Diagram of nanoplastic particles with SPIONs and agglomerates (top). (Bottom) Photos showing magnetic removal from left: nanoplastic dispersion; after addition of SPIONs; and after contact with a magnet.Our dependence on plastics has led to the emergence of a new type of waste. Microplastics waste – micro- or nanosized plastic particles used as filling materials or broken-down fragments of larger items – are now being detected around the globe in soil and water. Removing microplastics waste from water, however, is no easy task as conventional remediation approaches are unsuitable.
Now researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany have designed functionalized superparamagnetic iron oxide nanoparticles or SPIONs that capture plastic waste particles [Sarcletti et al., Materials Today (2021), https://doi.org/10.1016/j.mattod.2021.02.020 ]. Sticky SPIONs clump together with microplastics waste, forming agglomerates that can be simply removed from water magnetically.
“Microplastics… are strongly suspected to be detrimental to our health, have been found everywhere from the Antarctic to the depths of the ocean and, although they have been detected in organisms, are particularly prevalent in water,” explains Marcus Halik, who led the work.
Up to 12,000 million tons of plastic waste could have accumulated in the environment by 2050 according to estimates and would persist for decades. Natural processes and weathering break plastic waste down into microplastics, between 1 mm and 1 micron in size, and nanoplastics less than 1 micron in size. The waste is made up of a range of materials, including polystyrene (PS), poly(methyl methacrylate) (PMMA), and polyvinyl chloride, in different forms like particles or fibers.
The researchers designed core-shell magnetite and hematite SPIONs with different molecular surface monolayers based on phosphonic acid molecules to produce positively or negatively charged particles. These tiny particles stick to microplastics via electrostatic and van der Waals interactions, acting as a glue to bond smaller plastic particles into larger aggregates. Differently charged SPIONs attract different types of microplastics, in this case two of the most common plastics – PS and PMMA, working in harmony to remove a mixture of the two materials from water. A dispersion of coated SPIONs is simply added to water contaminated with microplastics and stirred. The resulting clumps of microplastics and SPIONs can then be removed magnetically.
“The ground-breaking approach behind this concept is that the surface functionalization of SPIONs can be adjusted to attach predominantly to specific types of plastics,” points out Halik. “The concept is flexible enough that it can be adapted to work efficiently with a wide range of nanoplastic mixtures, reflecting the situation in the environment,” he adds.
Crucially, the molecules on the surface of SPIONs attach specifically to plastic particles rather than inorganic sediment, which would reduce the efficiency of the remediation process. SPIONs, which are already used in medicine for cancer treatment and contrast imaging, are nontoxic, according to the researchers, and they are now working on a practical remediation system.