To remove microplastics from water, researchers have developed tiny robots that bind to the plastic particles and break them down with the help of sunlight. The robots are only about the size of a bacterial cell and, thanks to a clever combination of materials, can move independently and specifically bind to microplastic particles. There they act as a catalyst and accelerate the degradation through sunlight. The feasibility study shows that the principle works in principle. If it also proves itself in practice, the microrobots could help to reduce microplastic pollution in the oceans.
Microplastics are ubiquitous in the environment – in urban areas as well as in arctic ice and the oceans. It has also been detected in numerous foods, drinking water and even in human stool. It is still unclear what impact these tiny plastic particles have on human health. Microplastics are mainly created when larger plastic parts break down into smaller parts due to the effects of the weather. Once in the environment, it has so far been almost impossible to remove the particles, which are less than five millimeters in size. It takes hundreds of years to decompose naturally. It goes a little faster through the influence of sunlight. However, this also takes several years or even decades.
Microrobots on the hunt for plastic
A team led by Seyyed Mohsen Beladi-Mousavi from the University of Chemistry and Technology in Prague has now tested a way of using sunlight effectively to decompose microplastics. The key to this are so-called photocatalysts, which use the energy of sunlight to generate highly reactive compounds that are able to chemically break down plastic into its components. The problem with previous approaches was to bring the catalysts into contact with the plastic particles. In the past, this required extensive pre-treatments or bulky mechanical agitators. Such systems could not be used in the wild – especially since some of the catalysts themselves were toxic and would have further polluted the environment.
Mohsen Baladi-Mousavi and his colleagues have now found a possible solution to these problems: They constructed microrobots from photocatalyst materials that swim independently to the microplastic particles, bind to them and decompose them. At four to eight micrometers, the robots are only about the size of a bacterial cell. Its main component is a star-shaped structure made of the semiconductor material bismuth vanadate, which acts as a photocatalyst. Each particle is coated with magnetic iron oxide. The microrobots are powered by visible light and hydrogen peroxide in a concentration that occurs naturally in water.
Plastic surface becomes holey
“The intelligent, visible light-powered microrobots can capture and break down microplastics as they swim past,” the researchers write. In their experiments, they made the microrobots swim through a maze of canals containing four different types of plastic. The robots interacted particularly well with particles of the plastics polylactic acid (PLA) and polycaprolactone (PCL). Here they caught around 70 percent of the particles. They were somewhat less effective for polyethylene terephthalate (PET) and polypropylene (PP) – probably because the surface of these plastics is particularly water-repellent.
For seven days, the researchers let the robots interact with the plastic particles, illuminating them continuously. Before and after the treatment, the team analyzed the weight and surface structure of the microplastic. The result: after a week of contact with the microrobots, the particles had lost up to three percent of their weight. The degradation worked best at PCL. PET particles, on the other hand, only lost about one percent of their weight. “It is noteworthy that the morphology of all types of microplastic has changed dramatically, from smooth surfaces to rough, uneven structures, with a few holes in the case of PCL,” the researchers write. They also found degradation products of all plastics in the surrounding solution. “The microrobots are basically able to further decompose the monomers and oligomers obtained; however, this study only focused on the initial degradation of plastic parts, ”the authors say.
Use in the environment is conceivable
Unlike previous systems, the microrobots could also be suitable for environmentally friendly use in nature. “Thanks to their magnetic properties, the microrobots can be collected after the experiments, which is beneficial in terms of recyclability,” the researchers report. It is also possible to steer the robots to the desired location with the help of magnetic fields and to remove them safely from the environment. Whether they can actually be recycled afterwards depends on further developments: In the current experiment, their performance had already decreased significantly after seven days.
“How to counter the increasing pollution with microplastics is an important research focus, especially in view of the fact that appalling amounts of microplastics are discovered in the environment and there are potential risks to marine systems and human health,” the researchers write. “The novel approach of this study with the additional element of autonomous movement enables the efficient collection and degradation of microplastics and can pave the way to new intelligent systems that can be used in difficult-to-access environments.”
Source: Seyyed Mohsen Beladi-Mousavi (University of Chemistry and Technology, Prague, Czech Republic) et al., ACS Applied Materials & Interfaces 2021, doi: 10.1021 / acsami.1c04559