Anti-fouling TiO2 nanowires membrane for oil/water separation: Synergetic effects of wettability and pore size

Super hydrophilic and super oleophobic carbon nanotube/TiO2 composite membranes for efficient separation of algal-derived oil/water emulsions

The separation of oil from microalgae aqueous emulsions is a critical step in producing algal-derived biofuels and nutraceuticals. This study presents the development of super hydrophilic and super oleophobic composite membranes to efficiently separate algal oil from oil/water emulsions. Carbon nanotubes (CNTs) were functionalized with polydopamine (PDA), polyethylene glycol (PEG), and titanium dioxide (TiO2) nanoparticles and coated onto a mixed cellulose ester (MCE) substrate to fabricate the composite membranes. Two distinct incorporation methods were employed for TiO2; direct nanoparticle incorporation and surface coating onto the CNT/PDA network. The membranes were comprehensively characterized using FTIR, SEM, EDS, contact angle measurements, and AFM analysis. The synthesized MCE@CNT/PDA/NP-TiO2 membrane exhibited super hydrophilicity with a water contact angle of 6.3° and underwater super oleophobicity with oil contact angles up to 172°. Membrane performance evaluation using a Nannochloropsis salina microalgae oil/water emulsion revealed excellent flux up to 9238 L m-2 h-1 bar-1 and oil rejection as high as 98.6 % for the TiO2-incorporated membranes. Additionally, these membranes demonstrated superior antifouling properties, maintaining over 90 % of initial flux even after five separation cycles. Incorporating TiO2 nanoparticles significantly enhanced the membrane's hydrophilicity, oleophobicity, antifouling capability, and stability under extreme pH conditions. The developed composite membranes show great potential for efficient and cost-effective separation of algal oil from microalgae cultivation systems.

Enhancing Membrane Materials for Efficient Li Recycling and Recovery

Rapid uptake of lithium-centric technology, e.g., electric vehicles and large-scale energy storage, is increasing the demand for efficient technologies for lithium extraction from aqueous sources. Among various lithium-extraction technologies, membrane processes hold great promise due to energy efficiency and flexible operation in a continuous process with potential commercial viability. However, membrane separators face challenges such as the extraction efficiency due to the limited selectivity toward lithium relative to other species. Low selectivity can be ascribed to the uncontrollable selective channels and inefficient exclusion functions. However, recent selectivity enhancements for other membrane applications, such as in gas separation and energy storage, suggest that this may also be possible for lithium extraction. This review article focuses on the innovations in the membrane chemistries based on rational design following separation principles and unveiling the theories behind enhanced selectivity. Furthermore, recent progress in membrane-based lithium extraction technologies is summarized with the emphasis on inorganic, organic, and composite materials. The challenges and opportunities for developing the next generation of selective membranes for lithium recovery are also pointed out.

Bioinspired superwetting oil-water separation strategy: toward the era of openness

Bioinspired superwetting oil-water separation strategies have received significant attention for their potential in addressing global water scarcity and aquatic pollution challenges. Over the past two decades, the field has rapidly developed, reaching a pivotal phase of innovation in the oil-water separation process. However, many groundbreaking studies have not received extensive scientific recognition. In this review, we systematically examine the application of bioinspired superwetting materials for complex multiscale oil-water separation. We discuss the development of 2D membrane filtration and 3D sponge adsorption materials in confined spaces, summarizing the core separation mechanisms, key research findings, and the evolutionary logic of these materials. Additionally, we highlight emerging open-space separation strategies, emphasizing several novel dynamic separation devices of significant importance. We evaluate and compare the design concepts, separation principles, materials used, comprehensive performance, and existing challenges of these diverse strategies. Finally, we summarize these advantages, critical bottlenecks, and prospects of this field and propose potential solutions for real oil-water separation processes from a general perspective.

Superhydrophobic and environmentally friendly bovine bone biomass based cellulose membrane for oil-water separation

With the development of superhydrophobic materials for oil-water separation, there is an urgent need to develop environmentally friendly, low-cost, and novel hydrophobic materials. In this paper, based on bovine bone biomass raw materials, bone ash particles are obtained by calcination and grinding, and then bovine bone ash/cotton fabric cellulose membranes are prepared by vacuum filtration and impregnation methods. The pore size of the membrane is regulated and the hydrophobicity of the material is enhanced by constructing the surface microstructures. Results indicate that the membranes possess good hydrophobicity with a contact angle of 161° and the flux can reach 53,635.2 L/m2h for light oil. The separation efficiencies for petroleum ether, cyclohexane, kerosene, and dichloromethane all reach >99 %. In addition, the separation efficiency of the cellulose membrane is still >99 % in the 40-day separation test and always exceeds 90 % for 30 cycling test, indicating that it has good stability and recoverability. Interestingly, the cellulose membrane is prepared from biodegradable and renewable raw materials, which reduces environmental pollution and effectively utilize natural resources.

A comprehensive review on preparation and functional application of the wood aerogel with natural cellulose framework

As the traditional aerogel has defects such as poor mechanical properties, complicated preparation process, high energy consumption and non-renewable, wood aerogel as a new generation of aerogel shows unique advantages. With a natural cellulose framework, wood aerogel is a novel nano-porous material exhibiting exceptional properties such as light weight, high porosity, large specific surface area, and low thermal conductivity. Furthermore, its adaptability to further functionalization enables versatile applications across diverse fields. Driven by the imperative for sustainable development, wood aerogel as a renewable and eco-friendly material, has garnered significant attention from researchers. This review introduces preparation methods of wood aerogel based on the top-down strategy and analyzes the factors influencing their key properties intending to obtain wood aerogels with desirable properties. Avenues for realizing its functionality are also explored, and research progress across various domains are surveyed, including oil-water separation, conductivity and energy storage, as well as photothermal conversion. Finally, potential challenges associated with wood aerogel exploitation and utilization are addressed, alongside discussions on future prospects and research directions. The results emphasize the broad research value and future prospects of wood aerogels, which are poised to drive high-value utilization of wood and foster the development of green multifunctional aerogels.

Ultra-high flux mesh membranes coated with tannic acid-ZIF-8@MXene composites for efficient oil-water separation

Oil/water separation has become a global concern due to the increasing discharge of multi-component harmful oily wastewater. Super wetting membranes have been shown to be an effective material for oil/water separation. Ultra-high flux stainless-steel meshes (SSM) with superhydrophilicity and underwater superoleophobicity were fabricated by tannic acid (TA) modified ZIF-8 nanoparticles (TZIF-8) and two-dimensional MXene materials for oil/water separation. The TZIF-8 increased the interlayer space of MXene, enhancing the flux permeation (69,093 L m-2h-1) and rejection of the composite membrane (TZIF-8@MXene/SSM). The TZIF-8@MXene/SSM membrane showed an underwater oil contact angle of 154.2°. The membrane maintained underwater superoleophobic after stability and durability tests, including various pH solutions, organic solvents, reusability, etc. In addition, the oil/water separation efficiency of TZIF-8@MXene/SSM membranes was higher than 99% after treatment in harsh conditions and recycling. The outstanding anti-fouling, stability, durability, and recyclability properties of TZIF-8@MXene/SSM membrane highlight the remarkable potential of membranes for complex oil/water separation process.

A comprehensive review on the behavior and evolution of oil droplets during oil/water separation by membranes

Membrane separation technology has significant advantages for treating oil-in-water emulsions. Understanding the evolution of oil droplets could reveal the interfacial and colloidal interactions, facilitate the design of advanced membranes, and improve the separation performances. This review on the characteristic behavior and evolution of oil droplets focuses on the advanced analytical techniques, and the subsequent fouling as well as demulsification effects during membrane separation. A detailed introduction is provided on microscopic observations and numerical simulations of the dynamic evolution of oil droplets, featuring real-time in-situ visualization and accurate reconstruction, respectively. Characteristic behaviors of these oil droplets include attachment, pinning, wetting, spreading, blockage, intrusion, coalescence, and detachment, which have been quantified by specific proposed parameters and criteria. The fouling process can be evaluated using Hermia and resistance models. The related adhesion force and intrusion pressure as well as droplet-droplet/membrane interfacial interactions can be accurately quantified using various force analysis methods and advanced force measurement techniques. It is encouraging to note that oil coalescence has been achieved through various effects such as electrostatic interactions, mechanical actions, Laplace pressure/surface free energy gradients, and synergistic effects on functional membranes. When oil droplets become destabilized and coalesce into larger ones, the functional membranes can overcome the limitations of size-sieving effect to attain higher separation efficiency. This not only bypasses the trade-off between permeability and rejection, but also significantly reduces membrane fouling. Finally, the challenges and potential research directions in membrane separation are proposed. We hope this review will support the engineering of advanced materials for oil/water separation and research on interface science in general.

Robust Janus Superwetting Textile with Large Pore Sizes for Oil-in-Water Emulsion Separation

Developing advanced oil-water separation technology is significant for environmental conservation. According to the synergetic effects of the size-sieving mechanism, superwetting materials with small pore sizes have been designed to realize high-efficiency separation for oil-water emulsions. However, the separation flux limited by the pore size and the weakness of the superwetting material impede its practical application severely. Herein, we construct a robust Janus superwetting textile with large pore sizes for oil-in-water emulsion separation. The pristine textile is coated by the as-prepared CuO nanoparticles as the bottom layer with superhydrophilicity and then grafted by 1-octadecanethiol as the top layer with superhydrophobicity to construct the Janus textile. When used as a filter, the superhydrophobic layer acts as the nucleation site to coalesce the small oil droplets facilely. Then, the coalesced oil fills the pores of the superhydrophobic layer and selectively permeates it but is blocked by the superhydrophilic layer with large pore sizes. Utilizing the unique separation mechanism, the Janus textile realizes efficient and rapid separation. Even after multicycle separation, hot liquid immersion for 24 h, tribological test for 60 min, and sandpaper abrasion for 500 cycles, the Janus textile still retains the superwettability and excellent separation performance, manifesting outstanding stability to resist severe damage. This separation strategy provides a novel guideline for high-efficiency and high-flux emulsion separation and practical application.

A Self-Cleaning TiO2 Bacterial Cellulose Super-Hydrophilic Underwater Super-Oleophobic Composite Membrane for Efficient Oil-Water Separation

Due to the increasingly serious problem of offshore oil spills, research related to oil-water separation has attracted more and more attention. Here, we prepared a super-hydrophilic/underwater super-oleophobic membrane (hereinafter referred to as BTA) using poly-dopamine (PDA) to adhesive TiO₂ nanoparticles on the surface of bacterial cellulose, coated with sodium alienate by vacuum-assisted filtration technique. This demonstrates its excellent underwater super-oleophobic property. Its contact angle is about 153°. Remarkably, BTA has 99% separation efficiency. More importantly, BTA still showed excellent anti-pollution property under ultraviolet light after 20 cycles. BTA has the advantages of low cost, environmentally friendliness and good anti-fouling performance. We believe it can play an important role in dealing with problems related to oily wastewater.

Blow-Spun Nanofibrous Membrane for Simultaneous Treatment of Emulsified Oil/Water Mixtures, Dyes, and Bacteria

Membrane separation is of great significance due to its unique performance in treating wastewater. However, the simultaneous treatment of oily emulsions and other complex pollutants in water remains challenging. Herein, we have proposed a simple strategy to prepare a multifunctional titanium dioxide/silver nanoparticles/polyacrylonitrile (TiO₂/AgNPs/PAN) nanofibrous membrane. The experimental results showed that the combination of the hierarchical structure composed of PAN nanofibers and Ag/TiO₂ nanoprotrusions contributed to the superhydrophilicity and superoleophobicity (UOCA = 153.3 ± 2.0°). Further, the nanofibrous membrane exhibited a rapid gravity-driven permeate flux (>1829.37 ± 83.51 L m^-2 h^-1) and an ultrahigh separation efficiency (>99.9%) for the surfactant-stabilized oil/water emulsions. Moreover, due to the synergistic effect between the PAN fibers and TiO₂/Ag heterojunction, Rhodamine B dye in water can be removed quickly and efficiently (up to 97.67% in 90 min). More importantly, the obtained nanofibrous membrane exhibited ultrahigh stability in different harsh environments. The design of superoleophobic nanofiber membrane with a high separation efficiency and high photocatalytic activity has great potential for practical applications in the purification of oily wastewater.

Slippery Mechanism for Enhancing Separation and Anti-fouling of the Superhydrophobic Membrane in a Water-in-Oil Emulsion: Evaluating Water Adhesion of the Membrane Surface

Water removal from water-in-oil emulsions with superhydrophobic microporous membranes is an important industrial process, where the interface property between the membrane and feed becomes critical. Here, superhydrophobic isotactic polypropylene (iPP) microporous membranes with the "lotus effect" and "rose-petal effect" were prepared via utilizing micromolding phase separation, where the former surface exhibited a water contact angle of 153° and a sliding angle of 3.2°, while the latter surface exhibited a water contact angle of 151° and adhesive characteristics. Surface topography and wettability analysis revealed that surface hydrophobicity and water adhesion could be improved by reducing the periodic distance and diameter and increasing the height of the micron-scale structure. When treating both water-in-oil emulsions and water-in-oil emulsions containing BSA pollutants, the iPP membrane with the "lotus effect" was superior to that with the "rose-petal effect" in terms of oil permeate flux, separation efficiency, anti-fouling ability, and recyclability (20 cycles). To explain this phenomenon, a "slippery" mechanism was introduced that correlated the sliding angle to the slippery surface of the iPP membrane with the "lotus effect" and its anti-water adhesion property. This work proposed a theoretical platform for investigating the effect of water adhesion on superhydrophobic membranes in terms of oil-water separation efficiency and anti-fouling ability, thereby providing a definite basis for preparing superhydrophobic membranes with efficient separation and fouling resistance capabilities.

Silicon carbide microfiltration membranes for oil-water separation: Pore structure-dependent wettability matters

Both the pore size and surface properties of silicon carbide (SiC) membranes are demonstrated to significantly affect their separation efficiency when used for oily water treatment. However, the potential influences of open porosity together with the pore size of SiC membranes on their surface properties and oil-water separation performance have rarely been investigated. In this work, porous SiC ceramic membranes with tunable open porosity and pore size were purposely prepared and selected to systematically study the effect of pore structure-dependent wettability on the oil-water separation performance. The measured pure water flux of selected membranes as a function of open porosity (34-48%) and pore size (0.43-0.67 μm) was well-fitted by using a modified H-P equation. Interestingly, the hydrophilicity of SiC membranes was improved with the increase in open porosity and pore size, as evidenced by the gradually decreased dynamic water contact angle and underwater adhesion of oil droplets. Further, the open porosity of SiC membranes was found to contribute more to the improved surface wettability. As a result, the stable flux of SiC membranes in oil-in-water (O/W) emulsions was increased by 24% with the increased open porosity while the oil rejection rate remained above 90%. This work quantitatively reveals the contributions of the pore structure to the surface wettability of ceramic membranes, and thus provides an effective pathway to improve their performance in oil-water separation.

Poly(ethylene-co-vinyl alcohol) Electrospun Nanofiber Membranes for Gravity-Driven Oil/Water Separation

Fabrication of highly efficient oil/water separation membranes is attractive and challenging work for the actual application of the membranes in the treatment of oily wastewater and cleaning up oil spills/oil leakage accidents. In this study, hydrophilic poly(ethylene-co-polyvinyl alcohol) (EVOH) nanofiber membranes were made using an electrospinning technique for oil/water separation. The as-prepared EVOH electrospun nanofiber membranes (ENMs) exhibited a super-hydrophilic property (water contact angle 33.74°) without further treatment. As prepared, ENMs can provide continuous separation of surfactant-free and surfactant-stabilized water-in-oil emulsions with high efficiency (i.e., flux 8200 L m−2 h−1 (LMH), separation efficiency: >99.9%). In addition, their high stability (i.e., reusable, mechanically robust) would broaden the conditions under which they can be employed in the real field oil/water separation applications. Various characterization techniques (including morphology investigation, pore size, porosity, mechanical properties, and performance test) for gravity-driven oil/water separation were employed to evaluate the newly prepared EVOH ENMs.

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.

Superlyophilic Shape Memory Porous Sponge for Smart Liquid Permeation

Recently, smart liquid permeation has aroused much attention. However, existing strategies to achieve such a goal are often based on reversibly controlling hydrophobicity/hydrophilicity on static porous structures, which are unsuitable for oils with low surface tension, and meanwhile they cannot realize tunable permeation flux since the pore sizes are constant. Herein, we report a superlyophilic shape memory porous sponge (SSMS) that can demonstrate tunable pore size from about 28 nm to 900 μm as the material's shape is changed. Based on the controllability in pore size, not only ON/OFF penetration but also precisely tunable permeation flux can be obtained for both water and oil. Furthermore, by using the SSMS, an application in accurate release of small-molecule rhodamine B was also demonstrated. This work reports a material with tunable pore size for controllable liquid permeation, which provides some ideas for designing smart superwetting permeation materials.

Dimensional Nanofillers in Mixed Matrix Membranes for Pervaporation Separations: A Review

Pervaporation (PV) has been an intriguing membrane technology for separating liquid mixtures since its commercialization in the 1980s. The design of highly permselective materials used in this respect has made significant improvements in separation properties, such as selectivity, permeability, and long-term stability. Mixed-matrix membranes (MMMs), featuring inorganic fillers dispersed in a polymer matrix to form an organic-inorganic hybrid, have opened up a new avenue to facilely obtain high-performance PV membranes. The combination of inorganic fillers in a polymer matrix endows high flexibility in designing the required separation properties of the membranes, in which various fillers provide specific functions correlated to the separation process. This review discusses recent advances in the use of nanofillers in PV MMMs categorized by dimensions including zero-, one-, two- and three-dimensional nanomaterials. Furthermore, the impact of the nanofillers on the polymer matrix is described to provide in-depth understanding of the structure-performance relationship. Finally, the applications of nanofillers in MMMs for PV separation are summarized.

Zwitterionic Polymer-Grafted Superhydrophilic and Superoleophobic Silk Fabrics for Anti-Oil Applications

A highly anti-oil fabric membrane is synthesized by surface grafting of zwitterionic poly(sulfobetaine methacrylate) (PSBMA) onto the fabric surface. The fabric membrane is first enzymatically modified to create more reactive amine groups on the surface. A surface-initiated atom transfer radical polymerization (SI-ATRP) reaction is then performed to modify the fabric membrane surface with a dense PSBMA brush layer. Surface characterization indicates that the brush-grafted fabric membrane exhibits increased surface roughness and improved superhydrophilicity. The PSBMA-modified silk fabrics show a very large contact angle for oil droplets in water, and have excellent oil resistance in air and in water-oil mixtures.

Ultrasonication favors TiO2 nano-particles dispersion in PVDF ultrafiltration membrane to effectively enhance membrane hydrophilicity and anti-fouling capability

The influence of ultrasonication on membrane performance was investigated by two ultrasonication modes, direct and indirect ultrasonication as pretreatment, and simply improved PVDF-TiO₂ membranes' performance was systematically compared. Ultrasound intensity of 100% and ultrasonication time ranged from 1 to 2 h positively affect membrane permeability. Characterization results manifested that membrane structure was eventually optimized with an even nano-TiO₂ dispersion by direct ultrasonication. Analysis of surface roughness reflected that PVDF-TiO₂ (MS-U2) surface morphological pattern was peak-valley structure that resisted fouling greatly. A good fitting of experimental result and Tansel's simulation illustrated that anti-fouling ability was realized direct ultrasonication modified membrane. PVDF-TiO₂ (MS-U2) membrane showing the lowest |τ| reflecting the time required to reach a certain level of the fouling degree was the lowest. Relying upon modified Hermia's model analysis, protein blockage within the membrane pore was one major fouling mechanism; surface blockage degree of PVDF-TiO₂ (MS-U2) was relative slight. Fouling mechanism analyzed by two models reflected that PVDF-TiO₂ (MS-U2) membrane exhibited a higher anti-protein fouling ability during cross-flow filtration process.

Morphology-controllable synthesis and application of TiO2 nanotube arrays with “photocatalysis and self-cleaning” synergism

The integration of photocatalytic materials and self-cleaning superhydrophobic materials provides a possibility of combining decontamination and antifouling.

Electrostatic Assembly of a Titanium Dioxide@Hydrophilic Poly(phenylene sulfide) Porous Membrane with Enhanced Wetting Selectivity for Separation of Strongly Corrosive Oil-Water Emulsions

The efficient treatment of oil-water emulsions in extreme environments, such as strongly acidic and alkaline media, remains a widespread concern. Poly(phenylene sulfide) (PPS)-based porous membranes with excellent resistance to chemicals and solvents are promising for settling this challenge. However, the limited hydrophilicity and the poor hydrated ability of the hydrophilic PPS (h-PPS) membranes reported in the literature prevents them from separating oil-water emulsions with high efficiency, large fluxes, and good antifouling performances. In this study, a firm rough TiO₂ layer is constructed on a h-PPS membrane via electrostatic assembly to improve the surface hydrophilization. The introduction of the TiO₂ layer increases the wetting selectivity and decreases the oil adhesion, which makes it capable to efficiently treat oil-in-water emulsions (efficiency > 98%). Most importantly, the underwater critical oil intrusion pressure almost doubled after the incorporation of the TiO₂ layer, which allows the membrane to withstand pressurized filtration, achieving a high flux of ∼4000 L m^-2 h^-1. This is more than 2 orders of magnitude larger than the flux of the reported h-PPS. Furthermore, the TiO₂@h-PPS membrane displays long-term stability in separating oil-water emulsions in strong acid and strong alkali, showing a promising prospect for the treatment of strongly corrosive emulsions.