Access to clean water is strained as the human population increases, and contamination impacts freshwater sources. Devices currently in development that clean up dirty water using sunlight can only produce up to a few gallons of water each day. But now, research published in ACS Central Science report how loofah sponges inspired a sunlight-powered porous hydrogel that could potentially purify enough water to satisfy someone’s daily needs — even when it’s cloudy.
Previously, researchers have suggested that sunlight-driven evaporation could be a low-energy way to purify water, but this approach doesn’t work well when it’s cloudy. One solution could be temperature-responsive hydrogels, specifically poly(N-isopropyl acrylamide) (PNIPAm), that switch from absorbing water at cooler temperatures to repelling it when heated.
However, because of their closed-off pores, conventional PNIPAm gels can’t generate clean water fast enough to meet people’s daily needs. Conversely, natural loofahs, which many people use to exfoliate in the shower, have large, open, interconnected pores.
So, Rodney Priestley, Xiaohui Xu, and colleagues from the Department of Chemical and Biological Engineering at Princeton University wanted to replicate the loofah’s structure in a PNIPAm-based hydrogel. It would yield a material that could rapidly absorb water at room temperature. It would then release purified water when heated by the sun’s rays under bright or cloudy conditions.
The researchers used a water and ethylene glycol mixture as a uniquely different polymerisation medium. They made a PNIPAm hydrogel with an open pore structure, similar to a natural loofah. The researchers coated the opaque hydrogel’s inner pores with polydopamine (PDA) and poly(sulfobetaine methacrylate) (PSMBA). They tested this material using an artificial light equivalent to the sun’s power.
It absorbed water at room temperature. When heated by artificial light, it released 70 per cent of its stored water in 10 minutes. This rate is four times greater than the one for a previously reported absorber gel.
The researchers say that, at this rate, the material has the potential to meet a person’s daily demand. Under lower light conditions, replicating partly cloudy skies, it took 15 to 20 minutes for the material to release a similar amount of stored water.
Finally, the new loofah-like material was tested on samples polluted with organic dyes, heavy metals, oil and microplastics. In all of the tests, the gel made the water substantially cleaner. For example, in two treatment cycles, water samples with around 40 parts per million (ppm) chromium were absorbed and released with less than 0.07 ppm chromium. That represents the allowable limit for drinking water. The researchers say their unique hydrogel structure could be helpful for applications. They include drug delivery, intelligent sensors and chemical separations.
Their paper is titled Quick-Release Antifouling Hydrogels for Solar-Driven Water Purification.
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