Researchers from University College London have demonstrated a graphene nanomesh membrane that possesses high hydrophilicity, super-oleophobicity and low oil adhesion underwater.
The researchers in this work have put a nature-inspired spin on the fabrication of high-performance graphene membranes for tricky oil/water separations – even in stable emulsions. They demonstrated graphene nanomesh membranes within a wide pH range at impressive water permeance (close to 4000 L m–2 h–1 bar–1) under a very low trans-membrane pressure difference.
The researchers explained that they used chitosan-functionalized graphene nanomesh membranes to achieve this superior water flow rate with very high selectivity, resulting in a water recovery ratio of 98.7%. The chitosan repels pollutants at the membrane surface and the nanomesh reduces the path length for water molecules, which rapidly slip along and through the graphene layers covered with nano-holes.
The team said that its nature-inspired chemical engineering (NICE) approach, with its systematic nature-inspired solution methodology, allows to leverage fundamental mechanisms underpinning desired properties in natural systems – like scalability, efficiency, and resilience – for use within the context of an engineering application. "We already have demonstrated the success of this approach in projects in fuel cells, sustainable manufacturing, healthcare engineering applications, and more."
The researchers' inspiration for this work comes from the structure of cell membranes, specifically aquaporins. Aquaporins are proteins embedded in cell walls that act as biological channels. They keep cells alive by selectively regulating flow of water, gasses, ions, and other solutes into and out of cells in a manner unmatched by anything humans have made. The reason that aquaporins are so highly effective is that their channel walls repel water (i.e., they are hydrophobic), and that they are quite narrow, having a sub-nanometer diameter. This narrowness forces water to move through the channel in a single-file line at an amazing speed of 3 billion water molecules per second.
Inspired by nature's elegant and highly effective design, the team introduced 'nano-holes' through graphene oxide sheets to create a nanomesh. These nanopores reduce the distance that water has to travel through the membrane and also benefit from the slip along the graphene nanosheets. Combined with the low friction between graphene nanosheets and water molecules, this results in high permeance of almost 4000 L m–2 h–1 bar–1, which is about 260 times the permeance of a graphene oxide membrane.
Fouling is an inevitable problem in membrane separation, where blockages occur in the pores of a membrane, stopping the flow and preventing the membrane from functioning normally. Fouling is an especially severe issue for oil separation technology due to how easily the oil droplets stick onto the membrane surface.
Nature provided the inspiration in this case as well. Cell membranes have a natural anti-fouling mechanism thanks to hydrophilic and charged groups that create a hydration layer on the membrane. Having similar functional hydroxyl and amino groups, chitosan has been proposed to functionalize surfaces to be anti-fouling.
Combining these ideas synergistically, the researchers used chitosan with hydrophilic hydroxyl groups and amino groups to modify their graphene nanomesh to increase its hydrophilicity and induce the formation of an anti-fouling hydration layer on the membrane surface.
The next stage in this research effort is to scale it up to larger-scale membrane separation modules and to test the long-term stability of the membranes under various practical situations. The researchers also plan to investigate other methods to achieve robust, broad anti-fouling properties of their membranes.