Nanofibre membrane renders seawater drinkable in minutes
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Researchers from the Korea Institute of Civil Engineering and Building Technology (KICT) have developed an electrospun nanofibre membrane which turns seawater into drinking water through membrane distillation.
Despite the vast quantity of water on Earth, just 2.5 per cent of it is freshwater and an estimated 785 million people lack a clean source of drinking water.
One of the ways to provide safe drinking water is through desalination of seawater. Now, a team of KICT engineers has presented an effective new desalination solution which takes just minutes to make seawater drinkable.
Membrane distillation is a separation process driven by phase change. Membranes normally use static pressure difference to separate materials, but in membrane distillation the hydrophobic membrane blocks liquid water while allowing free water molecules (in water vapour) through its pores.
Among the most challenging issues in membrane distillation is membrane wetting; this is the direct permeation of liquid through the pores, compromising the trapping of salt. If a membrane exhibits wetting during use, it must be replaced because a fully wetted membrane means poor performance and low-quality permeate.
This new desalination technology can prevent wetting issues and improve stability in membrane distillation in the long term. It uses membranes fabricated by electrospinning: a fibre production method which uses electrostatic forces to stretch out charged nanoscale threads. This is a popular approach to creating membranes for membrane distillation, resulting in high surface area; high hydrophobicity; high porosity, and a good level of control over pore size and membrane thickness.
The KICT researchers, led by Professor Yunchul Woo, approached fabrication with a co-axial electrospinning technique. For the core, they used the polymer PVD; for the sheath, they used a silica aerogel mixed with a low concentration of the same polymer. This allows them to produce a composite membrane with a rough, highly porous and superhydrophobic surface ideal for desalination.
The silica aerogel sheath had a much lower thermal conductivity compared with conventional polymers; the reduction of heat loss by conduction meant increased water vapour flux during distillation.
While most studies using electrospinning to create nanofibre membranes for distillation operated for less then 50 hours with high water vapour flux, Woo and his team were able to maintain stable membrane distillation for 30 days. Their membrane performed at 99.99 per cent salt rejection for this period, retaining its excellent water vapour flux without wetting and fouling issues thanks to its physical properties.
“The coaxial electrospun nanofibre membrane has strong potential for the treatment of seawater solutions without suffering from wetting issues and may be the appropriate membrane for pilot-scale and real-scale membrane distillation applications,” commented Woo.
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