To create a sponge, just add acetic acid

Canadian scientists have chanced upon a soft and spongy type of calcium carbonate, a material that is usually found in harder forms in limestone, chalk, marble, and organisms such as shellfish and coral.

Blue mussel shells after initial treatment with acetic acid. Credit: Jennifer Murphy

They were exploring new uses for mussel shells leftover by the aquaculture industry at the time, attempting to create a less corrosive substance to remove ice from vehicles and roads.

Instead, they discovered that when combined with acetic acid – found in food-grade white vinegar – the shells form a strange, white spongy material.

“We thought somebody was playing a joke on us, putting bits of filter paper or something into the solution, because it didn’t look like anything that we put in there,” says lead researcher Francesca Kerton, from Memorial University of Newfoundland.

“So we scooped out some of the material and ran X-ray diffraction on it, and that told us it was calcite.”

Calcium carbonate is usually extracted by mining and quarrying, rather than from food waste products. It is an extremely versatile material, used in an incredible range of applications such as in paper, plastics, paints and coatings; in dietary supplements, toothpaste, wine and baking powder; in wastewater treatment and animal feed; and in building materials including mortar, steel and glass.

But this new sponge-like form could open up even more potential applications.

Subsequent tests by Kerton and team revealed that it was highly absorbent to oils and dyes, raising hopes that it could be used in marine pollution clean-up.

They acknowledge that the scalability and cost of making this sponge will limit that application but believe it could have a future in biomedical technologies.

“We’re interested in whether it can take up drugs or active pharmaceutical ingredients or help control acid in the body,” Kerton says. “Biological medicine might be the area where this is going to make the most impact.”

In their paper in the journal Matter, the researchers note that as the global population grows, so too does the demand for protein.

Aquaculture is arguably more sustainable than other protein sources, but increased production means increased waste – so it is important to find a use for leftover shells, relieving strain on landfill and contributing to the goals of a circular economy.

“Our results demonstrate how a natural structure can be modified in a sustainable fashion to yield a higher-value material,” the authors conclude.

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