Graphene Nanocomposite to Improve Desalination Processes

The use of reverse osmosis desalination technology has gathered more and more usage and interest over the last few years. It is responsible for producing a large amount of fresh water for the growing populations around the world.

Despite their widespread usage, there are still fundamental issues that need to be addressed, and in an effort to expand this technology to more desalination plants worldwide, a team of Researchers from Australia and Egypt have created a new thin film nano-composite (TFNC) membrane to address the issues surrounding water flux, salt rejection and biofouling in these processes.

Currently, reverse osmosis (RO) desalination technology is used in more than 50% of the world’s desalination plants for the production of fresh water. Within these technologies, thin-film composite (TFC) membranes are the most common material utilized for nanofiltration processes.

However, even though this technology is used across most of our drinking water purification processes, they are still privy to some drawbacks, namely a trade-off between both water flux and salt rejection, chlorine degradation and biofouling- all of which lead to the loss of membrane flux and salt rejection performance.

Biofouling is currently the biggest challenge facing desalination plants. Biofouling in these desalination processes has been linked to microorganisms that attach themselves to the filter membrane, where the membrane(ligand)-organism(receptor) interactions cause the formation of extracellular polymeric substances which increase the adherence of bacteria to the membrane.

To combat this, the Researchers required a material with a large (and smooth) surface area for filtering processes, which also possessed biocidal properties.

Naturally, a derivative of graphene is the obvious choice and the Researchers decided upon graphene oxide (GO) nanosheets that help improve the flux, selectivity and antibacterial properties of TFNC membranes.

The Researchers created the composite by incorporating the graphene oxide nanosheets into a thin polyamide (PA) active layer, in the form of poly tannic acid-functionalized graphene oxide nanosheets (pTA-f-GO).

The layers were produced through interfacial polymerization. The graphene oxide was first functionalized with tannic acid (TA) followed by polyethyleneimine (PEI). The tannic acid groups were found to bind tightly to the graphene oxide surface whilst the PEI groups provided free amine groups which helped to facilitate crosslinking to both the tannic acid groups and the polyamide active layer.

The crosslinking chains were found to interact very strongly with the graphene oxide sheets and tightly integrate them into the nanocomposite matrix.

The Researchers characterized the new TNFC using Transmission electron microscopy (TEM, FEI Tecnai G2 Spirit), atomic force microscopy (AFM, NT-MDT NTEGRA SPM), Fourier-transform infrared spectroscopy (FTIR, Nicolet Nexus 8700 FTIR Spectrophotometer, Thermo Electron Corporation) with a smart orbit attenuated total reflectance probe, X-ray photoelectron spectroscopy (XPS, Kratos Axis-Ultra DLD, Kratos Analytical) with CasaXPS software (Neal Fairly), electrokinetic analysis methods (Anton Paar) and captive bubble techniques.

By incorporating the pTA-f-GO layer into the TFNC membranes, the Researchers achieved a filtration material with a thinner PA layer, lower surface roughness and a higher hydrophobicity. The presence of such properties increased both the membrane water flux by up to 40% and the salt rejection by 8%.

In addition, the biocidal properties of the graphene sheet within the active layer improved the antibacterial properties of the membrane by 80% compared to standard non-composite membranes.

The process of fabrication was also found to be practical, scalable, versatile, of lower energy consumption, have an improved performance and possess an increased cost-efficiency against current methods. Such production benefits lend the nanocomposite membranes to be implemented across a wide range of applications.

Couple this with the TFNC’s excellent separation and anti-biofouling properties, and the material is one that could easily see itself become a commercially used membrane in the near future, and will perhaps help to increase the number of desalination plants around the world which use of reverse osmosis desalination methods.