Engineering superwetting membranes through polyphenol-polycation-metal complexation for high-efficient oil/water separation: From polyphenol to tailored nanostructures

Development of an EOR-produced petroleum wastewater treatment system through integrated polyacrylonitrile membrane and ZrO2/sericin technologies: revelation of interactive mechanism based on synchrotron and XDLVO analyses

Ultrafiltration technology is one of the most efficient methods to address the issues of enhanced oil recovery-produced petroleum wastewater (EOR-PW) treatment. However, membrane fouling significantly impairs the efficiency of PW treatment. Moreover, the impacts of the complex components (e.g., salt ions, heavy metal ions, and pH level) in PW on membrane performance and the underlying mechanisms (i.e., fouling modes and interactive force) need further exploration. Herin, a novel ZrO2/sericin polyacrylonitrile (ZrSS) ultrafiltration membrane was developed for PW treatment, and the impacts and mechanisms of contaminants in PW on membrane filtration performance were systematically investigated using synchrotron-based technology and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis. The synchrotron-based characterization results indicate the successful fabrication of the ZrSS membrane and the uniform distribution of ZrO2/sericin nanocomposites (ZrSS NCs) within the membrane matrix. Optimization results show that the 3ZrSS membrane exhibits the highest water flux of 337.21 LMH and oil rejection of 99.80%. There are 67.58% and 11.04% improvements compared to the pristine PAN (polyacrylonitrile) membrane. Under alkaline pH, high salt ion (NaCl) strength, and low heavy metal ion (Ba2+) concentration, the 3ZrSS membrane experienced the least fouling (22.68% water flux decline). XDLVO theory elucidates that, under such conditions, there is a strong repulsive UTOT (total interaction force) between oil droplets and the 3ZrSS membrane, which is demonstrated via the strong repulsive EL (electrostatic double layer) force. The 3ZrSS membrane maintained 84.84% of its initial water flux after a 72 h long-term filtration. After four cycled filtration, the 3ZrSS membrane kept an extremely high FRR (flux recovery rate) of 98.83%. This study is anticipated to offer technical, theoretical, and practical insights for the on-demand PW treatment.

New Antifouling Dual Iron-Based Photo-Fenton Catalytic Membrane Dedicated to Long-Term Treatment of Oils and Dyes

Combining the photo-Fenton catalytic reaction with membrane separation technology can effectively treat oils and dyes in water. However, single iron-based catalysts suffer from limited photogenerated carrier utilization and slow Fe3+/Fe2+ redox reactions, leading to membrane contamination. In this study, iron oxyhydroxide and Prussian blue were grown on a PVDF membrane, optimizing electron transfer to improve the catalytic efficiency (the methylene blue degradation efficiency was nearly 50% in 10 min and the reaction rate constant was 0.094 min-1). The membrane flux for multicomponent oily wastewater reached 965.3 L·m-2 h-1, with an over 99.9% dye removal rate. The flux of actual printing and dyeing wastewater after continuous filtration for 60 min reached 373.6 L·m-2 h-1. Moreover, the catalytic and filtration mechanisms were explained in detail. The strategy provided in this study has certain practical application value in printing and dyeing wastewater treatment.

Nanofiltration Membranes Containing a Metal-Polyphenol Network Layer: Using Casting Solution pH as a Tool to Tailor the Separation Performance

Thin-film composite (TFC) membranes containing metal-polyphenol network (MPN) selective layers were fabricated using a supramolecular self-assembly between tannic acid (TA) and ferric ion (Fe3+). The TA-Fe3+ thin film was coated on a porous polyacrylonitrile support using aqueous solutions of TA and FeCl3 via a layer-by-layer deposition technique. The pH of the TA solution was used as a tool to alter the membrane characteristics. The surface porosity and water contact angle of the fabricated membranes gradually decreased as the pH of TA casting solutions was increased from 3 to 8.5 for both single-layered and double-layered TA-Fe3+ TFC membranes. This allowed us to tune the water permeance and the retentions of water-soluble neutral and anionic molecules by the MPN membranes by varying the pH of the casting solution. It has been shown that the water permeance decreased from 184 to 156 L·m-2·h-1·bar-1 for single TA-Fe3+ layer coated membranes when the pH was increased from 3 to 8.5, while it declined from 51 to 17 for the double TA-Fe3+ layer. Anionic solutes in aqueous solutions were highly retained compared to neutral components as the TFC membranes had a negative surface charge. Retentions of 95 and 90% were achieved for naphthol green B and orange II dyes by a double-layered M4 membrane fabricated at pH 8.5, while only 13% retention was found for the neutral riboflavin. The neutral dye riboflavin permeated 30.8 times higher than the anionic dye naphthol green B during a mixed dye filtration test through the TFC membrane prepared by using a TA solution of pH 8.5. To the best of our knowledge, this is the highest selectivity of a neutral/anionic dye pair so far reported for a TFC membrane having an MPN selective layer. Moreover, fouling tests have demonstrated that the MPN separation layers exhibit robust stability and adequate antifouling performance with a flux recovery ratio as high as 82%.

Preparation of hydrophilic PVDF membranes through in situ assembly of phytate-polyethyleneimine-Fe3+ for efficient separation of herbal volatile oil from oily water

In the realm of oil-water separation technologies, membrane-based separation emerges as an efficacious approach. Nevertheless, crafting a hydrophilic membrane capable of effectively segregating herbal volatile oil remains a formidable challenge. Our study introduces a facile in situ assembly strategy for fabricating a double-crosslinked composite coating comprising phytate (PA)-polyethyleneimine (PEI) polyelectrolyte complexes and PA-Fe3⁺ assemblies. The PA within the PA-PEI/Fe3⁺ coatings form a double cross-linking layer through interactions with amine groups and metal ions, thereby enhancing interfacial interactions and structural integrity of the membranes. Consequently, the resultant PVDF/PA-PEI/Fe3⁺ membranes exhibit improved coating stability, pronounced hydrophilicity, and exceptional antifouling capabilities, rendering them highly suitable for the separation of diverse herbal volatile oil-in-water emulsions. Furthermore, they possess the capability for reuse with an average retention ratio exceeding 90% and a pure water flux reaching up to 3200 L·m⁻2·h⁻1. Additionally, they demonstrate long-term stability and resistance to corrosion. With a simplistic yet efficient preparation process, the PVDF/PA-PEI/Fe3⁺ membrane holds significant potential for the extraction of oils from herbal volatile oil-in-water emulsions.

Dialdehyde cellulose (DAC) and polyethyleneimine (PEI) coated polyvinylidene fluoride (PVDF) membrane for simultaneously removing emulsified oils and anionic dyes

Developing high-efficiency membrane for oil and dye removal is very urgent, because wastewater containing them can cause great damage to human and environment. In this study, a coated membrane was fabricated by applying DAC and PEI onto the commercial PVDF microfiltration membrane for supplying the demand. The coated membrane presents superhydrophlic and superoleophobic properties with a water contact angle of 0o and underwater oil contact angle exceed 150°, as well as excellent low underwater oil adhesion performance. The coated membrane shows high separation efficiency exceeded 99.0% and flux 350.0 L·m-2·h-1 when used for separating for six kinds of oil including pump oil, sunflower oil, n-hexadecane, soybean oil, diesel and kerosene in water emulsions. Additionally, the coated membrane can effectively remove anionic dyes, achieving rejection rates of 94.7%, 93.4%, 92.3%, 90.7% for the CR, MB, RB5, AR66, respectively. More importantly, the membrane was able to simultaneously remove emulsified oil and soluble anionic dyes in wastewater containing both of them. Therefore, this novel coated membrane can be a promising candidate for treating complex wastewater.

Zero-Oil-Fouling Membrane With High Coverage of Grafted Zwitterionic Polymer for Separation of Oil-in-Water Emulsions

Current hydrophilic modification strategies improve the antifouling ability of membranes but fail to completely eliminate the fouling of emulsified oil droplets with a wide size distribution. Constructing membranes with superior anti-oil-fouling ability to resist various oil droplets especially at high permeation fluxes is challenging. Here, the fabrication of a zero-oil-fouling membrane by grafting considerably high coverage of zwitterionic polymer and building defect-free hydration defense barrier on the surface is reported. A uniform layer of protocatechuic acid with COOH as abundant as existing in every molecule is stably deposited on the membrane so as to provide sufficient reactive sites and achieve dense grafting of the zwitterionic polymer. The coverage of zwitterionic polymer on the membrane plays a crucial role in promoting the antifouling ability to emulsified oil droplets. The poly(vinylidene fluoride) membrane with 93% coverage of the zwitterionic polymer exhibits zero oil fouling when separating multitudinous oil-in-water emulsions with ≈0% flux decline, ≈100% flux recovery, and a high water flux of ≈800 L m-2 h-1 bar-1. This membrane outperforms almost all of the reported membranes in terms of the comprehensive antifouling performance. This work provides a feasible route for manufacturing super-antifouling membranes toward oil/water separation application.

Rapid Construction of Caffeic Acid/p-Phenylenediamine Antifouling Hydrophilic Coating on a PVDF Membrane for Emulsion Separation

The current methods of constructing modification strategies for hydrophilic membranes are time-consuming, complex in operation, and poor in universality, which limit their application on membranes. In this work, inspired by the adhesion properties and versatility of caffeic acid (CA) and p-phenylenediamine (PPDA), a simple, rapid, and universal method was designed for the separation of oil-in-water emulsion by preparing a stable hydrophilic coating separation membrane. The preparation time of the membrane was shortened to 40 min. The developed PVDF-PCA/PPDA membrane showed superhydrophilic and underwater superoleophobic properties. When applied to petroleum ether-in-water emulsion, isooctane-in-water emulsion, and dodecane-in-water emulsion separation, the oil rejection was more than 99.0%. In the circulating separation of 10 g/L soybean oil-in-water emulsion, the oil rejection was more than 99.3%, and the highest flux was 1036 L·m-2·h-1. The prepared PVDF-PCA/PPDA membrane performed well in the separation test of oily wastewater. The proposed strategy is simple and rapid; it may become a universal method for preparing membranes with super strong antifouling properties against viscous oil and accelerate the research progress of membrane separation of oil-in-water emulsions.

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.

Recent advances in eco-friendly fabrics with special wettability for oil/water separation

Considering the serious damage to aquatic ecosystems and marine life caused by oil spills and oily wastewater discharge, efficient, environment-friendly and sustainable oil/water separation technology has become an inevitable trend for current development. Herein, fabrics are recognized as eco-friendly materials for water treatment due to their good degradability and low cost. Particularly, fabrics with rough structures and natural hydrophilicity/oleophilicity enable the construction of superwetting surfaces for the selective separation of oil/water mixtures and even complex emulsions. Therefore, superwetting fabrics for efficiently solving oil spills and purifying oily wastewater have received extensive attention. Especially, Janus and smart fabrics are highly anticipated to enable the on-demand and sustainable treatment of oil spills and oily wastewater due to their changeable wettability. Moreover, the fabrication of superwetting fabrics with multifunctional performances for oily wastewater purification can further promote their practical industrial applications, such as photocatalytic, self-cleaning, and self-healing characteristics. However, some potential challenges still exist, which urgently need to be systematically summarized to guide the future development of this research field. In this review, firstly, the fundamental theories of wettability and the separation mechanisms based on special wettability are discussed. Then, superwetting fabrics for efficient oil/water separation are systematically reviewed, such as superhydrophobic/superoleophilic (SHB/SOL), superhydrophilic/superoleophobic (SHL/SOB), SHL/underwater superoleophobic (SHL/UWSOB), and UWSOB/underoil superoleophobic (UWSOB/UOSHB) fabrics. Most importantly, we highlight Janus, smart, and multifunctional fabrics based on their superwetting property. Correspondingly, the advantages and disadvantages of each superwetting fabric are comprehensively analyzed. Besides, super-antiwetting fabrics with superhydrophobic/superoleophobic (SHB/SOB) property are also introduced. Finally, the challenges and future research directions are explained.

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.

Facile ultrasonic preparation of a polypyrrole membrane as an absorbent for efficient oil-water separation and as an antimicrobial agent

Polypyrrole (PPY) spherical particles synthesized using carbon dots as an efficient catalyst were strongly embedded on fluorinated nonwoven fabric by ultrasonication to form a membrane with high hydrophilicity. An optimal amount of PPY adhered to the membrane after 30 min of sonication enhanced the overall membrane area with high hydrophilicity. Oil with high hydrophobicity was repelled by the resulting membrane, whereas water was freely penetrated and diffused from the membrane. The membrane exhibited good reusability and efficiency for the recovery of oil from a cooking oil-water mixture within 30 s. The incorporation of PPY in the fluorinated fabric imparts significant antibacterial properties against two common pathogens, Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive). The anti-biofouling membrane could pave the way for its potential application to separate spilled oil from contaminated waters, comprising different microorganisms and living species. The novelty of this manuscript is described in a new system, the fabrication of PPY membranes with two important properties: biocidal and oil/water separation.

Cellulose-based special wetting materials for oil/water separation: A review

Oil spill accidents and oily wastewater discharged by petrochemical industries have severely wasted water resources and damaged the environment. The use of special wetting materials to separate oil and water is efficient and environment-friendly. Cellulose is the most abundant renewable resource and has natural advantages in removing pollutants from oily wastewater. The application and modification of cellulose as special wetting materials have attracted considerable research attention. Therefore, we summarized cellulose-based superlipophilic/superhydrophobic and superhydrophilic/superoleophobic materials exhibiting special wetting properties for oil/water separation. The treatment mechanism, preparation technology, treatment effect, and representative projects of oil-bearing wastewater are discussed. Moreover, cellulose-based intelligent-responsive materials for application to oil/water separation and the removal of other pollutants from oily wastewater have also been summarized. The prospects and potential challenges of all the materials have been highlighted.