Photo-Fenton superwettable NiFe2O4/TA/PVDF composite membrane for organic pollutant degradation with successively oil-in-water separation

Development of inorganic and mixed matrix membranes for application in toxic dyes-contaminated industrial effluents with in-situ treatments

Dyes are the most ubiquitous organic pollutants in industrial effluents. They are highly toxic to both plants and animals; thus, their removal is paramount to the sustainability of ecosystem. However, they have shown resistance to photolysis and various biological, physical, and chemical wastewater remediation processes. Membrane removal technology has been vital for the filtration/separation of the dyes. In comparison to polymeric membranes, inorganic and mixed matrix (MM) membranes have shown potentials to the removal of dyes. The inorganic and MM membranes are particularly effective due to their high porosity, enhanced stability, improved permeability, higher enhanced selectivity and good stability and resistance to harsh chemical and thermal conditions. They have shown prospects in filtration/separation, adsorption, and catalytic degradation of the dyes. This review highlighted the advantages of the inorganic and MM membranes for the various removal techniques for the treatments of the dyes. Methods for the membranes production have been reviewed. Their application for the filtration/separation and adsorption have been critically analyzed. Their application as support for advanced oxidation processes such as persulfate, photo-Fenton and photocatalytic degradations have been highlighted. The mechanisms underscoring the efficiency of the processes have been cited. Lastly, comments were given on the prospects and challenges of both inorganic and MM membranes towards removal of the dyes from industrial effluents.

Construction of self-cleaning g-C3N4/Bi2MoO6/PVDF membrane and coupling with photo-Fenton-like reaction for sustainable removal of antibiotics in wastewater

Constructing a photocatalytic membrane and photo-Fenton reaction coupling system is a novel strategy to enhance the photocatalytic activity of the membrane and eliminate the problem of membrane contamination. Herein, a g-C3N4/Bi2MoO6/PVDF photocatalytic membrane was prepared using a tannic acid-assisted in-situ deposition method. The membrane was characterized by three advantages of photocatalytic, self-cleaning, and antibacterial properties. Under the photo-Fenton-like conditions, the membrane had superior photodegradation efficiency of 90.7% for tetracycline, one of the main antibiotic contaminants in the China's aquatic system. Moreover, the membrane had excellent photo-Fenton self-cleaning ability, its flux recovery rate was up to 96%-98% after the self-cleaning process. Photoluminescence spectra, diffuse UV-visible spectrum, transient photocurrent responses, and electrochemical AC impedance spectrum results show that the heterojunction structure formed by g-C3N4 and Bi2MoO6 could improve the separation efficiency of photogenerated electrons-hole pairs. Electron spin resonance spectroscopy confirmed the photo-electrons facilitated the formation of hydroxyl radical (·OH) in the existence of H2O2, which enhanced tetracycline degradation. Moreover, the superior photo-Fenton self-cleaning performance, which mainly relied on the active free radicals produced by the photo-Fenton-like membrane to remove dirt on the membrane surface or in the membrane pore channel. Our results may shed new light on the development of promising photocatalytic membrane systems by coupling with photo-Fenton-like processes, and facilitate their applications for wastewater treatment.

MoS2/polyaniline (PANI)/polyacrylonitrile (PAN)@BiFeO3 bilayer hollow nanofiber membrane: Photocatalytic filtration and piezoelectric effect enhancing degradation and disinfection

A novel MoS₂/polyaniline (PANI)/polyacrylonitrile (PAN)@BiFeO₃ bilayer hollow nanofiber membrane (PPBM-H) was successfully synthesized by coaxial electrospinning technique. In the nanofiber, BiFeO₃ nanoparticles (NPs) and MoS₂ nanosheets (NSs) were loaded in the middle and outer layers of the PANI/PAN composites, respectively, which constructs a type II heterojunction with spatially separated microtopography, thus significantly improving the charge separation in photocatalysis. Moreover, the hollow structure and the vast number of exposed groups on the surface of PPBM-H help to improve the mass transfer efficiency and pollutant adsorption performance in wastewater treatment. In addition, PPBM-H can generate H₂O₂ by in-situ activation of BiFeO₃/MoS₂ for photo-Fenton catalysis, enabling Fe³⁺ and Fe²⁺ recycling. Also, PPBM-H can produce piezoelectric polarisation under ultrasonic excitation, which can further enhance the efficiency of electron/hole separation and transfer, and induce the generation of active free radicals. Owing to its wonderful self-cleaning effect, the PPBM-H has good mechanical strength (2.95 Mpa), hydrophilicity (11.6°), water flux (1248 L·m^-2·h^-1), BSA rejection (98.8 %), and exhibits distinguished photocatalytic filtration efficiencies (99.5 % tetracycline hydrochloride (TCH) and 99.9 % methyl orange (MO) within 60 min), piezo-photocatalysis (99.2 % TCH within 2 h), disinfection performance for Escherichia coli (E. coli) (100 %, within 60 min).

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.

Efficient oil-water emulsion treatment via novel composite membranes fabricated by CaCO3-based biomineralization and TA-Ti(IV) coating strategy

Continuous increasing discharge of industrial oily wastewater and frequent occurrence of oil spill accidents have taken heavy tolls on global environment and human health. Organic-inorganic modifications can fabricate superhydrophilic/submerged superoleophobic membranes for efficient oil-water separation/treatment though they still suffer from complex operation, non-environmental friendliness, expensive cost or uneven distribution. Herein, a new strategy regarding tannic acid (TA)-Ti(IV) coating and CaCO₃-based biomineralization through simple inkjet printing processes was proposed to modify polyvinylidene fluoride (PVDF) membrane, endowing the membrane with high hydrophilicity (water contact angle (WCA) decreased from 86.01° to 14.94°) and underwater superoleophobicity (underwater contact angle (UOCA) > 155°). The optimized TA-Ti(IV)-CaCO₃ modified membrane possessed perfect water permeation to various oil/water emulsions (e.g., 355.7 L·m^-2·h^-1 for gasoline emulsion) under gravity with superior separation efficiency (>98.8 %), leading the way in oil/water emulsion separation performance of PVDF membranes modified with polyphenolic surfaces to our knowledge. Moreover, the modified membrane displayed rather high flux recovery after eight cycles of filtration while maintaining the original excellent separation efficiency. The modification process proposed in this study is almost independent of the nature of the substrate, and meets the demand for simple, inexpensive, rapid preparation of highly hydrophilic antifouling membranes, showing abroad application prospect for oil-water emulsion separation/treatment.

Designing of nanotextured inorganic-organic hybrid PVDF membrane for efficient separation of the oil-in-water emulsions

The separation of the emulsified oil/water is one of the critical environmental challenges. The PVDF membranes have been found helpful for separation, but rapid fouling makes them less attractive in treating oil-in-water emulsions. The design of antifouling membranes has become an area of deep interest. Herein, developing a novel modified PVDF ultrafiltration membrane was reported by doping the pyrrole and solidifying it in a ferric-containing coagulation bath, resulting in a unique nanotextured PVDF membrane (CCB-Fe/PP_np-PVDF) to separate the oil/water emulsions. The resultant CCB-Fe/PP_np-PVDF membrane was thoroughly characterized using the FTIR, FE-SEM, EDX, mapping, AFM, and contact analyzer. The hydrophilicity of the CCB-Fe/PP_np-PVDF was substantially improved, and the water contact angle was reduced from 81֯ ± 0.9֯ to 44֯ ± 1.7֯. The CCB-Fe/PPnp-PVDF membrane flux increased by 121% compared to the pristine PVDF membrane, with high separation efficiency of 99%. The hydrophilic nanotextured surface of the CCB-Fe/PP_np-PVDF membrane showed good antifouling behavior, with a flux recovery ratio (FRR) of more than 96%. Irreversible flux was just less than 4%. The high flux recovery ratio indicated that the nanotextured surface produced by the Fe/PP_np had prevented the blockage of the membrane pores and compact cake layer formation, which makes it an excellent membrane for oil/water emulsion separation. This strategy can be adopted for designing advanced membranes for separation applications.

NiFe2O4/g-C3N4 heterostructure with an enhanced ability for photocatalytic degradation of tetracycline hydrochloride and antibacterial performance

In this work, NiFe₂O₄/g-C₃N₄ heterostructure was prepared and used for the photocatalytic decomposition of tetracycline hydrochloride antibiotic and for inactivation of E. coli bacteria. The fabricated NiFe₂O₄/g-C₃N₄ composite displayed enhanced ability for photodegradation of organic pollutants and disinfection activities compared to the bare samples, because of the enhancement of visible light absorbance, heterojunction formation and photo-Fenton process. The optimized sample 10%-NiFe₂O₄/g-C₃N₄ has photodegraded 94.5% of tetracycline hydrochloride in 80 min. The active species trapping experiments revels that ·O₂^-, h⁺ and •OH are key decomposing species participated in the antibiotic degradation. It is hoped that the present study will provide a better understanding to fabricate efficient photocatalysts for the decomposition of organic pollutants and disinfection of bacteria.

Mn2+ doped AgInS2 photocatalyst for formaldehyde degradation and hydrogen production from water splitting by carbon tube enhancement

In this work, AgInS₂ and Mn²⁺ doped AgInS₂ (Mn-AgInS₂) with different Mn²⁺: (Ag⁺ + In³⁺) ratios were synthesized via a low temperature liquid method. The photocatalytic activity of the obtained samples was followed by taking formaldehyde as the target pollutant under visible light irradiation. The photocatalysts were passed through various characterization procedures to investigate their morphological, structural and photophysical characteristics. The optimal proportion sample [with the ratio n (Mn²⁺): n (Ag⁺ + In³⁺) = 1:100] photodegraded about 79% formaldehyde in 150 min. These upgraded activities are attributed to the enhanced visible light absorption and superior charge separation due to the presence of Mn²⁺ as confirmed site from charge separation measurements. In addition, a possible mechanism for the photodegradation of formaldehyde is proposed based on the experimental results. Furthermore, the photocatalytic water splitting performance of Mn-AgInS₂ and multi-walled carbon nanotubes (MWCNTs) modified Mn-AgInS₂ is investigated and compared under simulated sunlight irradiation, and remarkable hydrogen production is achieved (105 μmol h^-1 g^-1) by using the latter.

A robust air superhydrophilic/superoleophobic diatomite porous ceramic for high-performance continuous separation of oil-in-water emulsion

Three-dimensional (3D) porous architecture has attracted considerable attention in remediation of oil/water emulsion. In present work, an air superhydrophilic/superoleophobic diatomite porous ceramic (AS-DC) was prepared, using SiO₂ whiskers modified diatomite ceramic as the substrate and FS-50 as the modifier. The interconnected SiO₂ whiskers intertwined on the skeleton of ceramic block forming a 3D network structure, which not only improved the wettability of AS-DC, but also reinforced its mechanical property (about 2.5 MPa of compressive strength). The as-prepared AS-DC with intrinsically superoleophobicity (154°) and superhydrophilicity (0°) exhibited an underwater oil contact angle of 161°, suggesting a multifunctional separation capability. By simply assembling AS-DC with pipes and a pump, it could not only separate the surfactant-stabilized oil-in-water emulsion in a permeation flux as high as 107.8 kg min^-1 m^-2 with a selectivity of >95%, but also collect the clean water from the floating oil/water mixture in a flux of 197.4 kg min^-1 m^-2 and a selectivity of ∼99%. In addition, the AS-DC was resistant to the salt/acid/alkaline corrosion and temperature fluctuation. The mechanical/chemical firmness of AS-DC renders it tremendous potential as a robust 3D architecture in real application for purification of oil/water mixture.

A robust underwater superoleophobic aminated polyacrylonitrile membrane embedded with CNTs-COOH for durable oil/water and dyes/oil emulsions separation

Considering the emulsified oil and water-soluble dyes in wastewater, the exploitation of easy-manufacturing, energy-saving and high-efficiency separation materials is urgently required. In this work, integrating the positively charged polyethyleneimine (PEI) with negatively charged CNTs-COOH constructed the superhydrophilic Cassie-Baxter structure onto the electrospun polyacrylonitrile (PAN) membrane surface by ultrasonic, electrostatic interaction and thermal treatment. Based on it, the PEN@CNTs membrane achieved efficient separation for surfactant-free, tween 80-stabilized, SDS-stabilized, and CTAB-stabilized emulsions (the fluxes reached 508-3158 L m^-2 h^-1, the separation efficiency reached 99.42%) by the splendid water-penetration and oil-repellency, electrostatic interaction, and "aperture sieve". Moreover, because of the porosity and strong charged surface of PEN@CNTs membrane, the anionic dyes can be quickly removed by one-step filtrate method (∼403 L m^-2 h^-1). Meanwhile, the PEN@CNTs membrane also achieved synchronous and efficient remediation for oil/dye mixture emulsions after many cycles. More importantly, facing the complex physical and chemical environments, the combination of the stabilized PEN membrane, inactive CNTs-COOH layer, and the bond of embedding method between CNTs-COOH and PEN nanofibers made the PEN@CNTs membrane demonstrated robust stability and durable separation capability.

Recent Progress in Microfiltration/Ultrafiltration Membranes for Separation of Oil and Water Emulsions

Oily wastewater has become one of the leading causes of environmental pollution. A massive quantity of oily wastewater is released from industries, oil spills, and routine activities, endangering the ecosystem's sustainability. Due to the enormous negative impact, researchers put strenuous efforts into developing a sustainable solution to treat oily wastewater. Microfiltration/ultrafiltration membranes are considered an efficient solution to treat oily wastewater due to their low cost, small footprint, facile operation, and high separation efficiencies. However, membranes severely fouled during the separation process due to oil's adsorption and cake layer formation, which shortens the membranes' life. This review has critically discussed the microfiltration/ultrafiltration membrane synthesizing methods and their emulsion's separation performance. In the end, key challenges and their possible solutions are highlighted to provide future direction to synthesize next-generation membranes.

Sustainable Green Nanotechnologies for Innovative Purifications of Water: Synthesis of the Nanoparticles from Renewable Sources

Polluting the natural water resources is a serious global issue, which is confirmed by the fact that today at least 2 billion people consume water from contaminated sources. The conventional wastewater treatment methods cannot effectively remove the persistent pollutants (e.g., drugs, organic dyes, pesticides) from the aqueous environment. Heterogeneous photocatalysis is a promising and sustainable alternative for water remediation. It is based on the interaction between light irradiation and the semiconductors (e.g., TiO₂, ZnO) as photocatalysts, but these compounds, unfortunately, have some disadvantages. Hence, great attention has been paid to the nanotechnology as a possible way of improvement. Nanomaterials have extraordinary properties; however, their conventional synthesis is often difficult and requires a significant amount of dangerous chemicals. This concise topical review gives recent updates and trends in development of sustainable and green pathways in the synthesis of nanomaterials, as well as in their application for water remediation. In our review we put emphasis on the eco-friendly, mostly plant extract-based materials. The importance of this topic, including this study as well, is proved by the growing number of publications since 2018. Due to the current serious environmental issues (e.g., global warming, shortage of pure and quality water), it is necessary for the traditional TiO₂ and ZnO semiconductors to be replaced with the harmless, non-toxic, and more powerful nanocomposites as photocatalysts. Not only because of their higher efficiency as compared to the bulk semiconductors, but also because of the presence of biomolecules that can add up to the pollutant removal efficiency, which has been already confirmed in many researches. However, despite the fact that the application of heterogeneous photocatalysis together with green nanotechnology is absolutely the future in water purification, there are some challenges which have to be overcome. The exact effects of the biomolecules obtained from plants in the synthesis of nanoparticles, as well as in the photocatalytic processes, are not exactly known and require further investigation. Furthermore, heterogeneous photocatalysis is a well-known and commonly examined process; however, its practical use outside the laboratory is expensive and difficult. Thus, it has to be simplified and improved in order to be available for everyone. The aim of our review is to suggest and prove that using these bio-inspired compounds it is possible to reduce human footprint in the nature.

Tannic acid-based functional coating: surface engineering of membranes for oil-in-water emulsion separation

Recent progress in the tannic acid-based functional coating for surface engineering of membranes toward oil-in-water emulsion separation is summarized.