“We showed that the natural shell structure could be disrupted in a gentle way and that the new, higher surface area material had some useful properties”Francesca Kerton
Researchers from the Memorial University of Newfoundland have produced a spongey form of calcium carbonate from mussel shells that could find uses as components in biomedical materials or in energy storage devices. Although most forms of calcium carbonate – which is found in the shells of mussels and other shellfish, as well as limestone, chalk and marble – are hard minerals, this new form of the substance is both soft and absorbent.
The team discovered this new spongey material by accident while looking for ways to produce a less-corrosive de-icer that could be used in vehicles and on the roads, and also for possible new applications for the waste mussel shells from the aquaculture industry. Their approach is environmentally friendly as the mussel shells are obtained from aquaculture waste streams, and therefore helps reduce landfill food waste.
As described in the journal Matter [Murphy et al. Matter (2020) DOI: 10.1016/j.matt.2020.09.022], an enzyme was used to remove residual meat from the mussel shells, before the waste shells were heated in an oven at a moderate heat. The mussels were then treated with diluted acetic acid, which caused the crystals in the shells to break apart before they recombined in the presence of the natural biopolymer with gentle stirring that produced a white spongey material.
The researchers were surprised that such a relatively simple process could discover a new form of calcite. Once some of the material was examined by x-ray diffraction, it was confirmed to be calcite formed from extra calcium carbonate that had not fully reacted with the acetic acid. They had hoped the material could be applied to treat pollution in the ocean but, on testing the substance with crude oil and dyes, although it was found to be highly absorbent the scalability and cost of producing the sponge would inhibit such an application.
As senior author Francesca Kerton told Materials Today, “It was completely unexpected to take a hard, brittle shell and turn it into something soft and compressible”. She added: “We showed that the natural shell structure could be disrupted in a gentle way and that the new, higher surface area material had some useful properties”.
It is hoped the material could take up drugs or active pharmaceutical ingredients, or work to control acid in the body for uses in biological medicine. The team will continue to investigate some adsorption processes, as well as attempting to combine the material with hydrogels, biopolymers and other materials for possible biological and medical applications.
Scanning electron microscope images of the spongey material (credit: Jennifer Murphy)