Gravity-driven Oil-water Separation Research Articles

Novel conductive Janus membranes with water diode effect assisted by an electrostatic field for efficient mass transfer and oil-water separation

Membrane separation technology is effective for treating oily wastewater but is limited by the lack of membranes that are both highly permeable and fouling-resistant. Janus membranes, with asymmetric wettability and electrostatic repulsion of charged oil droplets, offer a potential strategy for oil-water separation. This study developed novel electrically enhanced conductive Janus membranes using metal-modified organic membranes to achieve high-efficiency treatment. Conductive metal Janus membranes were fabricated using a straightforward two-step method involving simple electroless nickel plating and surface peeling. The resulting nickel-coated surface of the membrane demonstrated extreme hydrophilicity, underwater oleophobicity, and excellent conductivity (approximately 4.7 Ω). Fascinatingly, the developed membranes exhibited a “water diode” effect, facilitating unidirectional liquid transport without external pressure. Gravity-driven oil-water separation achieved a flux rate of 4874 L m−2 h−1 and over 99 % oil removal efficiency, surpassing most existing studies. During electro-filtration of oil-water emulsions, the conductive Janus membrane as a cathode generated electrostatic repulsion, preventing emulsified oil droplet adhesion. Applying an external voltage increased the flux by approximately 13 times and oil removal efficiency to 99.6 %. The membrane's separation capability remained consistent across six cycle tests, demonstrating robust stability. As all, this work first synthesized conductive “water diode” Janus membranes with high oil-water separation efficiency, offering fresh perspectives on the design and fabrication of Janus membranes as well as on the processes for treating oily wastewater.

Enhanced superhydrophobic melamine sponge with bimetal organic framework for simultaneous oil-water separation and microplastic removal

The escalating problem of oily wastewater and microplastic pollution poses a significant threat to our environment. Addressing this challenge has been hampered by the lack of sustainable materials capable of handling multiple pollutants effectively. However, this research introduces a groundbreaking approach for designing a superhydrophobic composite sponge, MS@ZnCo-ZIF@HDTMS. Utilizing a straightforward layer-by-layer self-assembly strategy, this work achieved the robust integration of bimetallic organic framework (ZnCo-ZIF) onto the sponge surface. This integration was facilitated by the adhesive and ion exchange properties of polydopamine (PDA), thereby constructing intricate micro- and nanocomposite structures. The immense specific surface area of ZnCo-ZIF offered numerous absorption sites for the assembly of PDA and hexadecyltrimethoxysilane (HDTMS). Remarkably, the composite sponge exhibits superhydrophobic and superoleophilic properties, demonstrating exceptional oil absorption capacities ranging from 55 g/g to 96 g/g, even after enduring 50 cycles. Moreover, MS@ZnCo-ZIF@HDTMS possesses remarkable mechanical and chemical stability. Its gravity-driven oil-water separation capabilities are noteworthy, achieving a permeate flux of 10,000 L m-2 h-1 and a removal rate of up to 99.2 %. Crucially, the superhydrophobic nature of the composite sponge, coupled with capillary effect, Van der Waals forces, hydrogen bonding, and other interactions, enables MS@ZnCo-ZIF@HDTMS to capture a diverse range of microplastics from water relatively swiftly. Therefore, this work not only provides effective candidates for the sustainable treatment of oil and microplastics in water but also opens new avenues for the restoration of aquatic environments.

Tune wastepaper to hydrophobic membranes through metal-ion-induced lignocellulose nanofibril crosslinking for oil-water separation

Herein, we proposed a metal-ion-induced surface nano-engineering technology for transforming hydrophilic waste newspaper (WNP) into a hydrophobic material suitable for oil-water separation, without using hydrophobic chemicals/materials. WNPs were swelled in a dilute ferric ion solution (i.e., 50 mM FeCl3) followed by oven-drying to induce the coordination between ferric ions and hydroxyls of lignocellulose nanofibrils of the paper. This resulted in the crosslinking of lignocellulose nanofibrils and converted the hydrophilic WNP to the hydrophobic membrane (HM). The HM exhibited remarkable oil sorption capacity for various oils, up to 3.4 times its weight, and demonstrated good recyclability after undergoing 10 cycles of absorption-desorption-drying. Additionally, HM demonstrated the ability to effectively separate both immiscible oil-water mixtures and surfactant-stabilized water-in-oil emulsions in continuous gravity-driven oil-water separation processes. With the modification method and advantages in oil-water separation, the as-prepared HM exhibits great potential for application in oil spill treatment and environmental remediation.

Facile fabrication of super-wettable mesh membrane using locally-synthesized cobalt oxide nanoparticles and their application in efficient gravity driven oil/water separation

A functional cobalt (II, III) oxide (Co3O4) nanoparticle coated stainless steel mesh membrane for the application of oil-water separation was fabricated by facile spray coating, where the coating material Co3O4 nanoparticle was locally synthesized by co-precipitation method. The fabricated membrane exhibited the unique surface wettability of in-air superhydrophilicity with static contact angle, θWA= 0°, in-air superoleophilicity with static contact angle θOA= 0°, and underwater superoleophobicity with static contact angle θOW= 160°. The contrasting water and oil wettability of the membrane surface is quite congenial for the separation of oil and water in immiscible oil-water mixture. When this membrane was used as a filtering medium in gravity driven oil water separation system, oil phase retained on the feed side of the membrane, while the water phase permeated through the membrane. In this process, the oil-water separation efficiency as high as 99 % was achieved, which is quite significant considering the simplicity of the membrane fabrication. Also, the membrane remained quite intact in terms of retaining the same level of separation efficiency even after 10 consecutive oil-water separation cycles, which shows the mechanical, structural and chemical robustness of the membrane. The results of this oil-water separation process is explained in the light of the observed surface wettability, that emerge from the three different interfacial energies and hierarchical surface roughness brought about by the spray coating. FESEM images of the membrane surface confirmed the uniform distribution of Co3O4 nanoparticles with average particle size of ∼ 30–50 nm, and the XRD pattern revealed the typical rhombohedral crystal shape of Co3O4. The elemental and chemical compositions of the surface were further substantiated by XPS and FTIR spectra respectively.

Application of Superhydrophobic Mesh Coated by PDMS/TiO2 Nanocomposites for Oil/Water Separation.

Superhydrophobic materials have recently attracted great interest from both academia and industry due to their promising applications in self-cleaning, oil-water separation, etc. Here, we developed a facile method to prepare hybrid PDMS/TiO2 fiber for superhydrophobic coatings. TiO2 could be uniformly distributed into PDMS, forming a hierarchical micro/nano structure on the surface of the substrate. The contact angle of the superhydrophobic coating could reach as high as 155°. The superhydrophobic coating possessed good self-cleaning performance, corrosion resistance, and durability. It was found that gravity-driven oil-water separation was achieved using stainless steel mesh coated with the PDMS/TiO2 coating. More importantly, the coated filter paper could not only separate oil and pure water but also corrosive solutions, including the salt, acid, and alkali solution.

Mg-calcite CaCO3 particle: Rapid synthesis and application in fabrication of environmentally friendly hydrophobic polyurethane sponge for oil–water separation

The hydrophobic modification of polyurethane (PU) sponge is attracting considerable attention in the field of oil–water separation. A hydrophobic PU sponge with microsize cavities on the skeleton surface and interior was prepared with hierarchical structural Mg-calcite CaCO3 (MCC) particles as a template. Hydrophobic-modified MCC particles with controllable morphologies were firstly synthesized by a facile and rapid batch method under high-speed stirring for 1 min by the presence of cetyltrimethyl ammonium bromide. MCC particles were then blended into the starting foaming materials to fabricate hydrophobic PU/MCC sponge by in-situ polymerization method. Finally, MCC particles of PU/MCC sponge were etched by acid solution and microsize cavities were generated on the surface and interior of EPU/MCC sponge skeleton. The oil-adsorption capacity of obtained hydrophobic EPU/MCC sponge was 7.58 and 5.31 times higher than pristine PU sponge for engine oil and sunflower oil, respectively. This could be attributed to the microsize cavities on the skeleton surface and interior which would increase the skeleton flexibility and enhance adhesion level between the oil and sponge surface. The underwater CCl4 was completely adsorbed into the EPU/MCC sponge within 1 s, indicating its high adsorption speed. EPU/MCC sponge exhibited high separation selectivity and separation efficiency in gravity-driven oil-water separation. The results could provide a strategy to develop hydrophobic sponges with excellent oil-water separation property.

Facile fabrication of 2D MOF-Based membrane with hierarchical structures for ultrafast Oil-Water separation

The surface chemical composition and building unit structures of membrane are the key factors to realize highly-efficient oil–water separation. Here we report the synthesis of Cu-CAT-1 metal–organic frameworks (MOFs) with unique two-dimensional (2D) hierarchical structures grown directly on copper mesh (Cu-CAT-1@CM) for oil–water separation. The Cu-CAT-1@CM membrane with micro/nano- architecture exhibits robust superhydrophilicity and underwater superoleophobicity characteristics, allowing gravity-driven oil–water separation by achieving ultrahigh flux up to 329 kL∙m−2∙h−1. The membrane shows excellent separation performances by testing various oils, particularly actual crude oil and xylene, which are considered as the notorious pollutants in the water. The residual oil contents of six samples are all less than 24.6 mg∙L-1. The mechanism analysis demonstrated that the appropriate hierarchical structures of the membrane surface is the code for the extreme wettability that remarkably improved the separation efficiency. Moreover, such a membrane with high performance can be readily synthesized by electrochemical method at room temperature in only 20 min, which shows the advantages of facile and environmentally friendly fabrication process as well as low cost. Overall, the membrane with energy-saving of gravity-driven and ultra-high permeability allows the rapid separation of oil–water mixture, showing the potential applications for resolving the problem of oil pollution.

Nanosecond laser induced glass particle deposition over steel mesh for long-term superhydrophilicity and gravity driven oil water separation

In this report we show that the laser textured stainless steel meshes which serve as excellent oil water separator immediately after laser processing, lose their oil water separation capability after storage in ambient air. This was possibly due to the reactive nature of the metal oxides formed during laser processing which resulted in transition of the wettability of processed metallic meshes from superhydrophilic to superhydrophobic during storage. To overcome this issue, we show that by using a glass cover plate over the metal mesh during laser processing, micron/submicron sized glass particles can be deposited on to the mesh via a process known as laser induced plasma assisted ablation. Since the glass is inherently hydrophilic and inert, and therefore the glass particles coating produced a stable superhydrophilic/underwater-superoleophobic surface which was observed to maintain its superhydrophilicity for the tested duration of ~8 months and perform oil/water separation with an efficiency of ~96% for various oils. Further, the glass particles coated mesh was found to sustain several cycles of sandpaper abrasion before losing its oil water separation capability.

One-step synthesis of a steel-polymer wool for oil-water separation and absorption

Methods for the efficient and affordable remediation of oil spills and chemical leaks are crucially needed in today’s environment. In this study, we have developed a simple, magnetic, porous material based on polydimethylsiloxane (PDMS) and steel wool (SW) that can fulfill these needs. The PDMS-SW presented here is superhydrophobic, superoleophilic, and capable of absorbing and separating oils and organic solvents from water. The material is mechanically and chemically stable, even in salty environments, and can be magnetically guided. It exhibits good selectivity, recyclability, and sorption capacity, and can quickly and continuously absorb and remove large amounts of oils and organic solutions from stationary and turbulent water. In addition, PDMS-SW’s inherently high porosity enables direct, gravity-driven oil-water separation with permeate flux as high as ~32,000 L/m2·h and separation efficiency over 99%. The solution immersion process used to prepare the material is easily scalable and requires only a single step. Thus, with its demonstrated combination of affordability, efficiency, and ease of use, PDMS-SW has the potential to meet the demands of large-area oil and chemical clean-ups.

Electrospun frogspawn structured membrane for gravity-driven oil-water separation

The aim of this study is to prepare a fibrous membrane scaffold that possesses a frogspawn structure for high-efficiency oil-water separation. Polyamic acid was first electrospun onto a rotating wheel-collector to obtain the fibrous membrane. Subsequently, post-processing by immersion in a polydimethylsiloxane solution and a silica nanoparticles suspension, followed by a thermal treatment generated a frogspawn-structured fibrous membrane. The obtained membrane achieved superhydrophobicity and superoleophilicity, with the water contact angle as high as 155.75° and the oil contact angle lower than 10°. The separation efficiencies of the membrane were higher than 99.55% and the permeate flux was maintained at greater than 4400 L/m2∙h after 20 separation cycles. Additionally, the wettability studies suggested the membrane exhibits high stability because it can resist damages due to high temperature (150 °C), acid/basic conditions and organic/inorganic solvents. These findings indicated that this composite membrane has great potential for use in gravity-driven oil-water separation and can extend the range of its application for treatments of oil spills incident, oily wastewater and spent liquor.

Pouring‐type gravity‐driven oil–water separation without water bridge

Oil pollution brings great challenge to environmental protection. Utilising oil-water separation device to separate and recycle oils is an environment-friendly approach to dispose mixtures containing oils. Pouring-type gravity-driven oil-water separation method based on extremely wettable materials is an interesting area of research because of its high separation velocity, selectivity and purity. However, the presence of water bridge results in low separation efficiency. Herein, the authors first synthesised a facile etching method and a dipcoating method to fabricate contrary extreme wettability sponges including superhydrophilic-underwater-superoleophobic (SHI-U-SOO) sponge and superhydrophobic-superoleophilic (SHO-SOI) sponge. Accordingly, they designed a three-way oil-water separation system whose left and right bottom tubes were installed with the SHI-U-SOO sponge and the SHO-SOI sponge, respectively. Water and oil separately flow out from the left and right tubes, thereby eliminating the water bridge and increasing separation efficiency, and still having ideal separation selectivity and separation purity. The developed three-way oil-water separation system has promising application prospects in disposing oil spills, oil leakage from factories and industrial effluents.

Modified metal mesh with bipolar wettability for rapid and gravity-driven oil-water separation and oil collection

Oil contamination from oilfield geothermal water severely restricts the efficient utilization of geothermal energy. Oil-water separation pretreatment via removing oil from water may provide a kind of possibility to mitigate the oil pollution problem. Stainless steel filtration mesh with bipolar wettability was successfully fabricated to attempt the gravity-driven separation of various oil-water mixtures. The highly hydrophilic mesh was prepared by liquid phase deposition of TiO2, and the hydrophobic mesh was realized via the subsequent modification of fluorosilane on highly hydrophilic mesh deposited by TiO2. Only with relatively small pore size, the highly hydrophilic mesh and the hydrophobic mesh were applicable for separating oil-water mixtures. Besides, microscopic morphologies, surface chemical compositions, contact angles, water wetting behaviors, water-holding capacities, abrasion test and oil collection were also investigated. In general, the modified mesh with bipolar wettability presents gravity-driven oil-water separation for various oil-water mixtures, which may have potential industry application value such as the pretreatment of oilfield geothermal water.

Wrinkled Graphene Monoliths as Superabsorbing Building Blocks for Superhydrophobic and Superhydrophilic Surfaces.

Superhydrophobic and superhydrophilic surfaces are of great interest because of a large range of applications, for example, as antifogging and self-cleaning coatings, as antibiofouling paints for boats, in metal refining, and for water-oil separation. An aqueous ink based on three-dimensional graphene monoliths (Gr) can be used for constructing both superhydrophobic and superhydrophilic surfaces on arbitrary substrates with different surficial structures from the meso- to the macroscale. The surface wettability of a Gr-coated surface mainly depends on which additional layers (air for a superhydrophobic surface and water for a superhydrophilic surface) are adsorbed on the surface of the graphene sheets. Switching a Gr-coated surface between being superhydrophobic and superhydrophilic can thus be easily achieved by drying and prewetting with ethanol. The Gr-based superhydrophobic membranes or films should have great potential as efficient separators for fast and gravity-driven oil-water separation.

Gravity-driven hybrid membrane for oleophobic-superhydrophilic oil-water separation and water purification by graphene.

We prepared a simple, low-cost membrane suitable for gravity-driven oil-water separation and water purification. Composite membranes with selective wettability were fabricated from a mixture of aqueous poly(diallyldimethylammonium chloride) solution, sodium perfluorooctanoate, and silica nanoparticles. Simply dip-coating a stainless steel mesh using this mixture produced the oil-water separator. The contact angles (CAs) of hexadecane and water on the prepared composite membranes were 95 ± 2° and 0°, respectively, showing the oleophobicity and superhydrophilicity of the membrane. In addition, a graphene plug was stacked below the membrane to remove water-soluble organics by adsorption. As a result, this multifunctional device not only separates hexadecane from water, but also purifies water by the permeation of the separated water through the graphene plug. Here, methylene blue (MB) was removed as a demonstration. Membranes were characterized by high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy to elucidate the origin of their selective wettability.

Underwater superoleophobic graphene oxide coated meshes for the separation of oil and water.

Hydrophilic graphene oxide (GO) nanosheets can be easily coated onto stainless steel meshes. Compared to neat meshes, GO coated meshes become more hydrophilic in air and superoleophobic under water. Taking advantage of this completely opposite wettability, GO coated meshes were used for gravity-driven oil-water separation.