaimed wastewater has been recognized as a sustainable water management strategy since it represents an alternative water resource for non-potable or (indirect) potable use. The conventional wastewater remediation approaches for the removal of different emerging contaminants (pharmaceuticals, dyes, metal ions, etc.) are unable to remove/destroy them completely. Hybrid membrane processes (HMPs) are a powerful solution for removing emerging pollutants from wastewater. On this aspect, the present paper focused on HMPs obtained by the synergic coupling of biological and/or chemical reaction driven processes with membrane processes, giving a critical overview and particular emphasis on some case studies reported in the pertinent literature. By using these processes, a satisfactory quality of treated water can be achieved, permitting its sustainable reuse in the hydrologic cycle while minimizing environmental and economic impact.

Ceramic-based photocatalytic membrane reactors for water treatment – where to next?

Ceramic-based photocatalytic membrane reactors (cPMRs) are becoming increasingly popular among researchers and will soon be seen on the water/wastewater-treatment market. This review provides a thorough analysis of the available data on cPMRs fabricated to date based on coating method, support and coating materials, membrane design, pore size and model compounds used to evaluate process efficiency and light source. While all of the studies describe cPMR preparation in great detail, over half do not provide any information about their performance. The rest used various dyes that can be conveniently detected by spectrophotometry/fluorimetry, or micropollutants that require analytical equipment available only in specialized laboratories. In addition, cPMRs are viewed as a convenient way of incorporating a photocatalyst on an inert surface assuming that the surface itself, i.e. the membrane, does not participate in the treatment process. A unified test for cPMR performance should be developed and implemented for all cPMRs that have the potential for commercialization. There is a need for standardization in cPMR testing; only then can the true performance of cPMRs be evaluated and compared. Such testing will also answer the question of whether the cPMR membrane is indeed an inert support or an active part of the treatment process.

A Self-Cleaning Heterostructured Membrane for Efficient Oil-in-Water Emulsion Separation with Stable Flux

Lack of clean water is a major global challenge. Membrane separation technology is an ideal choice for the treatment of industrial, domestic sewage owing to its low energy consumption and cost. However, membranes are highly susceptible to contamination, particularly during wastewater treatment, which has limited their practical applications in this field. Similarly, the flux of the membrane decreases with prolonged use due to its reduced interlayer spacing. Preparation of membranes with anticontamination properties and stable flux is the key to addressing this problem. In this study, a 2D heterostructure membrane with visible-light-driven self-cleaning performance is prepared via a self-assembly process. Notably, the addition of palygorskite increases the interlayer spacing of the graphene and heterojunction structures, which increases the flux of the membrane and avoids a decrease of the interlayer spacing of the membrane under pressure. The presence of a heterojunction with visible light catalytic properties effectively avoids membrane fouling and avoids a sharp decrease of the permeation flux. Importantly, the prepared 2D membrane has excellent separation performance for oil-water emulsions with both high flux and efficiency. These features suggest great potential for the prepared 2D membrane in wastewater treatment applications.

Critical Issues and Guidelines to Improve the Performance of Photocatalytic Polymeric Membranes

Photocatalytic membrane reactors (PMR), with immobilized photocatalysts, play an important role in process intensification strategies; this approach offers a simple solution to the typical catalyst recovery problem of photocatalytic processes and, by simultaneous filtration and photocatalysis of the aqueous streams, facilitates clean water production in a single unit. The synthesis of polymer photocatalytic membranes has been widely explored, while studies focused on ceramic photocatalytic membranes represent a minority. However, previous reports have identified that the successful synthesis of polymeric photocatalytic membranes still faces certain challenges that demand further research, e.g., (i) reduced photocatalytic activity, (ii) photocatalyst stability, and (iii) membrane aging, to achieve technological competitiveness with respect to suspended photocatalytic systems. The novelty of this review is to go a step further to preceding literature by first, critically analyzing the factors behind these major limitations and second, establishing useful guidelines. This information will help researchers in the field in the selection of the membrane materials and synthesis methodology for a better performance of polymeric photocatalytic membranes with targeted functionality; special attention is focused on factors affecting membrane aging and photocatalyst stability.

Ceramic nanocomposite membranes and membrane fouling: A review

Membrane technologies have broad applications in the removal of contaminants from drinking water and wastewater. In recent decades, ceramic membrane has made rapid progress in industrial/municipal wastewater treatment and drinking water treatment owing to their advantageous properties over conventional polymeric membrane. The beneficial characteristics of ceramic membranes include fouling resistance, high permeability, good recoverability, chemical stability, and long life time, which have found applications with the recent innovations in both fabrication methods and nanotechnology. Therefore, ceramic membranes hold great promise for potential applications in water treatment. This paper mainly reviews the progress in the research and development of ceramic membranes, with key focus on porous ceramic membranes and nanomaterial-functionalized ceramic membranes for nanofiltration or catalysis. The current state of the available ceramic membranes in industry and academia, and their potential advantages, limitations and applications are reviewed. The last section of the review focuses on ceramic membrane fouling and the efforts towards ceramic membrane fouling mitigation. The advances in ceramic membrane technologies have rarely been widely reviewed before, therefore, this review could be served as a guide for the new entrants to the field, as well to the established researchers.