Microbes could be used to collect microplastics from wastewater

Microplastics are fragments of plastic less than half a centimetre in length, including fragments of larger plastic objects. Much remains unknown about the movement of microplastics in the environment, although they have been found at high levels in all sorts of environments. Due to their size, conventional methods for disposal such as incineration and storage in landfill are limited.

“They are not easily biodegradable, where they remain in ecosystems for prolonged durations,” said Professor Yang Liu, who led the research. “This results in the uptake of microplastics by organisms, leading to transfer and retention of microplastics down the food chain.

“Due to their huge surface area and absorption capacity, microplastics can absorb toxic pollutants such as pesticides, heavy metals, and drug residues at high concentrations. This leads to biological and chemical toxicity to organisms in ecosystems and humans after prolonged unintended consumption of such microplastics. Moreover, microplastics are also difficult to remove in wastewater plants, resulting in their undesired release into the environment.”

The Hong Kong Polytechnic University researchers have proposed using bacterial biofilms (a sticky shield formed by clumps of bacteria to protect against external threats such as antibiotics) to trap microplastics. The biofilm is then processed and dispersed, releasing the microplastic particles for processing.

Liu and his colleagues experimented with capturing microplastics using the bacterium Pseudonomas aeruginosa. This species of bacteria is found in all environments, and has been shown to colonise microplastics. It causes microplastics to aggregate together, eventually causing them to sink and making them more convenient to collect.

Once the microplastics had been captured by the biofilms and sunk to the bottom of the bioreactor used for the demonstration, the researchers used a biofilm-dispersal gene, which caused the biofilm to release the microplastics.

“[This] allows convenient release of microplastics from the biofilm matrix, which is otherwise difficult and expensive to degrade, so that the microplastics can be later recovered for recycling,” said Liu.

Next, the researchers will move from the proof-of-concept stage in the lab to an environmental setting. They will isolate and identify natural pro-biofilm forming and dispersing bacterial isolates either from sewage of aquatic environments, where they show heightened abilities to colonise and form biofilms on microplastics. They hope that eventually the technique could be used in wastewater plants to stop microplastics escaping into the ocean.

“It is imperative to develop effective solutions that trap, collect, and even recycle these microplastics to stop the 'plastification' of our natural environments,” said Liu.