Sugarcane-based superhydrophilic and underwater superoleophobic membrane for efficient oil-in-water emulsions separation

The development of ecological, low cost, easy preparation, especially high performance materials for emulsions separation is of great importance due to the rise in pollution of oil-water emulsions from industrial production and domestic waste. Straws as agricultural wastes, including plenty of hydrophilic groups and multi-level pore structures, can be prepared as biomass membranes for oil-water emulsion separation. Herein, a novel super-hydrophilic sugarcane-based (SHS) membrane was prepared using a facile and eco-friendly method including chemical treatment and freeze-drying. The as-prepared SHS membrane has unique wettabilities due to the hydrophilic property of the internal cellulose and the micro-nano pores, including superhydrophilicity (water contact angle of 0°) and underwater superoleophobicity (underwater oil contact angles of over 150°). The SHS membrane has good durability and stability against ultraviolet (UV) irradiation, corrosion by acids and alkalis, mechanical abrasion and especially mould adhesion. Importantly, the SHS membrane can be used for separation of various oil-in-water emulsions, and exhibits excellent separation performances such as high separation efficiency (> 99 %) and good separation flux (above 891 L m-2 h-1 bar-1). The SHS membrane also exhibits excellent recyclability over 10 continuous separation cycles. Furthermore, the SHS membrane can be utilized to selectively absorb water from oils as a water absorbent material. Hence, SHS membrane is a promising and practical material for applications in treatment of wastewater containing oil-water emulsions.

Biomimetic materials in oil/water separation: Focusing on switchable wettabilities and applications

Clean water resources are crucial for human society, as the leakage and discharge of oily wastewater not only harm the economy but also disrupt our living environment. Therefore, there is an urgent need for efficient oil-water separation technology. Surfaces with switchable superwetting behavior have garnered significant attention due to their importance in both fundamental research and practical applications. This review introduces the fundamental principles of wettability in the oil-water separation process, the basic theory of switchable wettability, and the mechanisms involved in oil-water separation. Subsequently, the review discusses the research progress, challenges, and issues associated with three conventional types of special wettability materials: superhydrophobic/superoleophilic materials, superhydrophilic/superoleophobic materials, and superhydrophilic/underwater superoleophobic materials. Most importantly, it provides a detailed exploration of recent advancements in switchable wettability smart materials, which combine elements of traditional special wettability materials. These include stimulus-responsive smart materials, pre-wetting-induced materials, and Janus materials. The discussion covers key response factors, detailed examples of representative works, design concepts, and fabrication strategies. Finally, the review offers a comprehensive summary of switchable superwetting smart materials, encompassing their advantages and disadvantages, persistent challenges, and future prospects.

A review of self-cleaning photocatalytic surface: Effect of surface characteristics on photocatalytic activity for NO

Self-cleaning surface has attracted much attention in the field of photocatalytic degradation of NO due to its dirt pickup resistance and self-cleaning effect under the action of rainwater. In this review, the factors affecting NO degradation efficiency were analyzed in terms of photocatalyst characteristics and environmental conditions combined with the photocatalytic degradation mechanism. The feasibility of photocatalytic degradation of NO on superhydrophilic, superhydrophobic and superamphiphobic surfaces was discussed. Furthermore, the effect of special surface characteristics of self-cleaning on photocatalytic NO was highlighted and the improvement of the long-term effect using three self-cleaning surfaces on photocatalytic NO was evaluated and summarized. Finally, the conclusion and outlook were proposed related to the self-cleaning surface for photocatalytic degradation of NO. In future research, the comprehensive effects of the characteristics of photocatalytic materials, self-cleaning characteristics and environmental factors on the photocatalytic degradation of NO and the actual application effects of such self-cleaning photocatalytic surfaces should be further clarified in combination with the engineering. It is believed that this review can provide some theoretical basis and support for the development of self-cleaning surfaces in the field of photocatalytic degradation of NO.

Polyester fabrics coated with cupric hydroxide and cellulose for the treatment of kitchen oily wastewater

In recent years, kitchen oily wastewater has received much attention because of its harmful effects on the ecological environment. Therefore, separation of oil from kitchen oily wastewater has become an urgent issue. In this study, this problem could be solved using polyester fabrics covered with cupric hydroxide and cellulose. The functional fabric was obtained by the dipping-rolling-drying process which is an easy and practical way to prepare the fabric and could improve the hydrophilicity of polyester. The functional polyester fabric could separate oil/water mixtures completely under the force of gravity with a high water flux of 2079 L m^-2 h^-1-3620 L m^-2 h^-1 and high separation efficiency of 99.6%. Because kitchen oily wastewater contains floating oil and emulsified oil, we also tested the separation of oil-in-water emulsions. The functional polyester fabric could successfully separate the emulsions with the water flux of 1210 L m^-2 h^-1-2018 L m^-2 h^-1 and a separation efficiency of 99.0%. Moreover, the water flux and separation efficiency of functional polyester fabric remained unchanged after the immersion in salt, alkali, and acid solutions, indicating that the functional polyester fabric exhibited commendable environmental stability. The oil in Chongqing Street Noodles soup with a high oil content was separated to simulate real-life oil/water separation, confirming that the functional polyester fabric could be applied to the treatment of kitchen oily wastewater.

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.

One-pot route to synthesize HNTs@PVDF membrane for rapid and effective separation of emulsion-oil and dyes from waste water

Water-purification membranes with high water flux, excellent recycling ability, good anti-fouling property, wide range for different oil/water emulsions separation, and strong water-soluble dyes removal ability are highly desirable for water purification but still a challenge to be realized. In this work, we show a simple method to address this challenge, in which halloysite nanotubes (HNTs) have been decorated on the poly (vinylidene fluoride) (PVDF) membrane surface through one-pot method via the aid of KH550 and dopamine. The HNTs@PVDF composite membrane exhibits superhydrophilic and under-water superoleophobic performance. In addition, our membrane can separate wide oil/water emulsions (including surfactant-stabilized and surfactant-free emulsions) from wastewater with high efficiency (>99.65%), outstanding recycle ability with good flux recovery rate (FRR > 95%) and good anti-fouling performance (the underwater oil contact angle (OCA) is 155.8 ± 1.8° with low oil adhesion) and high water flux (7994 ± 150 L m^-2 h^-1). Interestingly, our membrane also removes different water-soluble dyes (e.g., Congo red, methylene blue, and rhodamine B) from waste water by simply filtering. Those outstanding properties make our membrane hold great potential applications in real-world water purification and environmental protection.

A versatile CeO2/Co3O4 coated mesh for food wastewater treatment: Simultaneous oil removal and UV catalysis of food additives

Food waste water is one of the most urgent environmental problems for the close connection between food and our daily life. Herein, we use a simple hydrothermal method to prepare a highly efficient catalyst-CeO₂/Co₃O₄ compound on the stainless steel mesh, aiming for food waste water treatment. Possessing the superhydrophilic property and catalytic ability under ultraviolet light, CeO₂/Co₃O₄ coated mesh has successfully processed three representative contaminants in food wastewater, which are soybean oil (food oil), AR (food dye) and VA (food flavor) simultaneously with an one-step filtration. Besides, the mesh is stable in a wide pH range and performs well in reusability. Therefore, such a multifunctional material with simple preparation method, high processing efficiency and facile operation shows a promising prospect for practical production and application for food wastewater treatment.

Membrane fouling and wetting in membrane distillation and their mitigation by novel membranes with special wettability

Membrane distillation (MD) has been identified as a promising technology to desalinate the hypersaline wastewaters from fracking and other industries. However, conventional hydrophobic MD membranes are highly susceptible to fouling and/or wetting by the hydrophobic and/or amphiphilic constituents in these wastewaters of complex compositions. This study systematically investigates the impact of the surface wetting properties on the membrane wetting and/or fouling behaviors in MD. Specifically, we compare the wetting and fouling resistance of three types of membranes of different wetting properties, including hydrophobic and omniphobic membranes as well as composite membranes with a hydrophobic substrate and a superhydrophilic top surface. We challenged the MD membranes with hypersaline feed solutions that contained a relatively high concentration of crude oil with and without added synthetic surfactants, Triton X-100. We found that the composite membranes with superhydrophilic top surface were robustly resistant to oil fouling in the absence of Triton X-100, but were subject to pore wetting in the presence of Triton X-100. On the other hand, the omniphobic membranes were easily fouled by oil-in-water emulsion without Triton X-100, but successfully sustained stable MD performance with Triton X-100 stabilized oil-in-water emulsion as the feed solution. In contrast, the conventional hydrophobic membranes failed readily regardless whether Triton X-100 was present, although via different mechanisms. These findings are corroborated by contact angle measures as well as oil-probe force spectroscopy. This study provides a holistic picture regarding how a hydrophobic membrane fails in MD and how we can leverage membranes with special wettability to prevent membrane failure in MD operations.