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‘A nanoscale solution to a gigaton pollution problem’

Researchers invent a lightweight, reusable sponge that can soak up phosphorus pollution - and release it for recycling and reuse later on
June 4, 2021

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Researchers have developed a unique sponge that uses nanotechnology to scrub phosphate pollution out of water, and possibly repurpose this precious material as fertilizer back on agricultural fields.

The invention could provide a means to recycle phosphorus on a large scale, instead of us continually mining this dwindling, non-renewable resource out of earth’s crust.  

Phosphorus is a chemical element that’s often described as a ‘building block’ of life, because it is crucial for the growth of every living thing; it’s part of cell membranes and is embedded in our DNA. This ingredient has also become particularly important for our global crop systems, which account for the large majority of all phosphorus use on earth, via humans’ enthusiastic application of fertilizer.  On farms around the world, phosphorus-enriched fertilizers are spread liberally over soil to fuel plant growth and boost productivity. But this touchstone of modern agricultural production leads to a problem downstream, when run-off transports excess phosphorus into rivers, lakes, and the sea. There, it causes phosphate pollution—triggering massive algal blooms that suffocate the waters and leave them devoid of life.

Meanwhile, as this excess of phosphorus plagues earth’s freshwater bodies, phosphorus reserves globally are in a general state of decline. Estimates vary, but some suggest that we have between a few decades to a few hundred years of raw, mineable phosphorus left on earth—largely thanks to agriculture’s voracious appetite for this resource. 

What often gets left out of this equation, however, is the potential to recycle existing phosphorus, and extend the lifespan of this important resource. This is where the new invention—which hails from Northwestern University—comes in. 

The sponge-like structure, called the ‘Phosphate Elimination and Recovery Lightweight membrane’ (PEARL) builds on pre-existing technology to extract oil from water bodies. This new iteration uses nanostructures, incorporated into the material of the sponge, to attract phosphate ions in water, effectively ‘soaking’ them into the material.

But the real novelty is that the researchers are then able to ‘tune’ the material to make the sponge release these ions, by placing it in a solution that has a lower pH. This facilitates the release and recovery of the phosphorus within – and also enables reuse of the PEARL membrane, itself. “We can make use of this pH sensitivity to reuse the PEARL membrane for multiple cycles,” says Stephanie Ribet, lead author on the study, and PhD researcher at Northwestern University. “Ability to adjust pH is a very standard – and cost-effective – and is often innate in wastewater treatment,” adds Vinayak Dravid, professor of material science and engineering at Northwestern University, and the study’s corresponding author.

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So far, the material has been tested in the laboratory on real samples from the surrounding Chicago water system, and was able to soak up 99% of the phosphate ions in the water samples, in just a matter of minutes, suggesting it could work at-scale in the real world. The researchers envision that the material could be deployed as absorbent pads, floating booms, or filter-like structures at inlet and outlet sources in lake systems and other water bodies.

If these were put to use on a large scale, the idea is that the captured phosphate could then be released, gathered, and recycled, Ribet says. “We aim to capture these essential nutrients for further reuse, such as in agricultural applications.” This would keep the material in circulation, instead of perpetuating the unsustainable pattern of use and disposal that we’ve followed for so long. What’s more, says Ribet, “By modulating the nanocomposite coating, other types of contaminants can also be targeted. Moreover, a combination of different coating materials can be used to remove and recover multiple pollutants, at the same time.”

Still, the sponge isn’t a silver bullet: fully solving the problem of phosphate pollution will also require other on-farm solutions alongside this new technology, to directly limit the excess of fertilizer that’s applied to soil. In fact, this is one of the primary goals of precision agriculture, which seeks to reduce excess nutrient leaching with more targeted applications of fertilizer that only deliver to plants what they will consume. 

But, with a problem as expansive as phosphate pollution, the best chance of success will be to tackle it from multiple angles – and this new invention could be one particularly powerful approach. As Dravid described it, the invention is a “nanoscale solution to a gigaton problem.” 

Ribet et. al. “Phosphate Elimination and Recovery Lightweight (PEARL) Membrane: A Sustainable Environmental Remediation Approach. Proceedings of the National Academy of Science. 2021.

Image courtesy Northwestern University

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