Reducing membrane fouling with clay

Materials World magazine
,
31 Mar 2020

A 2D clay material could solve the biggest headache in membrane technology – fouling. Idha Valeur discovers how it works. 

2D flakes of the clay mineral, vermiculite, could improve the antifouling properties of membranes, making them more efficient in separating oil and water. 

University of Manchester, UK, Professor Rahul Raveendran Nair, told Materials World, that a membrane with high hydrophilic properties is the preferred option for separating oil from a water-rich system. ‘Vermiculite flake is by-far the most hydrophilic 2D material we have found. In addition to its super-hydrophilicity, it also holds or captures water firmly, helping to reduce the membrane fouling for an extended period,’ he said. 

Nair has created the membrane from already commercially available polyamide, coated with vermiculite flakes suspended in water. ‘The coating exhibits stability of the hydration layer upon prolonged exposure to a fouling environment, making it a promising candidate for antifouling coating. A thin layer of vermiculite coating, therefore, enhances the hydrophilicity of the polyamide support and significantly reduces the oil adhesion,’ he added. 

Applying a super-hydrophilic coating to a membrane to reduce fouling is a common approach, ‘However, most of these materials only perform for a short period of time since the antifouling property decreases over time. This is mainly due to the instability of adsorbed water on the surface of the membrane when it is exposed to a fouling agent. Here we show that the water adsorbed on the vermiculite is exceptionally stable even after exposure to a pure oil environment,’ Nair explained. 

Changing cations

The work demonstrated that the wetting properties of vermiculite membranes can be tuned from super-hydrophilic to hydrophobic by changing the cations that are either adsorbed on the surface of the vermiculite or between the layers of the silicate. ‘We found that the wetting properties of the vermiculite membranes are highly dependent on the nature of the cations. For example, lithium-exchanged vermiculite shows super-hydrophilicity, whereas a tin-exchanged membrane shows hydrophobicity. The change in wetting properties of the vermiculite membrane after ion exchange is due to the difference in the hydrated structure of different cations on vermiculite,’ Nair said. Using lithium-exchanged vermiculite is also beneficial for use as a coating layer to reduce fouling in commercial microfiltration membranes.

Biomedical applications.

While the research is currently only at a proof-of-concept stage, Nair believes that the vermiculite membranes would also work for biofiltration. He added that they are currently uncertain on how this would work, but that they are collaborating with biomedical colleagues to perform initial experiments.