Biodegradable hydrogels can clear microplastics from water
Microplastics are small particles of plastic under 5 mm. Widely used for a range of products from cosmetics to clothes, they have become dispersed throughout the environment. It has been estimated that 1.3 million tonnes of microplastics enter the ocean annually. Alongside a reduction in microplastics use, solutions are needed to remove those already present in the environment. However, their small size makes them difficult to remove, especially from sewerage and wastewater systems. Added to that, the synthetic filtration and/or coagulation processes typically used can be environmentally damaging. The Marie Skłodowska-Curie Actions (MSCA) funded MICROPLASTINE project has developed biodegradable biopolymer hydrogels that offer an environmentally friendly alternative. The biodegradable biopolymer hydrogels trap and aggregate microplastic contaminants, enabling them to be easily removed from wastewater. “Our biodegradable gelatine hydrogels could trap and remove over 98 % of microplastic contaminants in a model wastewater solution, and over 70 % in highly saline environments, such as seawater,” notes project coordinator Tim J. Wooster. The project demonstrated that a wide range of microplastic contaminants could be trapped including polystyrene, polymethacrylate and polyethylene. Laboratory testing also confirmed the high biodegradability of the biopolymer hydrogel particles in lake and seawater.
The unique properties of hydrogels
Biodegradable gelatine hydrogels naturally have a positive surface charge that can attract negatively charged microplastic contaminants to their surface. The more microplastic contaminants that stick to the hydrogel’s surface, the heavier it becomes causing it to sink and settle at the bottom of wastewater, from where it can be separated and removed. Juliette Behra, MICROPLASTINE’s MSCA postdoctoral fellow, benefited from the expertise of project host Nestlé Research and their understanding of natural biopolymers to create the biodegradable hydrogels for the study. The microstructure of these biopolymer hydrogels was analysed using rheology and advanced light scattering techniques, leading to a discovery about how they change their microstructure upon cooling. “Studying the hydrogel structure during chemistry (syneresis), when liquids leave gels, has advanced our understanding of how stress builds up in hydrogels, helping to make robust structures,” adds Behra.
When hydrogel meets microplastic
Laboratory experiments were conducted to study how the biopolymer hydrogels interacted with model microplastics of varying charges, to determine how best to maximise microplastic binding. Experiments indicated that the biopolymer hydrogels were capable of successfully removing around 98.5 % of contaminants, with the hydrogels easily removed from the bottom of wastewater containers. As the charge of the hydrogels is pH-sensitive, trapped microplastics can be quickly released by changing the pH, meaning that the hydrogel beads can easily be regenerated after removal from the wastewater source. The ability of these biopolymer hydrogels to bind and release weathered microplastics was then also successfully tested. Finally, the biodegradability of the biopolymer hydrogels in actual sea and lake water was lab tested, revealing a high degree of biodegradability. “And crucially, initial laboratory testing suggested that the process is over 80 % reversible, meaning that the hydrogels could potentially be regenerated for reuse,” explains Behra.
Supporting EU commitments to reduce microplastic contamination
MICROPLASTINE’s results support EU commitments to reduce the amount of microplastics in the environment, protecting marine, animal and plant life. “This is an emerging technology that shows very promising initial laboratory results. However, a systematic approach is needed to address potential challenges during upscaling and translation to actual wastewater treatment plants,” says Wooster. Towards this end, the team is now working to secure further investment alongside future research collaborations.
Keywords
MICROPLASTINE, microplastics, hydrogel, wastewater, biodegradable, sewerage, biopolymer
