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While the application of graphene as a desalination tool is not a new research initiative, a group of Researchers at the National Graphene Institute (NGI) at The University of Manchester have revolutionized how graphene is used for this purpose.
By assembling the smallest possible man-made holes into graphene, the Researchers have mimicked naturally occurring ion transport systems to selectively allow for specific ions in water to pass through this material.
Ion Passage Through Water
The selective permeability of materials to remove unwanted particles and ions for drinking purposes is a highly specific process that has been successful through the application of various materials. To transport an ion, the nonflexible and hydrated radius of this particle can only move through a material if its shape allows, rather than through any influence of its charge onto the given filter material.
Previous Graphene Desalination Materials
When used for membrane applications, graphene-based materials can be applied either as a functional coating, lamellar structures with nano-channels or by permitting selective permeation through the structural defects present within both single-layer graphene or graphene oxide. Various research initiatives around the world continue to work towards achieving the best possible application of graphene for this use. One example of a revolutionary application of graphene for this purpose includes a graphene-based sieve used to transform seawater to drinking water, conducted by Dr. Rahul Nair at The University of Manchester.
The Sub-Nanometer Slits
To visualize the way in which the Manchester Researchers created sub-nanometer slits within graphene, Sir Andre Geim of Manchester University, one of the Physicists who was awarded the 2010 Novel Prize in 2010 on graphene, states that it is similar to the stacking of books with matchsticks standing between each book. In this case, the books represent 100 nanometer (nm) thick crystal slabs of graphite that measure several microns in length. Between each of the two-dimensional (2D) crystal layers, a monolayer of molybdenum disulphide (MoS2) is placed at each edge of the slabs to allow for an equidistant space between each of the graphite layers. The spaces created by the MoS2 monolayer measures at just several angstroms, or 0.1 nm in diameter.
Confirmation of Desalination Activity
Similar to the way in which the membrane protein aquaporin acts as a biological water channel to facilitate water transport between cells, the MoS2 addition to graphite layers showed that this is the smallest possible pore that can be achieved for a water desalination membrane. To allow for the flow of ions through MoS2 layers, the Researchers applied a voltage across the membrane following the submersion of the material within an ionic chloride solution.
By measuring the ionic conductivity of the solution as it moved through the MoS2 layer, the Researchers determined that the ions act like tennis balls, bouncing between the layer until eventually passing completely through. This activity is preferred over other desalination methods that instead trap ions and other large diameter particles within the material as billiard balls, unable to pass through completely. The ability to filter ions through water by manipulating their ionic activity has not been conducted in any previous research studies.
The development of the MoS2 graphite desalination material by The University of Manchester Scientists provides an ideal platform for future initiatives looking to further nanofluidic systems and other molecular separation technologies.
Image Credit:
Egorov Artem/ Shutterstock.com
References:
- “Size effect in ion transport through angrstrom-scale slits” A. Esfandiar, B. Radha, et al. Science. (2017). DOI: 10.1126.science.aan5275.
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