Evaporation is one of the most enduring methods of purifying water to make it drinkable. Now researchers at the University of Science and Technology of China have developed a novel device made of wood that can do just that, by employing bacteria to help build key nanostructures.
The basic idea of an evaporative water purifier is something most of us would remember from science class at school. Concentrating sunshine onto the device, the water heats up and evaporates into steam, leaving behind any nasty contaminants. The steam can then be guided into a separate container where it re-condenses into clean, drinkable water.
We’ve seen this basic concept put to work many times in the past, using a variety of materials and configurations. That includes machines that float on the surface of a lake, absorbing water from below and cleaning it up, sponges encased in bubble wrap, foamy structures made of graphite flakes, solar stills made with carbon-dipped paper, and cellulose aerogels.
For this study, the new device is made up of several layers. The top is made of carbon nanotubes, which are efficient at absorbing heat from sunlight. In the middle sits a layer of tiny glass bubbles, forming a heat-insulating aerogel. Below that is a block of wood, with the water sitting underneath that.
But the key ingredient is bacteria. The team started by applying the bugs to the surface of the wood, leaving them to ferment. When the glass bubbles and carbon nanotube layers are added, the bacteria build cellulose nanofibers around them, which holds the whole thing together.
With this design, water is transported up through the wood, using the natural porous structure that helps trees stay hydrated. When it reaches the light-absorbing carbon nanotube layer on top, it heats up and evaporates. The glassy aerogel layer, meanwhile, acts as an insulator, keeping the heat from dissipating downwards.
The team says that this solar evaporator design is more effective than most. It boasts an evaporation rate of 2.9 kg m–2 h–1, and a solar-to-vapor efficiency of 80 percent. As an added bonus, wood and carbon nanotubes are relatively cheap and abundant materials.
The research was published in the journal Nano Letters.
Source: American Chemical Society
