Yingze Cao

Special wettable materials for oil/water separation

Oil/water separation is an important field, not only for scientific research but also for practical applications aiming to resolve industrial oily wastewater and oil-spill pollution, as well as environmental protection. Recently, research into the role of special wettability for oil/water separation has attracted much attention. In this review we summarize the design, fabrication, applications and recent developments of special wettable materials for oil/water separation. Based on the different types of separation, we organize this review into three parts: “oil-removing” type materials with superhydrophobicity and superoleophilicity (that selectively filter or absorb oil from oil/water mixtures), “water-removing” type materials with superhydrophilicity and superoleophobicity (that selectively separate water from oil/water mixtures), and smart controllable separation materials. In each section, we present in detail the representative work, introduce the design idea, outline their fabrication methods, and discuss the role of special wettability on the separation. Finally, the challenges and outlook for the future of this subject are discussed.

Mussel-inspired chemistry and Michael addition reaction for efficient oil/water separation.

An oil/water separation mesh with high separation efficiency and intrusion pressure of water has been successfully developed by combining mussel-inspired chemistry and Michael addition reaction. The substrate of the stainless steel mesh was first coated with the adhesive polydopamine (PDA) film by simple immersion in an aqueous solution of dopamine at pH of 8.5. Then n-dodecyl mercaptan (NDM) was conjugated with PDA film through Michael addition reaction at ambient temperature. The as-prepared mesh showed highly hydrophobicity with the water contact angle of 144° and superoleophilicity with the oil contact angle of 0°. It can be used to separate a series of oil/water mixtures like gasoline, diesel, etc. The separation efficiency remains high after 30 times use (99.95% for hexane/water mixture). More importantly, the relatively high intrusion pressure (2.2 kPa) gives the opportunity to separation of large amount of oil and water mixtures. This study provides a new prospect to simply introduce multiple molecules on the adhesive PDA-based mesh to achieve various functional oil/water separation materials.

Integrated oil separation and water purification by a double-layer TiO2-based mesh

We report a simple and inexpensive method for fabricating double-layer TiO2-based mesh. The upper layer is a TiO2 coated mesh film with micro- and nano-structures, while the lower is the same TiO2 mesh film but modified with ODP (octadecyl phosphonic acid) to impart its superhydrophobic and superoleophilic properties. Such a mesh can be successfully used for the separation of insoluble oil from water due to its special wettability, as well as the degradation of soluble pollutants in water under UV light because of the photocatalytic abilities of TiO2, making it a promising candidate for water purification.

Thermo and pH Dual-Responsive Materials for Controllable Oil/Water Separation

Thermo and pH dual-controllable oil/water separation materials are successfully fabricated by photo initiated free radical polymerization of dimethylamino ethyl methacrylate (DMAEMA). The PDMAEMA hydrogel coated mesh shows superhydrophilicity and underwater superoleophobicity at certain temperature and pH. Due to the double responsiveness of PDMAEMA hydrogel, the as-prepared mesh can selectively separate water from oil/water mixtures and make water and oil permeate through the mesh orderly and be collected separately by adjusting the temperature or pH. Water can pass through the as-prepared mesh under 55 °C (pH 7) and pH less than 13 (T = 25 °C) while oil is kept on the mesh. When the temperature is above 55 °C or pH is larger than 13, the water retention capacity of PDMAEMA hydrogel is significantly reduced and the swelling volume is decreased. Therefore, oil can permeate through the mesh and be collected in situ. Additionally, this material has excellent potential to be used in practical applications and has created a new field for water/oil separation in which the process can be diversified and more intelligent.

Mercury ion responsive wettability and oil/water separation.

A novel Hg(2+) responsive oil/water separation mesh with poly(acrylic acid) hydrogel coating is reported. The mesh can separate oil and water because of the superhydrophilicity of the poly(acrylic acid) hydrogel coating on the mesh, and switch the wettability based on the chelation between Hg(2+) and poly(acrylic acid) . The reversible change in oil contact angle of as-prepared mesh is about 149° after immersion in Hg(2+) solution. This mesh is an ideal candidate for oil-polluted water purification, especially for water that contains Hg(2+) contaminant.

A Facile Solvent-Manipulated Mesh for Reversible Oil/Water Separation

A controllable oil/water separation mesh has been successfully developed and easily manipulated by immersion in a stearic acid ethanol solution and tetrahydrofuran with a very short period of time. The superhydrophilic and underwater superoleophobic mesh is first obtained via a one-step chemical oxidation and subsequently converts to superhydrophobic after it is immersed in an ethanol solution of stearic acid for 5 min. The surface wettability is regained to superhydrophilic quickly by immersion in tetrahydrofuran for 5 min. More importantly, the reversible superhydrophobic-and-superhydrophilic switching can be repeated multiple times with almost no visible morphology variation. Therefore, this approach provides potential application in controllable oil/water separation and opens up new perspectives in manipulation of various metallic oxide substrates.

Straightforward oxidation of a copper substrate produces an underwater superoleophobic mesh for oil/water separation.

A superhydrophilic and underwater superoleophobic Cu(OH)2-covered mesh with micro- and nanoscale hierarchical composite structures is successfully fabricated through a one-step chemical oxidation of a smooth-copper mesh. Such mesh, without any further modification, can selectively separate water from oil/water mixtures with high separation efficiency, and possess excellent stability even after 60 uses. This method provides a simple, low-cost, and scalable strategy for the purification of oily wastewater.

Superoleophilic and superhydrophobic biodegradable material with porous structures for oil absorption and oil–water separation

In this work, novel biodegradable poly(lactic acid) oil absorption and filtration materials with superhydrophobic and superoleophilic properties were successfully fabricated. The as-prepared materials can separate oil from oil–water mixtures with high selectivity, high separation efficiency, stable performance, and easy recyclability, and are ideal candidates for applications in the cleanup of oil pollutants and the separation of oil and water. The used materials can be easily disposed because of their biodegradable essence and thus overcome the hard disposal and environmental contaminant limitations of traditional oil–water separation materials. This work provides a new design idea to prepare oil–water separation materials with consideration of both excellent separation ability and environmental friendliness, which shows attractive potential applications in industrial oily wastewater and oil spill cleanup.

A pure inorganic ZnO-Co3O4 overlapped membrane for efficient oil/water emulsions separation.

The earths environmental problems, especially for water remediation, need effective methods to solve. Materials with special wettability are developed for the separation of oil/water mixtures. However, the separation of emulsified oil/water mixtures can be a real challenge. There is still much deficiencies, on account of the surfactant, which could link water molecules and oil molecules to form a stabilized system. Here we report a pure inorganic ZnO-Co3O4 overlapped membrane to give a brand new solution to emulsified oil/water mixture separation. Fabricated by an easy and cost-efficient way, such a membrane combines the properties of under-water superoleophobicity and under-oil superhydrophobicity, which can be successfully used for the efficient separation of both surfactant-free and surfactant-stabilized emulsions, solely driven by gravity. This ZnO-Co3O4 overlapped membrane shows great potential applications to industrial wastewater treatment, domestic sewage purification and other water remediation.

Mussel-inspired chemistry and Stöber method for highly stabilized water-in-oil emulsions separation

A novel superhydrophobic–superoleophilic membrane for the separation of oil/water emulsions has been developed by combining mussel-inspired chemistry and Stober method. The membrane can be applied to various and highly stabilized water-in-oil emulsions. Separation process is achieved by one step under gravity with high efficiency. Moreover, the membrane is thermally stable, easily stored, and producible in large scale.