With accessing clean water being increasingly difficult due to population rise and greater contamination of freshwater sources, researchers at Princeton University have developed a porous solar absorber gel for water purification inspired by the loofah sponge. Their sunlight-powered hydrogel absorbs polluted water at room temperature before quickly releasing purified water when heated, enough for a person’s daily requirements.
While other sunlight-driven evaporation processes can purify water, they are less effective when it is cloudy, and tend to produce only a few gallons of water per day. One approach has been to use temperature-responsive hydrogels such as poly(N-isopropyl acrylamide) (PNIPAm), which switch from absorbing water at cooler temperatures to repelling it on being heated. However, standard PNIPAm gels can’t produce clean water sufficiently quickly due to their closed-off pores.
As reported in ACS Central Science [Xu et al. ACS Cent. Sci. (2023) DOI: 10.1021/acscentsci.2c01245], this led the team to investigate copying the structure of natural loofahs due to their large, open and interconnected pores in a PNIPAm-based hydrogel. A mixture of water and ethylene glycol was used as a polymerization medium to produce a PNIPAm hydrogel with an interconnected open pore structure, before coating its inner pores with polydopamine (PDA) and poly(sulfobetaine methacrylate).
The resulting hydrogel was found to absorb water at room temperature and, after being heated by the artificial light with a heat equivalent to that of the sun, released 70% of its stored water in 10 minutes, four times faster than a previously reported absorber gel. Under lower light conditions to mimic a cloudy sky, it took up to 20 minutes for the material to release a similar amount of stored water. When the new porous material was tested on water contaminated with organic dyes, heavy metals, oil and microplastics, it also made the water significantly cleaner, a breakthrough that could help in treatment of wastewater by industry.
As the structure is completely different from a conventional hydrogel with a closed-cell structure, it significantly improved the water transport rate. As Xiaohui Xu told Materials Today, “We noticed natural loofah has rapid liquid permeation, this is attributed to its open-pore network interconnected by cellulose fibers. Inspired by the loofah, we developed this second-generation hydrogel for water purification.”
The team now hope their hydrogel will find applications in other areas, including drug delivery, smart sensors and chemical separations, and are exploring the development of an antibacterial hydrogel able to efficiently kill water-born bacteria. They also hope to test the gel’s ability to purify water contaminated with toxic per- and polyfluoroalkyl substances (PFAS), known as “forever chemicals”.
“We noticed natural loofah has rapid liquid permeation, this is attributed to its open-pore network interconnected by cellulose fibers. Inspired by the loofah, we developed this second-generation hydrogel for water purification.”Xiaohui Xu
New porous solar absorber gel for water purification