Reuseable polymer membrane quickly filters oil and water

Materials World magazine
,
10 May 2019

A membrane has been developed that can separate oil from water without ruining the filter, to clean up large oil spills and improve wastewater treatments. Idha Valeur finds out more.

Separating oil from water without clogging the filter can be enhanced with a low-cost membrane developed by researchers from Tufts University, USA. Tufts University Assistant Professor and author of the study, Ayse Asatekin, told Materials World that the filter works by allowing oil and other organic solvents to pass through the membrane pores while preventing the passage of water.

‘This way, you can selectively remove oil and collect it elsewhere,’ Asatekin said. ‘The membrane has high porosity, and is made of a material that is superoleophilic i.e. loves oil/solvents, but hydrophobic, i.e. dislikes water. Water balls up on it like on a Teflon pan, and because of this, water cannot get in. In contrast, oils spread on it and get absorbed instantly. This allows it to pass through.’

The filter has two components – polyvinylidene fluoride (PVDF), which is a fluoropolymer often used in membranes, and a copolymer, made with both methyl methacrylate (MMA) and a highly fluorinated monomer, 1H, 1H, 2H, 2H-perfluorodecyl methacrylate (PFDMA).

‘They are like short polytetrafluoroethylene (PTFE) molecules attached to the main chain. MMA units anchor the copolymer to the PVDF, and improve its mechanical properties,’ Asatekin said. ‘As a result, the membranes combine the beneficial properties of PVDF –  mechanical properties, manufacturability and cost – with improved performance for oil-water separation.’

Cleaning waters

The filter is intended for clearing up oil spills. Technologies such as skimmers and booms can be expensive and, in the case of booms, are single-use. In comparison, the membrane is both fairly inexpensive and reusable. It also keeps a high flow rate without being fouled by accumulating oil.

The membrane would be part of a system, where modules would hold the filter in place on the water surface, allowing the contaminated water to be pumped through.

‘Oil would pass through, get collected in a tank, and be taken to the shore. Water would likely be returned to the ocean,’ Asatekin explained. ‘This would be more effective than skimming. Unlike absorbents, you could use the same membrane for a while, recovering just the oil.’

Membranes commonly used for oil filtration may foul when drops of water get stuck in the pores of the membrane or between fibres making it impossible for the oil to pass. With Asatekin’s membrane, this would be avoided due to it being hydrophobic.

‘Water droplets would roll off of the membrane surface. So when oil and water is fed to it, even if water droplets get to the membrane, they don’t stick and clog pores,’ Asatekin said.

The product has been tested in a lab environment for an hour, but according to Asatekin, to get an idea of the filter’s longevity it would need to get tested in real life applications, which are hard to simulate in a clinical environment.

‘Still, we would expect these membranes to be more resilient than others considered for this use, both because of their fouling resistance and their good mechanical strength.’

Creating the membrane

The research team created the membrane using electrospinning, where a polymer is dissolved in a solvent, then pushed through a needle, under a strong electric field.

‘This electric field causes the polymer solution to stretch and deposit onto an electrode on the other side in the form of very fine fibres – if done well and correctly, creating a non-woven mat or membrane,’ Asatekin said. ‘It is capable of creating membranes with very high porosity, which is crucial for achieving high flow rates. It can also create very small pores, but the fibre and pore size is also tunable, further expanding its applicability. I believe there are a handful of groups who have scaled up this process to roll-to-roll manufacturing, so it is also potentially scalable,’ she added.

Stepping up

Due to the unconventional scale-up process needed for Asatekin’s membrane, the research is still at an early stage.

‘We synthesised our specialty polymer, a random acrylic copolymer that includes fluorinated groups, in the lab. Its manufacture would need to be scaled up as well. It is prepared by a quite common method, but it is still a specialty polymer,’ Asatekin said.

‘Furthermore, the manufacturing process would need to be optimised for homogeneity, mechanical properties etc. The membrane would then need to be incorporated into a system that would feed it contaminated water and allow it to separate. Each of these processes can be plugged into existing manufacturing methods, so I believe this technology has good scale-up potential – but that is still a lot of work. So overall, the path from lab scale research to a commercial product is long – but it is feasible. Many products have made their way through this.’