Co-authors Vinayak Dravid and Stephanie Ribet examine their phosphate elimination and recovery substrate. Credit: Northwestern University
New research suggests that a membrane-embedded with specific nanostructures could provide a ‘plug & play’ solution to removing pollutants from aquatic ecosystems.
While concentrations of phosphates in rivers, lakes and other bodies of water continue to soar, reaching dangerous levels, the global farming industry is having the opposite problem. Reserves of phosphate fertilizers — the key to farming that forms at least half of the world’s food chain — are dwindling.
This means that non-renewable phosphates are causing eutrophication — the enriching of bodies of water with minerals and nutrients that result in the propagation of harmful blooms of algae — in many lakes. The irony of this situation is something that hasn’t been missed by science.
While historic attempts to deal with the ecological problem of phosphate pollution in waterways have focused on just eliminating such contamination, recently interest in methods that could recover and reuse this natural resource has grown.
A development pioneered by a Northwestern University team of scientists could hold the answer to tackling this problem. The team has developed a way to remove phosphate from waterways and reuse it. The system hinges on a porous flexible membrane that can grab up to 99% of phosphate ions from polluted water.
The team has dubbed their system Phosphate Elimination and Recovery Lightweight (PEARL) and says that as an added benefit, the system can be employed repeatedly. This is because the surface of the membrane is studded with nanostructures that bind to phosphate. These nanostructures also allow the PEARL system to be switched from absorbing to releasing nutrients.
The researchers describe their breakthrough as a ‘Swiss Army Knife’ for tackling pollution. By changing the nanostructures, the system can be tailored to absorb and release other pollutants and phosphate.
Phosphorous-containing chemicals were the natural choice for ‘proof of concept’ testing due to how ubiquitous they are and that they are desperately needed in other areas.
“We used to reuse phosphate a lot more. Now we just pull it out of the ground, use it once, and flush it away into water sources after use,” says Stephanie Ribet, a Materials Science and Engineering Ph.D. candidate at Northwestern University. “So, it’s a pollution problem, a sustainability problem, and a circular economy problem.”
Ribet is the lead author on a paper documenting PEARL to be published in the journal Proceedings of the National Academy of Science.
The Importance of Phosphate Control
Phosphorous is something vital not just for farming and the world’s food supply, but to all living things. Not only does this element form an important part of cell membranes, and the scaffolding of DNA, it is a vital part of the bones and skeleton structures of vertebrates.
Whilst other organic molecules vital for life — such as oxygen and nitrogen — can be found in the atmosphere, phosphorous is mostly absent from this source. And whilst small amounts of phosphorous can be recovered from Earth’s crust, the weathering process which releases it can take millions of years, and reverses are rapidly running out.
Clearly, this lack of phosphorous is a problem, but an overabundance of the same can be just as problematic. Phosphates are vital to all living things and algae is no exception. Excess phosphates in bodies of water can cause algae growth to accelerate and lead to algal blooms that turn waterways green. And, beyond this cosmetic effect, algal blooms also de-oxygenate aquatic ecosystems changing the pH balance and proving a threat to the living things that exist within them.
The PEARL system can control these pH levels by absorbing or releasing phosphates, thus helping to maintain the delicate balance of ecosystems.
Testing PEARL in Real Water Systems
The team hopes that PEARL can fulfill several important and desirable criteria for a phosphorous recovery and reuse system. Boxes which, in the past, no single system has managed to check off in a ‘real world’ scenario.
“One can always do certain things in a laboratory setting,” says Vinayak Dravid, Abraham Harris Professor of Materials Science and Engineering at Northwestern University, and the study’s corresponding 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.
Vinayak Dravid, Abraham Harris Professor of Materials Science and Engineering, Northwestern University
Another issue with current systems is that they produce a great deal of waste and are based on complex and lengthy processes which consist of multiple steps. PEARL, on the other hand, consists of a single-step process that generates zero physical waste.
The team tested PEARL using water samples collected from Chicago’s Water Reclamation District, which processes almost 500 billion gallons of wastewater across seven plants each year. Using real water ensured that PEARL was being tested against samples that are as complex as those it would be pitted against in the ‘real world.’
“We often call this a ‘nanoscale solution to a gigaton problem,’” Dravid said. “In many ways, the nanoscale interactions that we study have implications for macrolevel remediation.”
The series of tests conducted by the team with PEARL — based upon an earlier oil-removal sponge system devised by Dravid — shows it to be effective on scales ranging from milligrams to kilograms suggesting that it should be viable for scaling-up.
The team will now adapt the PEARL system to tackle another type of pollution that troubles global waterways — heavy metals. This can be done by switching out the nanostructures embedded into the sponge-like substrate at the heart of the system.
The team hopes that this ‘plug & play’ approach to materials science could lend itself to multiple different pollutants beyond oil, phosphates and heavy metals.
This water remediation challenge hits so close to home. The western basin of Lake Erie is one of the main areas you think of when it comes to eutrophication, and I was inspired by learning more about the water remediation challenges in our Great Lakes neighborhood.
Stephanie Ribet, Materials Science and Engineering Ph.D. candidate, Northwestern University
References
Ribet. S. M., et al, [2021], “Phosphate Elimination and Recovery Lightweight (PEARL) membrane: A sustainable environmental remediation approach,” PNAS, [www.pnas.org/cgi/doi/10.1073/pnas.2102583118]
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