As water stress increases worldwide and 2 billion people still lack access to safe drinking water at home, researchers are developing an inexpensive sponge that can clean any water source simply by exposing it to the sun.
In 2020, around one in four people worldwide – or 2 billion people – did not have access to safe drinking water at home. According to the WHO-UNICEF report “Progress in Household Drinking Water, Sanitation and Hygiene, 2000-2020” (Joint Monitoring Program on Water, Sanitation and Hygiene), 1.6 billion people will still have no drinking water at home when the current trend continues . The reason: the lack of infrastructure for water treatment and distribution. There are several individual solutions, however, they remain quite expensive or somewhat risky.
Princeton University researchers have developed a new generation of sun-absorbing gel, a device that could hold the key to bringing clean water to people around the world.
This device, the first version of which dates back to 2021, works with the sun a bit like a sponge, only letting through purified and drinkable water, leaving impurities behind. This spongy gel is inexpensive, easy to use and only requires sunlight to filter pollutants such as heavy metals, oils, microplastics and certain bacteria from water, making it a reliable alternative to off-grid water purification.
The latest device developed shows a filtration rate almost four times higher than that of the first generation technology. One square meter of the one centimeter thick material can produce around 4 liters of water in just 10 minutes. Details of the new sun-absorbing gel were published in ACS Central Science on February 8, 2023.
“Much effort has been expended to develop technologies that use solar energy to produce clean, potable drinking water, but they often do not produce enough water to meet daily needs,” said Rodney Priestley, Dean of Graduate School, Pomeroy and Betty Perry.
At the heart of this device’s spongy appearance is a gel formed from a polymer known as poly(N-isopropylacrylamide) or PNIPAm, which can either absorb or release water depending on the temperature.
Below 33°C, this hydrogel acts like a sponge, absorbing water from a source such as a lake. But when the hydrogel is taken out of the water and heated to a temperature above 33°C by sunlight, it starts to release the water. By adding polymers such as polydopamine (PDA) to the gel surface, the device can filter pollutants such as oils, heavy metals, microplastics and certain types of bacteria from the water.
This gel is very easy and inexpensive to use compared to systems based on evaporation: simply place the sponge in a water source until saturated with water. Then it must be removed and then exposed to the sun while waiting for the filtered water to be released in a container. Under the midday sun, the gel can release about 70% of the water absorbed in just ten minutes.
“Our first sun-absorbing gel already performed well,” said Xiaohui Xu, first author of the study. “But we wanted to make the device even more efficient at filtering water,” including a significant increase in filtration speed. This was made possible by modifying the hydrogel’s honeycomb structure for an interconnected fibrous structure of the new hydrogel to that of a ripe loofah fruit, commonly used as a scouring pad in bathrooms and kitchens.
In addition to its improved filtration rate, Néhémie Guillomaitre, study co-author and PhD student in chemical and bioengineering, added other improvements such as antifouling properties by adding another polymer, poly (sulfobetaine methacrylate) (PSBMA), to the surface the gel. Not only does the PSBMA help the device filter impurities out of the water even more effectively, it also bonds tightly with the water molecules on the surface of the gel to form a layer of moisture that repels oil and stains bacteria, allowing the device to be self-cleaning .
“Stain-resistant properties help the gel last longer,” says Guillomaitre, who adds, “You don’t have to worry as much about oils and bacterial films building up on the gel’s surface over time and reducing its effectiveness.”
This new absorbing solar gel could become an attractive option for household-scale water purification, providing off-grid drinking water at a lower cost. Therefore, prototypes are being tested to demonstrate that this device can be adapted for domestic use, increasing our resilience in the face of a world on the brink of WW3.
However, this gel could already be used in emergency situations, for example in the case of major earthquakes such as that of February 6, 2023 in Turkey.
With support from the National Science Foundation, the research has led to the creation of a startup that Priestley co-founded, AquaPao, which will continue to iterate and improve the sun-absorbing gel’s design, test its long-term durability, and identify opportunities for technology scaling. “This work is a wonderful example of how academic research can be translated into the startup world,” said Priestley.
references
- Xu, X., Ozden, S., Bizmark, N., Arnold, CB, Datta, SS, Priestley, RD, A bio-inspired elastic hydrogel for solar-powered water purification. Adult Mater. 2021, 33, 2007833.
- Manning SW, Kocik C, Lorentzen B et al. A severe multi-year drought coincides with the collapse of the Hittites around 1198–1196 BC. together. Nature (2023).
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