Several methods have been proposed for laboratory-scale filtration of NFC/MFC, such as vacuum filtration, centrifugation, freeze-drying and spray-drying.

Among these methods, vacuum filtration is considered to be the most effective one for obtaining uniform and compact NFC/MFC films with high solid content and low energy consumption. Vacuum filtration involves applying a pressure difference between the NFC/MFC suspension and a porous filter medium, such as a membrane or a paper.

The water passes through the filter medium while the nanofibers are retained on the surface, forming a thin film.

The film can be peeled off from the filter medium after drying or transferred to another substrate for further processing.

Vacuum filtration can be performed using different types of filter media, such as cellulose acetate, polyethersulfone, polyvinylidene fluoride and nylon membranes, depending on the desired properties of the NFC/MFC film. Some factors that affect the quality and performance of the vacuum filtration method are the concentration and viscosity of the NFC/MFC suspension, the pore size and surface chemistry of the filter medium, the applied pressure and the drying conditions.

Abnet Mengesha Dube Thank you for your answer! Do you have practical experiences based on a published method. I would appreciate if you can share!

Totally agreed Abnet Mengesha Dube. Vacuum filtration is the most effective in filtering out larger-sized particles. But what if there are other impurities? Commercially available CNCs could have cations (Na+) and other impurities such as sugar molecules or monomers that could affect the properties of the composite materials developed by adding CNCs. Are there any other ways to remove these types of impurities in nanocellulose, especially CNCs? Dialysis could be an option but it is time-consuming.

NC may also contain cations (such as Na+), sugar molecules, monomers, or other contaminants that originate from the biomass source or the extraction process. These impurities could interfere with the surface chemistry, crystallinity, rheology, or stability of NC and its applications.

Therefore, it is important to find effective and sustainable methods for purifying NC from these impurities. One possible method is dialysis, which involves passing the NC suspension through a semipermeable membrane that allows only water and small molecules to pass through, while retaining the NC particles. However, dialysis is time-consuming and may require large amounts of water and energy. Another possible method is chemical modification, which involves attaching functional groups or polymers to the NC surface to enhance its adsorption or photocatalytic performance. However, chemical modification may also alter the intrinsic properties of NC or introduce new environmental concerns.

A more promising method for purifying NC is based on exploiting its unique physical properties, such as size, shape, charge, or optical properties. For example, centrifugation can be used to separate NC particles based on their size and density. Electrostatic precipitation can be used to separate NC particles based on their charge and polarity. Magnetic separation can be used to separate NC particles that are coated with magnetic nanoparticles . Photocatalytic degradation can be used to degrade organic contaminants that are adsorbed on NC particles that are sensitized with photocatalysts. These methods are more efficient, selective, and environmentally friendly than conventional methods for purifying NC.

In conclusion, vacuum filtration is not enough for removing all types of impurities in NC suspensions. Other methods based on physical properties of NC are more suitable for achieving high-purity NC for various applications.