Inspired by naturally-occurring mats and balls of seaweed, researchers have created highly-porous, superadhesive meshes that are capable of capturing both large and small microplastic particles – a longstanding challenge in the field. The mesh can clean microplastics from both saltwater and freshwater and is made from sustainable and widely available biopolymers.
Microplastics – a catch-all term that refers to plastic particles less than five millimeters in size – represent a major pollution problem that poses risks to both human health and the environment. One area of particular concern is the impact of microplastics on aquatic ecosystems , leading to a wide range of efforts aimed at removing these pollutants from water.
Efforts to capture plastic microparticles in water have faced a significant challenge. Some methods can capture larger microparticles – around a millimeter in size. Others can capture smaller microparticles – those measured in micrometers or nanometers. But efficiently capturing both in a single process has been a challenge.
"Our goal here was to develop a multiscale structure that allows us to capture the full range of plastic microparticles," says Orlin Velev, corresponding author of a journal article on the new work and S. Frank and Doris Culberson Distinguished Professor of Chemical and Biomolecular Engineering at North Carolina State University.
Velev and his collaborators drew inspiration from floating mats of seaweed and so-called "Neptune balls" – spherical balls of tangled seaweed – which have been shown to collect microplastics .
"We wanted to create structures that mimicked what the tangled seaweed is already doing," Velev says.
The researchers created "cleaners" consisting of a mesh of porous fibers made from biopolymers alginate and chitosan, which are derived from seaweed and crustacean shells. The surface of the mesh is covered in a layer of very fine chitosan fibers. This surface layer consists of soft dendritic colloids – structures that branch repeatedly into finer and finer filaments, ending in a tuft-like crown of nanofibers. This structure allows the soft dendritic colloids to stick to almost any surface and to directly capture polymer microparticles and nanoparticles from water.
"What you end up with looks like a fluffy net," says Velev. "The 'net' part of the structure is a mesh capable of capturing the larger plastic microparticles – a millimeter or larger in size. Further, the individual strands of the net are 'fluffy' because they are coated with soft dendritic colloids, which are able to capture by adhesion even very small plastic microparticles – down to tens of nanometers in size."
In proof-of-concept testing, the researchers found their superadhesive meshes were effective at capturing lab-produced model nanoparticles and real-world microplastics across a wide range of sizes, in both freshwater and saltwater.
So, what happens once the fluffy mesh has captured its harvest of microplastics?
The researchers say the loaded mesh could be swept up and reprocessed. One possibility would use microbial digestion to break down the microplastics and the mesh to biosynthesize more of the biopolymer material – for making more fluffy nets.
"We've demonstrated that this design works," Velev says. "And the materials we used are of natural origin and relatively inexpensive. So, it may present a viable path forward. Can it be used on a large scale? That depends on the extent to which we want to invest in scaling up such cleanup approaches."
The paper, "Artificial Neptune balls: Superadhesive biomimetic networks for broad size microplastics capture and removal," will be published August 5 in the open access journal Science Advances. First author of the paper is Haeleen Hong, a recent Ph.D. graduate from NC State. The paper was co-authored by Byeunggon Kim and Mesbah Ahmad, both Ph.D. students at NC State.
This work was done with support from the National Science Foundation, under grants 2029327, 2233399 and 2243104.
