Thin-film composite membranes are widely used in applications such as wastewater treatment, gas separation, and chemical production. They comprise a porous support surmounted by an ultrathin layer containing nanometric pores. These pores can trap molecules and tiny particles while allowing liquid solvents to pass through.
Most of these membranes are made from materials derived from fossil fuels, some of which are toxic. Thus, a KAUST research team led by Gyorgy Szekely set out to redesign these membranes using environmentally friendly materials and processes.
The team made the porous backing using recycled plastic and coated it with a non-toxic natural polymer called chitosan, which is derived from shrimp shells. The National Aquaculture Group (Naqua) in Saudi Arabia produces around 50,000 tonnes of shrimp shell waste per year, which is used to produce 135 tonnes of chitosan per year.
To turn chitosan into a nanoporous membrane, the team cross-linked its polymer chains using 2,5-furandicarboxaldehyde (FDA), a molecule derived from plant waste via green processes. The researchers selected eucalyptol, produced from eucalyptus leaves, as the solvent for this reaction. They also used a catalyst called TMG, a greener alternative to the harsh compounds typically used to speed up crosslinking.
“Converting abundantly available waste biomass into value-added materials, like this membrane, not only solves a waste management problem, but also generates a value-added product,” says Szekely. Using waste also means the new membrane has a similar cost to conventional membranes, he adds.
After optimizing the membrane preparation process, the researchers tested the membranes using acetone, which carried polystyrene molecules of varying lengths, as well as a smaller molecule called methylstyrene dimer.
The membrane allowed acetone to flow at a rate similar to that of conventional membranes. “It can also filter out molecules equivalent in size to dyes or active pharmaceutical ingredients,” explains Cong Yang, a PhD student in the team. “Therefore, this membrane is practically applicable to biomedical, textile, pharmaceutical or food industries.”
The researchers also showed that they could fine-tune the properties of the membrane with a non-toxic solvent called TamiSolve. They now hope to work with local shrimp farms to ensure a sustainable supply of chitosan, as well as develop processes to manufacture the membranes on a larger scale.