Adaptable sponge soaks up phosphate from polluted waterways for reuse
The phosphate found in the Earth's crust is a key ingredient for the growth of aquatic plants and animals, but human activity is washing far too much of it into our waterways. One consequence of this is the algae blooms that coat the surface of some lakes and rivers, which starve other marine organisms of the oxygen they need to survive. A new sponge-like material likened to a Swiss Army knife could help scientists tackle this problem, with its ability to not just capture phosphate from water sources but allow it to be collected for reuse.
Phosphate is commonly used in fertilizers for agriculture, and on lawns and gardens in urban areas, and is also found in pet and wildlife manure. As levels build up in the soil, much of it washes away and enters creeks, rivers and other waterways to give rise to the damaging algae blooms.
Scientists at Northwestern University sought to tackle this problem with a new sponge they've called the Phosphate Elimination and Recovery Lightweight (PEARL) membrane. It is a porous and flexible material that is coated with nanostructures that bind to phosphate ions, and is therefore able to selectively sequester 99 percent of them from polluted water. By fine-tuning the pH levels in the material, it can then be made to release the compound and enable the sponge to be reused multiple times.
These capabilities were demonstrated in lab tests on real-world water samples collected from around Chicago, with the sponge proving effective on scales ranging from milligrams to kilograms. Applied to the real-world, which the researchers acknowledge is still a way down the road, this technology could offer a far more efficient way to clean up phosphate pollution than current approaches, which are costly, complicated and generate waste.
"One can always do certain things in a laboratory setting," says Vinayak Dravid, study author. "But there's a Venn Diagram when it comes to scaling up, where you need to be able to scale the technology, you want it to be effective and you want it to be affordable. There was nothing in that intersection of the three before, but our sponge seems to be a platform that meets all these criteria."
Because the sponge is also capable of releasing the captured phosphate and a huge portion of the world's crops rely on it for growth, the scientists also see it as a potential solution to the shortage of the resource moving forward.
"We used to reuse phosphate a lot more," says Stephanie Ribet, the paper's first author. "Now we just pull it out of the ground, use it once and flush it away into water sources after use. So, it's a pollution problem, a sustainability problem and a circular economy problem."
The PEARL membrane actually builds on a previous version developed by the same team, which was able to selectively remove and recover oil from polluted water. By altering the nanomaterial in the coating, the team hopes to further adapt the sponge to clean up heavy metals, and say that more of these tweaks to the design could enable it to tackle multiple pollutants at the same time – hence the team likening the technology to a potential Swiss Army knife for pollution remediation.