There is an increasing concern towards the treatment of oily wastewater generated by many industrial processes. There is a continuous search and demand for simple, cost-effective, eco-friendly and easily scalable solutions to fabricate materials that can efficiently separate oil and water mixtures/emulsions.
Frequent oil-based catastrophic events (i.e. spillages) from oil tankers has resulted in a loss of 5.72 million tons of oil over the last 45 years. Many techniques currently used have a low efficiency and a high cost, which are not able to separate oil from oil/water emulsions.
A group of Taiwanese researchers has developed a superhydrophilic and underwater superoleophobic PVP-modified cotton that is highly efficient in separating oil/water mixtures, oil/water emulsions, and oil/corrosive solutions.
Oil and water are generally two immiscible liquids unless present together in an emulsion. A mixture of both oil and water leads to a density phase-separation where the oil will float on top of the water. However, emulsions are a lot trickier to separate. In emulsions, there is a continuous and a discontinuous phase.
There are also, on a basic level, two types of emulsion- oil-in-water (o/w) and water-in-oil (w/o). The first term in the name corresponds to the discontinuous phase and the secondary term refers to the continuous phase, i.e. oil-in-water emulsions contain discontinuous oil with a continuous water phase, and vice versa for water-in-oil emulsions. The integrated nature of emulsions makes them much harder to separate than layered oil and water.
There has been a focus recently on using superhydrophobic and superoleophilic materials to separate oil and water mixtures. However, many of these produced have been easily fouled or become blocked up with oil due to possessing an intrinsic oleophilicity.
Supersolvophilicity and phobicity (be it hydro or oleo) describe the interaction through the 3-phase contact angle and hysteresis between the solvent and a surface. Superhydrophilicity is when a ‘water loving’ surface promotes extreme wetting, where the contact angle between water and the surface becomes 0°, causing a thin film and the molecules adhere to the surface.
Hydrophobicity is when a ‘water hating’ surface does not want to interact with the water molecule. As such, the water droplets spaces itself as far away as possible from the surface, resulting in a near 180° contact angle, where the drop essentially sits upright on top of the surface. A water droplet will often roll of a superhydrophobic surface. The principles are the same for oil droplets on superoleophobic/philic surfaces.
The researchers have developed a superhydrophilic and underwater-superoleophobic PVP-coated cotton mesh. Many materials that are inefficient predecessors used of the wetting behavior exhibited by fish scales to produce materials.
The researchers employed a simple and eco-friendly dipping method to fabricate the material. Cotton was chosen due to its being common, in abundance and biodegradable; PVP is thermally stable and non-toxic.
The novel material exhibits a high separation capacity with continuous separation of oil and water mixtures for up to 20 hours, without any increase of oil in filtrate. The PVP-cotton material can also effectively separate surfactant-free and stabilized o/w emulsions, with high efficiencies and fluxes up to 23,900 Lm-2 h-1 bar-1.
This is one to two orders of magnitudes greater than ultrafiltration membranes with similar rejection properties. The material, unlike any of its predecessors, also possess anti-biofouling properties.
The material can separate oil contents in filtrated water to as low as 7 ppm, even for emulsions. Thus, it has the potential to eradicate oil-based samples and emulsions produced by industrial processes and everyday life. It can also be used to separate a wide range of oil-based molecules including, but not limited to, n-hexane, n-hexadecane, isooctane, and diesel.
The PVP-cotton separation material has been shown to exhibit a great performance in oil/water separations through an eco-friendly process. There is a great potential for this material, where its principles and approach could be used to further academic research. It’s cost-effectiveness, low toxicity, green approach and scalability make it ideal for industrial and commercial purposes. All in all, it is a simple but effective material.
Wang C-F., Yang S-Y., Kuo S-W., Eco-Friendly Superwetting Material for Highly Effective Separations of Oil/Water Mixtures and Oil-in-Water Emulsions, Scientific Reports, 2017, 7, 43053