Scientists have developed new sponge materials to repeatedly remove and reuse phosphate in polluted waters

Phosphate in the Earth's crust is a key ingredient in the growth of aquatic plants and animals, but human activities have washed too much of it into our waterways. One consequence is algae blooms on the surface of some lakes and rivers, depriving other Marine life of the oxygen it needs to survive. A new type of sponge could help scientists solve that problem by not only capturing phosphates from water sources, but also collecting them for reuse.

Phosphates are commonly used as fertilizers in agriculture, as well as lawns and gardens in urban areas, and are also found in the droppings of pets and wildlife. As the amount of phosphorus in the soil increases, much of it is washed away and finds its way into streams, rivers and other waterways, leading to destructive algal blooms.

Scientists at Northwestern University are trying to solve this problem with a new type of sponge they call Phosphate Elimination and Recovery Lightweight (PEARL) membranes. It is a porous, flexible material coated with nanostructures bound to phosphate ions, so it can selectively store up to 99 percent of phosphate ions from contaminated water. By tweaking the pH in the material, it can release compounds that can be reused many times.

These abilities have been demonstrated in laboratory tests on real world water samples collected from around Chicago, where sponges have been shown to be effective in the range from milligrams to kilograms. The researchers acknowledge that real-world applications of the technology are still some way off and could provide a more efficient way to clean up phosphate pollution than current methods, which are costly, complex and produce waste.

Study author Vinayak Dravid said: "People can always do certain things ina laboratory environment. But when scaling up, there's a Venn diagram, you need to be able to scale up the technology, you want it to be efficient, you want it to be affordable. Previously there was nothing at the intersection of all three, but our sponge seems to be a platform that meets all these criteria."

Because the sponge also releases trapped phosphates and relies on it for much of the world's crops, scientists also see it as a potential future solution to resource shortages. "We used to reuse more phosphates," said Stephanie Ribet, the paper's lead author. Now we just pull it out of the ground, use it once, and then flush it into the water. So it is a pollution issue, a sustainability issue and a circular economy issue."

The Pearl membrane actually builds on a previous version developed by the same team that selectively removes and recovers oil from contaminated water. By changing the nanomaterials in the coating, the team hopes to further tweak the sponge to clean up heavy metals, and says more tweaks to the design could allow it to handle multiple contaminants simultaneously -- hence the team's comparison of the technology to a potential "Swiss Army knife" for pollution remediation.