Hydrophilic and underwater superoleophobic porous graphitic carbon nitride (g-C3N4) membranes with photo-Fenton self-cleaning ability for efficient oil/water separation

Illuminating for purity: Photocatalytic and photothermal membranes for sustainable oil-water separation

The integration of photocatalytic and photothermal materials with oil-water separation membranes marks a significant advancement in sustainable separation technologies. These hybrid membranes exhibit exceptional functionalities, including resistance to oil fouling, self-cleaning, antibacterial properties, and reduced oil viscosity. Based on their reaction mechanisms, current photocatalytic and photothermal membranes are categorized into four types, i.e., photocatalytic membranes, photo-Fenton membranes, PMS-assisted photocatalytic membranes, and photothermal membranes. Under light irradiation, photocatalytically functionalized membranes generate reactive oxygen species (ROS) that degrade organic pollutants and inactivate bacteria on the membrane surfaces, enabling in-situ cleaning and regeneration. In addition to the above benefits, photothermal membranes achieve reduction of oil viscosity for higher membrane permeation and removal of light oil from membrane surfaces through light-induced heating. This review first explores the mechanisms underlying light-driven advanced oxidation processes (AOPs) and photothermal effects, followed by an in-depth discussion on the fabrication methods of these membranes. Additionally, the applications of photocatalytic and photothermal membranes in oil-water separation are examined, with an emphasis on how the photocatalytic and photothermal materials contribute to membrane functionality. Finally, this review presents the challenges currently faced by photocatalytic and photothermal membranes and outlines future research directions.

High-permeance nanocellulose/ZnO hybrid membranes with photo-induced anti-fouling performance for wastewater purification

A hybrid ultrafiltration membrane based on nanocellulose and zinc oxide nanoparticles (ZnO NPs) was prepared by simple layered filtration without any chemical modification. Microscopic morphology analysis showed that the loading ZnO NPs significantly increased the membrane roughness, and wettability test demonstrated that the membrane surface possessed underwater superoleophobicity. Due to the "puncture effect" of the embedded ZnO NPs, abundant nanochannels were formed in the nanocellulose membrane and the highest water permeance of 5439.7 L·m-2·h-1·bar-1 was achieved. The hybrid membranes exhibited high rejection of nanoparticles larger than 20 nm and macromolecules with molecular weights higher than 100 kDa. Furthermore, ZnO NPs significantly improved the wet tensile strength of membrane. The hybrid membranes achieved high separation efficiency of nano-sized emulsions via size exclusion and demulsification effect, as well as the efficient removal of organic dyes and antibiotics via filtration-adsorption. The combination of underwater superoleophobicity and photocatalytic self-cleaning performance effectively solved the problem of a sharp decrease in permeance caused by oil contamination. This type of nanocellulose/ZnO hybrid membrane, which integrates high permeance, high filtration accuracy, and photocatalytic anti-fouling performance in one design, offers an innovative approach to the preparation of nanocellulose membranes for the treatment of organic wastewater.

Cellulose-based underwater superoleophobic coatings with robust anti-viscous oil-fouling property for complex oily wastewater remediation

Underwater superoleophobic coatings, known for their anti-oil-fouling properties, have garnered significant interest in the context of oily wastewater remediation. However, these coatings encounter challenges in preventing viscous oil contamination and structural damage, and easily become ineffective when treating crude oil/water pollutants. Additionally, the non-renewable and non-biodegradable components pose a huge risk to environmental safety and sustainable development. Herein, a cellulose-based coating that combines robust underwater superoleophobicity with anti-viscous oil-fouling characteristic is designed via the extraction of micro/nanoscale heteromorphic cellulose crystals (EHCC) and subsequent crosslinking with carboxymethyl chitosan (CCS). Leveraging the hierarchical micro/nanostructures constructed by EHCC and intensified hydration capability facilitated by multiple hydrogen bonding interactions, the EHCC-CCS coating demonstrates excellent superhydrophilicity/underwater superoleophobicity and ultralow-viscous oil-adhesion property. Moreover, the EHCC-CCS coating exhibits robust chemical resistance and mechanical tolerance. Importantly, it adapts effectively to various flat and porous substrates, offering outstanding anti-oil-fouling and self-cleaning performances. Notably, the EHCC-CCS-coated textile is applied in separating immiscible oil/water mixtures with varying oil viscosities, and the EHCC-CCS-coated PVDF membrane achieves to purify surfactant-stabilized crude oil/water emulsion. The findings provide a straightforward and cost-effective approach for large-scale production of fully biobased coatings with durable underwater superoleophobicity and excellent anti-viscous oil-fouling capability for complex oily wastewater remediation.

Design of multiple anti-fouling and honeycomb-like NH2-AgBiS2 @g-C3N4 hydrogel layer onto PAN fiber membrane for multicomponent pollutant-oil-water emulsion treatment

Nano-structured hydrogel with unique anti-oil fouling property exhibits big advantage in oil/water separation, but its application in complex oily wastewater (contain oils, organic matter, bacteria, etc.) cleanup is hampered by the insufficient capabilities in multi-antifouling and synergistic treatment. Herein, we constructed the amino-rich NH2-AgBiS2/PANI (polyaniline)-g-C3N4 based multi-functional hydrogel functional layer onto the polyacrylonitrile (PAN) fiber membrane via polyphenol-mediated chitosan gelation and vacuum-assisted self-assembly techniques. The unique honeycomb-like structure and super-wetting feature synergistically contributed to the powerful oil resistance and flux breakthrough of composite membrane. Such membrane achieved superior permeability (up to 3558 L-1 m-2 h-1) for various SDS-stabilized oil-in-water emulsions and remarkable synergistic treatment efficiency of multicomponent pollutant-oil-water emulsion. The rational design of hydrogel layer on membrane surface intensified the photo-response ability and multiple electron transport channels, which offered the favorable photocatalytic self-cleaning performance towards degradation of organic dyes. According to the free radical quenching and EPR experiments, the photocatalytic mechanism was proposed. In addition, the inhibition rate of E. coli could reach 100 % under illumination of 24 h. Therefore, the integration of ultra-low oil adhesion, photocatalytic self-cleaning, and antibacterial features endows membrane with exceptional multiple anti-fouling performance, exhibiting unique advantages over traditional membranes in handling complex membrane fouling issues.

Facile fabrication of g-C3N4/Bi2S3 coated melamine foam for oil/water separation applications

Regular occurrences of oil leaks are recognized as a significant contributor to water pollution, resulting in substantial environmental and ecological challenges, as well as posing potential for fires and explosions. Therefore, it is imperative to create a cost-effective and exceptionally effective absorbent material for separating oil and water. Hydrophobic, foam-like materials have garnered considerable attention as potential absorbers for addressing oil spills and recovering oil from water sources. In this experimental study, simple, low-cost, environmentally friendly, highly hydrophobic, and super oleophilic g-C3N4/Bi2S3 nanocomposite-coated melamine foam was introduced for oily wastewater treatment. The g-C3N4 and Bi2S3 were synthesized by thermal decomposition and hydrothermal methods, and the g-C3N4/Bi2S3 composite-coated foam was prepared by a simple dip-coated method. The g-C3N4/Bi2S3 composite-coated melamine foam shows excellent absorption capacity, and it can absorb various oils and solvents and separate different oils and solvents from water. Hence, the developed g-C3N4/Bi2S3 foam absorbent has excellent potential in oil/water separation applications.

Z-scheme MnO2/Mn3O4 heterojunctions with efficient peroxymonosulfate activation for organic pollutant removal

The exploration of efficient heterogeneous catalysts for persistent organic pollutant removal is extremely attractive. In the present work, MnO2/Mn3O4 photo-Fenton catalysts were designed by a facile hydrothermal route to activate peroxymonosulfate (PMS) under visible light irradiation for organic pollutant degradation. The optimized MnO2/Mn3O4 heterojunction shows excellent Rhodamine B (RhB) removal efficiency, whose apparent kinetic constant is 11.9 and 5.36 times as high as the MnO2 and Mn3O4. Meanwhile, there is a neglectable attenuation in catalytic performance after 5 recycling runs. Based on the active species trapping experiments, the non-radical process contributes more than the radical process during RhB degradation. Moreover, factors including the dosage of PMS, initial RhB concentration, initial pH, the presence of various anions, different organic pollutants, and water sources are investigated. Systematical characterizations reveal that the enlarged specific surface areas and the efficient charge separation aroused from the Z-scheme mechanism are attributed to the enhanced photo-Fenton performance. The present work contributes to the construction of the Mn-based photo-Fenton catalyst with efficient PMS activation capacity for environmental remediation.

Synthesis and up-to-date applications of 2D microporous g-C3N4 nanomaterials for sustainable development

In recent years, with the development of industrial technology and the increase of people's environmental awareness, the research on sustainable materials and their applications has become a hot topic. Among two-dimensional (2D) materials that have been selected for sustainable research, graphitic phase carbon nitride (g-C3N4) has become a hot research topic because of its many outstanding advantages such as simple preparation, good electrochemical properties, excellent photochemical properties, and better thermal stability. Nevertheless, the inherent limitations of g-C3N4 due to its relatively poor specific surface area, rapid charge recombination, limited light absorption range, and inferior dispersion in aqueous and organic media have limited its practical application. In the review, we summarize and analyze the unique structure of the 2D microporous nanomaterial g-C3N4, its synthesis method, chemical modification method, and the latest application examples in various fields in recent years, highlighting its advantages and shortcomings, with a view to providing ideas for overcoming the difficulties in its application. Furthermore, the pressing challenges faced by g-C3N4 are briefly discussed, as well as an outlook on the application prospects of g-C3N4 materials. It is expected that the review in this paper will provide more theoretical strategies for the future practical application of g-C3N4-based materials, as well as contributing to nanomaterials in sustainable applications.

Robust self-cleaning membrane with superhydrophilicity and underwater superoleophobicity for oil-in-water separation

The discharge of oily wastewater has increased dramatically and will bring serious environmental problems. In this work, a self-cleaning and anti-fouling g-C₃N₄/TiO₂/PVDF composite membrane was fabricated via the layer-by-layer approach. The surface of as-prepared composite membrane displayed a superhydrophilic and underwater superoleophobic behavior under irradiation with visible light. Also, upon irradiation with visible light, the fabricated g-C₃N₄/TiO₂/PVDF composite membrane displayed enhanced permeation flux and improved oil removal efficiency as a result of the generation of hydroxyl free radicals during the photocatalytic filtration process. Significantly, irradiation with visible light remarkably improved reusability of the composite membrane by initiating photocatalytic decomposition of deposited oil foulants, which enabled removal of over 99.75% of oils, thus reaching a nearly 100% flux recovery ratio. Furthermore, the g-C₃N₄/TiO₂/PVDF composite membrane exhibited great anti-fouling behavior in photocatalysis-assisted filtration. The mechanistic study revealed that underwater superhydrophobicity and the generation of free hydroxyl radicals jointly contributed to membrane anti-fouling. The greatest advantages of this g-C₃N₄/TiO₂/PVDF composite membrane are that not only does it degrades the oil pollutants, but it also makes the membrane less vulnerable to fouling.

A solar-driven degradation-evaporation strategy for membrane self-cleaning in the efficient separation of viscous crude oil/water emulsions

Membrane separation techniques are promising methods for effectively treating hazardous emulsified oily wastewater, but membrane fouling remains a serious challenge because the high viscosity and complex composition of crude oil make it easy to adhere to membranes and difficult to be removed by conventional physical or chemical cleaning means. Herein, a two-stage solar-driven (photo-Fenton degradation/evaporation) strategy was proposed to realize the self-cleaning of membranes fouled by viscous crude oil (>60,000 mPa s), wherein the photo-Fenton process helped to degrade the heavy components into light components, and all light components removed during the solar-driven evaporation process. A 1D/2D heterostructure membrane with photo-Fenton activity and anti-crude-oil-fouling performance was prepared via a facile self-assembly vacuum-assist method. The addition of rod-like g-C₃N₄ (RCN) increased the interlayer distance of α-FeOOH/porous g-C₃N₄ (FPCN) nanosheets, resulting in a high permeation flux. The FPCN-RCN membrane exhibited both high permeation flux of 779 ± 19 L m^-2h^-1bar^-1 and a separation efficiency of 99.4% for highly viscous crude oil-in-water emulsion. Importantly, the viscous crude oil fouled on the membrane was completely removed by the photo-Fenton degradation/solar-driven evaporation strategy, and the flux recovery rate of the membrane was ∼100%. Therefore, the FPCN-RCN membrane combined with the novel self-cleaning strategy exhibits great potential for practical emulsified oily wastewater treatment.

Exploring the use of sodium caseinate-assisted responsive separation for the treatment of washing effluents in shoreline oil spill response

The produced effluents after shoreline washing contain a certain number of oil droplets and further treatment is necessary. In this study, the innocuous, widely available, and biodegradable sodium caseinate (NaCas) was deployed to capture oil pollutants from oily wastewater. Oil droplets can be effectively and rapidly captured by NaCas and subsequently removed after pH-triggered separation, producing a clean supernatant with low turbidity. The removal efficiency was enhanced by increasing NaCas concentration and separation time. The salinity inhibited the oil removal by increasing the interfacial tension of NaCas and reducing their sorption sites caused by the large particle size. Humic acid negatively influenced the oil separation performance of NaCas because of the competitive sorption and enhanced repulsion force between oil and NaCas. In addition, the increasing temperature was found to augment the oil removal. Factorial analysis revealed the individual factors and two-factor interactions that had significant effects on oil removal. Biotoxicity experiments proved that NaCas can fully offset the inhibitory effect of oil on the photosynthesis of algae and thus promote algae growth. Two post-treatment methods, namely thermal treatment, and biodegradation, can be used for the post-treatment of NaCas/oil precipitation residues. The use of NaCas-assisted responsive separation in the treatment of washing effluents can help achieve a sustainable shoreline oil spill response.

Application of Photo-Fenton-Membrane Technology in Wastewater Treatment: A Review

Photo-Fenton coupled with membrane (photo-Fenton-membrane) technology offers great potential benefits in future wastewater treatment because it can not only degrade refractory organics, but also separate different pollutants from water; additionally, it often has a membrane-self-cleaning ability. In this review, three key factors of photo-Fenton-membrane technology, photo-Fenton catalysts, membrane materials and reactor configuration, are presented. Fe-based photo-Fenton catalysts include zero-valent iron, iron oxides, Fe-metal oxides composites and Fe-based metal-organic frameworks. Non-Fe-based photo-Fenton catalysts are related to other metallic compounds and carbon-based materials. Polymeric and ceramic membranes used in photo-Fenton-membrane technology are discussed. Additionally, two kinds of reactor configurations, immobilized reactor and suspension reactor, are introduced. Moreover, we summarize the applications of photo-Fenton-membrane technology in wastewater, such as separation and degradation of pollutants, removal of Cr(VI) and disinfection. In the last section, the future prospects of photo-Fenton-membrane technology are discussed.

Surfactant-Free Emulsion of Epoxy Resin/Sodium Alginate for Achieving Robust Underwater Superoleophobic Coating via a Combination of Phase Separation and Biomineralization

Underwater superoleophobic coatings exhibit promising prospects in the field of oil contamination resistance. However, their poor durability, stemming from the fragile structures and unstable hydrophilicity, greatly restricted their development. In this report, we proposed a novel strategy of combination water-induced phase separation and biomineralization to prepare the robust underwater superoleophobic epoxy resin-calcium alginate (EP-CA) coating by utilizing a surfactant-free emulsion of epoxy resin/sodium alginate (EP/SA). The EP-CA coating not only exhibited excellent adhesion to various substrates, but also had remarkable resistance to the physical/chemical attacks such as abrasion, acid, alkali and salt. It could also protect the substrate (e.g., PET substrate) from the damage of organic solution and the fouling of crude oil. This report provides a new perspective to fabricate robust superhydrophilic coating with a facile way.

A pH-responsive phosphoprotein washing fluid for the removal of phenanthrene from contaminated peat moss in the cold region

Oil pollution is one of the major environmental concerns in the petroleum industry. In this study, a cheap food-grade sodium caseinate (NaCas) was used as a pH-responsive washing fluid in the remediation of phenanthrene (PHE) affected peat moss. The effects of environmental factors on the removal of PHE were systematically investigated. The results showed that increasing NaCas concentration and washing temperature improved the PHE mobilization, while high salinity and humic acid dosage displayed a negative effect. The factorial analysis revealed that three individual factors and two interactions exhibited significant effects on the washing performance. Due to the pH-responsive property of NaCas, the turbidity, total organic carbon (TOC), and chemical oxygen demand (COD) of the washing effluent were remarkably reduced by simply adjusting the solution acidity, improving the practical application of such a washing method. Significantly, the toxicity modeling proved that NaCas can reduce the binding energy between PHE and superoxide dismutase (SOD) of the selected marine organism, and thus relieve the toxicity of PHE to the organisms. Given these advantages, NaCas-assisted washing can be a viable option for the remediation of contaminated peat moss.

A biomimetic structured bio-based flame retardant coating on flexible polyurethane foam with low smoke release and antibacterial ability

Flexible polyurethane foam (FPUF) is widely used in our life, but it is inherent flammable. The demand for environmental-friendly multi-functional FPUF has been increasing rapidly in the last decade. In this work, a novel bio-based flame retardant coating was constructed by chemically reacting sodium alginate (OSA) and polydopamine (PDA) on the FPUF, followed by depositing nanorod-like β-FeOOH molecules through complexation reaction to form a biomimetic structure. The limiting oxygen index of the coated FPUF samples reached 25.5%. The peak heat release rate was reduced by 45.0%, and the smoke density of the coated sample was decreased by 69.1% compared to that of the control FPUF sample. It was proposed that the OSA-PDA-β-FeOOH decomposed during combustion to promote the formation of compact crosslinked char and released inert gases to dilute the combustible gases, and the β-FeOOH transferred to Fe₂O₃ to settled the smoke particles reducing the smoke release. Furthermore, the coating with shark skin like structure endowed FPUF antibacterial ability because of its good superoleophobicity underwater. This work provided a novel strategy to construct a biomimetic multifunctional coating on the FPUF.

Green and controllable synthesis of kelp-like carbon nitride nanosheets via an ultrasound-mediated self-assembly strategy

The application of graphite carbon nitride photocatalysts is hampered by their low specific surface areas, few active sites, and low photogenerated electron-hole transfer rates. Here, we report a green and controllable strategy for synthesizing kelp-like carbon nitride nanosheets through self-assembled materials prepared from melamine and trithiocyanuric acid using sonochemistry. The prepared carbon nitride nanosheets showed superior and long-lasting photocatalytic activity in hydrogen evolution and the degradation of tetracycline hydrochloride. The significantly enhanced photocatalytic performance of carbon nitride nanosheets is attributed to the curled porous nanosheet structure and the appropriate amount of O-doping. This work provides a new design strategy for preparing shape-controlled carbon nitride catalysts via a green, fast, sonochemically mediated self-assembly approach.

Magnetically recoverable poly (methyl methacrylate-acrylic acid)/iron oxide magnetic composites nanomaterials with hydrophilic wettability for efficient oil-water separation

Due to the environmental and production problems of emulsion, it is important to efficiently separate oil-water emulsion to meet the refinery requirement and clean up oil spills. Synthesis of a universal demulsifier is not an easy task because the physical properties of crude oil vary, which makes its characterization and demulsification procedure difficult. To overcome this problem, hydrophilic and magnetically recoverable poly (methyl methacrylate-acrylic acid)/iron oxide magnetic composite nanoparticles ((P(MMA-AA)/Fe₃O₄ NPs) were developed as an efficient and economical demulsifier via soap-free emulsion polymerization. To characterize the magnetic composite NPs for their appropriate surface morphology and magnetic domain, TEM, FTIR, VSM, and TGA analyses were carried out. The newly synthesized NPs displayed good hydrophilic properties as they migrated quickly to the aqueous emulsion phase, which was also reassured by their water contact angle of 75°. They exhibit strong magnetic characteristics (20 amu/g) in the oil-water emulsion, makings the hydrophilic wettability capable and attractive to the external magnet. Experimental results revealed that the prepared magnetic composite NPs separated 99% of the water from stable emulsion in 30 min and could be recycled 8 times through magnetic separation. The recycled magnetic composite NPs maintain their hydrophilic wettability and efficiency in separating oil-water emulsion, making them economical and commercially viable. The migration of magnetic composite NPs to the aqueous phase in the stable emulsion with a strong magnetic domain explains the coalescence of emulsified water droplets and their quick separation from the stable emulsions through the external magnet.

Exploring the characteristics, performance, and mechanisms of a magnetic-mediated washing fluid for the cleanup of oiled beach sand

In the present study, an innovative, environmentally benign recyclable, and magnetically mediated surface washing fluid based on water-dispersible magnetite nanoparticles has been designed and investigated for the cleanup of oiled beach sand. The characterization results showed that the as-prepared magnetite nanoparticles had a spherical morphology with an average diameter of around 250 nm and the particle surface was successfully functionalized with carboxyl groups. The magnetite nanoparticles could be easily re-dispersed by lightly shaking the dispersion after withdrawing the magnet. In addition, prolonging the magnetic field strength and response time promoted the oil recovery from the washing effluent. Thermodynamic modeling was applied to theoretically elucidate the mechanism and the results were in alignment with the experimental findings. Four mechanisms were identified to likely affect surface washing performance. The magnetic fluid had a relatively low operation cost and good reusability for a number of multiple cycles. In terms of other operational limitations, it was noted that washing performance declined as clay (kaolinite) concentrations and salinity values increased. Based on these findings, the proposed stable, low-cost magnetite fluid formulation warrants further investigation as the basis for an operational system for the cleanup of sand beaches contaminated by oil spills.

Cleanup of oiled shorelines using a dual responsive nanoclay/sodium alginate surface washing agent

Oil spills may affect ecosystems and endanger public health. In this study, we developed a novel and dual responsive nanoclay/sodium alginate (NS) washing fluid, and systematically evaluated its application potential in oiled shoreline cleanup. The characterization results demonstrated that sodium alginate combined with nanoclay via hydrogen bonds, and was inserted into the interlayer spacing of nanoclay. Adding sodium alginate reduced surface and interfacial tensions, while increasing the viscoelasticity of the washing fluid. Batch experiments were conducted to investigate oil removal performance under various conditions. Additionally, the factorial design analysis showed that three single factors (temperature, oil concentration, and salinity), and two interactive effects (temperature/salinity; and oil concentration/HA) displayed significant effects on the oil removal efficiency of the NS washing fluid. Compared to the commercial surfactants, the NS composite exhibited satisfactory removal efficiencies for treating oily sand. Green materials-stabilized Pickering emulsion can potentially be used for oil/water separation. The NS washing agent displayed excellent pH- and Ca²⁺- responsiveness, generating transparent supernatants with low oil concentration and turbidity. Our work opens an interesting avenue for designing economical, high performance, and green washing agents.