United Nations Newest Report of the Intergovernmental Panel on Climate Change paints a grim picture of the world’s water supply: of the world’s 7.8 billion people, around 4 billion do not have access to sufficiently clean water for at least one month each year. Although several water purification systems have been proposed, they consistently fail at a critical point – typically they are not stable, large or robust enough for real-world applications, according to researchers based at the Technical Institute of Physics and Chemistry from the Chinese Academy of Sciences. and Xiamen University in China who may have solved the problem.
In Nano Research, they reported the details of a new platform that uses sunlight to purify seawater with high energy efficiency compared to other similar approaches (over 90%) while avoiding common pitfalls.
“There is a huge demand for fresh water in households and for industrial, agricultural and other applications, so various water purification technologies have been developed to alleviate the scarcity of fresh water resources,” said the author of the article Miao Wang, from the College of Materials of Xiamen University. “Comparing pathways, solar purification of seawater or contaminated water by interfacial evaporation shows promise as a low-cost system.”
Such purification approaches use sunlight to heat the water on its surface, evaporating the liquid and separating the polluted or salty counterparts of the water molecules, which then escape into the air as vapor to enter in the natural condensation cycle to become clean, drinkable water. The problem, according to article author Xu Hou, of Xiamen University’s College of Chemistry and Chemical Engineering, and College of Physical Sciences and Technology, is that attempts to scale of this approach for real-world application have been hampered by oil contamination, instability, salt-crystallization, and complicated manufacturing processes.
“To generate more purified water under the same amount of sunlight, how do you acquire more energy to locally heat the water to improve the vaporization rate – without encountering the drawbacks of other approaches?” Hou asked. “We answered this question and designed an ultra-stable, salt-resistant evaporator platform that can maintain accelerated evaporation while still repelling oil to prevent contamination.”
The researchers combined two gels into an oil-in-water emulsion. The gel material, called an organohydrogel, can transition from one phase of matter to another, but is largely liquid contained in a network of molecular chains. By dosing the organohydrogel with carbon nanotubes, the researchers created “hot spots” that can locally focus sunlight on the surface of the water, preventing heat dissipation throughout the area. The branched structure of the material also helps prevent energy dissipation and facilitates the transfer of molecules to prevent salt crystallization.
The organohydrogel has a low density, so when it dips below the water surface level, it can float again for a long-lasting evaporation process. The lifted water surrounds the evaporator, creating a lateral capillary repulsion effect in which oil molecules peel away from the heated water, like mercury peeling off the glass of a thermometer.
In experimental tests, the rig evaporated around 2.4 kilograms of water per square meter per hour and could persist for 240 hours without further removal needed – even under conditions of “tremendous oil contamination”, it said. the author of the article Shutao Wang, from the Chinese Academy of Sciences (CAS) Key Laboratory of Bio-inspired Materials and Interface Sciences, Technical Institute of Physics and Chemistry.
“We have developed an anti-fouling oil heating platform with outstanding solar energy utilization, which shows great potential for practical solar-powered water purification even in the heavily contaminated water,” Wang said. “We hope that this economical and environmentally friendly approach will help to further alleviate the global scarcity of freshwater resources.