Providing access to safe, clean water is an urgent global challenge due to the expansion of industrialization, the growth of the world’s population and the pollution of freshwater resources. Many devices are currently under development that use sunlight to purify polluted water, but their output is often quite limited.

Now, researchers at Princeton University have designed a new sunlight-powered porous hydrogel inspired by loofah sponges that may be able to purify enough water to meet a person’s daily needs, even when it’s overcast.

Many existing solar-powered systems use sunlight-driven evaporation to purify water, but this approach doesn’t work well when it’s cloudy.

Temperature-sensitive hydrogels, mainly composed of poly(N-isopropylacrylamide) or PNIPAm, offer an alternative that absorbs dirty water at lower temperatures and then expels it in a purified form when heated.

However, according to Rodney Priestley, Xiaohui Xu and colleagues at Princeton University, the conventional PNIPAm gels cannot generate clean water fast enough to meet people’s daily needs because of their sealed pores.

To solve these problems, researchers tried to replicate the large, open and interconnected pore design of the natural loofah sponge in a PNIPAm-based hydrogel. They used a mixture of water and ethylene glycol as a uniquely different polymerization medium to create a PNIPAm hydrogel with an open pore structure, similar to a natural loofah. The researchers then coated the inner pores of the opaque hydrogel with two other polymers: polydopamine (PDA) and poly(sulfobetaine methacrylate) (PSMBA).

A porous hydrogel inspired by loofah sponges absorbs polluted water at room temperature, then quickly releases purified water when heated.
A porous hydrogel inspired by loofah sponges absorbs polluted water at room temperature, then quickly releases purified water when heated. Adapted from ACS Central Science 2023, DOI: 10.1021/acscentsci.2c01245

Xu and her team tested the resulting material with artificial light that matches the power of the sun. The material absorbed contaminated water at room temperature and then released 70% of the stored water in 10 minutes when heated by artificial light – this rate was four times higher than that of a previously reported absorbent gel.

In addition, under low-light conditions mimicking a partly cloudy sky, the loofah-inspired gel still released a similar amount of stored water within 15 to 20 minutes.

In the laboratory tests, the new loofah-like material was successfully used to remove contaminants such as organic dyes, heavy metals, oil and microplastics. In two treatment cycles, water samples with approximately 40 parts per million (ppm) of chromium were absorbed and then released with less than 0.07 ppm of chromium, which is the allowable limit for drinking water.

Xu says the unique hydrogel structure could be useful in additional applications, such as drug delivery, smart sensors and chemical separations.

Magazine reference:

  1. Xiaohui Xu, Néhémie Guillomaitre, Kofi SS Christie, R. Ko̅nane Bay, Navid Bizmark, Sujit S. Datta, Zhiyong Jason Ren, and Rodney D. Priestley. Quick-Release Antifouling Hydrogels for solar powered water purification. ACS Central Science 2023, DOI: 10.1021/acscentsci.2c01245