Schematic of the electrospinning device used to make this nanofiber membraneElsevier
The World Health Organization estimates that over 785 million people lack a clean source of drinking water. Even though 71 percent of the Earth’s surface is covered by seawater, we cannot drink this water. Scientists around the globe have been working to find efficient ways to desalinate seawater cheaply. Now, a team of Korean scientists may have found a way to do it in minutes.
Freshwater, needed for most human activities accounts for only 2.5 percent of the total water available on the planet. Changing climate conditions are resulting in changes in precipitation and dry rivers, prompting countries to declare water shortages for the first time in their history. It is hardly surprising that desalination of seawater is the easiest way to address this issue. But these processes have their own limitations. One such limitation is membrane wetting.
When using membranes to filter seawater, the membrane must remain dry for long periods of time. If the membrane becomes wet, the filtration process becomes ineffective and allows large amounts of salt to pass through the membrane. For long term operations, progressive membrane wetting has been observed regularly, which be resolved by changing the membrane.
Researcher Yunchul Woo and his team at the Korea Institute of Civil Engineering and Building Technology (KICT) have now developed a membrane that is less susceptible to wetting and is stable in the long term.
The membrane is made of nanofibres that have been fabricated into a three-dimensional hierarchical structure, This was achieved by using a type of nanotechnology called electrospinning. Using this technology, the researchers were able to fabricate a membrane that is highly hydrophobic — i.e. water repellent.
The hydrophobic nature of the membrane is helpful because it is designed to not allow water molecules to pass. Instead, a temperature difference is applied on the two sides of the membrane that causes water from one end to evaporate into water vapor. The membrane allows water vapor to pass, which then condenses onto the cooler side. Called, membrane distillation, this is a commonly used method of desalination using membranes. Since the salt particles are not converted to the gaseous state, they are left out on one side of the membrane, giving highly purified water on the other side.
The Korean researchers also used silica aerogel in their membrane fabrication process which further enhanced the flow of water vapor through the membrane, providing quicker access to desalinated water. The team tested their technology for continuous operation for 30 days and found that the membrane continued to filter out 99.9 percent salt without any wetting issues.
The new water-purification device features a top layer of light-absorbing carbon nanotubes, a middle layer of insulating glass bubbles and a bottom layer water-transporting wood. Photo by Qing-Fang Guan, et al. / Nano Letters 2020
July 8 (UPI) — Engineers have developed a new wood-based steam generator that can purify water using bacteria-produced nanomaterials and the sun’s energy.
Solar steam generators, which use the sun’s energy to separate water molecules from contaminants via evaporation, aren’t new, but the quest to make the technology as efficient as possible is never-ending.
When working to improve upon solar steam generators, scientists can focus one or more sources of inefficiencies: light absorption, heat management, water transport or evaporation.
Researchers at the University of Science and Technology of China developed a water purification device that improves on all four processes.
The team of scientists selected wood for its sustainability, and because its porosity allows for rapid water transport. To bind the device’s wood layers, researchers relied on long cellulose nanofibers, which are produced by bacteria.
After sterilizing the wood, scientists applied the bacteria strain Gluconacetobacter xylinus and allowed it to ferment on the back of a block of wood. Next, researchers sprayed on a layer of aerosolized glass bubbles, an excellent insulator.
“The glass bubbles became embedded in the cellulose nanofibers produced by the bacteria, forming a hydrogel,” researchers explained in a news release.
The cellulose nanofibers formed by the bacteria worked to bind together layers of the device together. To complete the device’s light absorbing top layer, researchers applied carbon nanotubes, which became intertwined with the cellulose nanofibers.
The device — detailed Wednesday in the journal Nano Letters — works by pulling and filtering water up throw the wood layers to the light absorbing top layer, which is heated by the sun. The purified water evaporates and is collected and condensed above and funneled to a holding tank.
The carbon nanotubes and cellulose nanofibers lower the energy required for water vaporization, while the layer of glass bubbles ensure the sun’s energy doesn’t dissipate down through the wood layers.