Tackling microplastics and water pollution with magnetic “rust”

by | Oct 20, 2023

Magnetic iron nanoparticles shown to eliminate persistent water pollution, such as microplastics and oils.
Plastic bag in the ocean.

Beyond the problem of greenhouse gas emissions and their contribution to the climate crisis, the widespread use of hydrocarbons, the fundamental components of fossil fuels, presents significant environmental challenges.

“Hydrocarbons represent the most abundant class of organic pollutants on Earth,” said Marcus Halik, professor in the Department of Materials Science and Engineering at Friedrich-Alexander-Universität. “Almost all organic pollutants originate from crude oil, and the simplest components of oil, with structures of (-(CH2)n-), are widely used as fuels for cars and airplanes, household heating fuels, or as polymer materials (which we all know as plastic).”

Despite the alarms being raised over the devastating impacts of climate change — many of which are already unfolding — the problem remains that fossil fuels still drive the global economy.

“The world production of crude oil as main source of liquid fuels still exceeds 4200 million tons (Mt) added by >4000 billion m3 of natural gas in 2021,” wrote Halik and his team in a paper recently published in Advanced Science.

Besides the staggering volumes of greenhouse gases released into the atmosphere, the pervasive presence of microplastic pollution derived from hydrocarbons has also raised alarm. Scientists have recently discovered microplastics in the most remote corners of our planet, such as the Arctic, and more recently, even within clouds.

“Hydrocarbons are chemically very stable and have a low tendency to degrade in the environment — this is called persistence,” added Halik. “[They] accumulate in the soil and are taken up by plants and animals but the major transportation pathway to distribute them around the world is water.”

Cleaning up plastic pollution is imperative

While the transition away from fossil fuels and the exploration of alternative options for plastic products are gaining momentum, the removal of the multitude of plastic pollution currently plaguing our environment requires an urgent solution.

“This means a simple, reliable method to remove all kind of hydrocarbons from water,” wrote the team. But options for removing micro- and nano-plastics are limited so far. “It is possible to remove portions of the microplastics by skimming and flocculation [the clumping together of small particles],” they continued. “However, the smaller pieces are still a huge challenge for today’s wastewater treatment plants.”

When mechanical and trapping methods proved unfeasible, Halik and his team took a different approach. They developed a universal remediation method to eliminate a variety of hydrocarbons, ranging from C15 to C∞, by employing superparamagnetic iron oxide nanoparticles, abbreviated as SPIONs.

“SPIONs are basically ‘rust’ of very small size (approximately 10 nm) with huge active surfaces,” explained Halik. “The superparamagnetism, as a size-related effect of the material, ensures that the particles do not attract each other but they can be [collected using] an external magnet.”

Halik also explained that the SPION are inexpensive and non-toxic, and their surface chemistry can be tuned, making it possible to capture a range of different hydrocarbons. For example, for low-weight, liquid hydrocarbons that are hydrophobic, meaning they don’t mix well or dissolve in water, the iron particles surfaces were fitted with similar carbon-based molecules that interact favorably with them.

“These similar chemical structures [of the hydrocarbon and the molecules on the SPION surface] ‘like’ each other more than the surrounding water,” said Halik. “Both are hydrophobic in water. As consequence, they find each other and the hydrocarbon droplets get loaded with SPIONs.”

In the case of solid hydrocarbons, the surface of the SPIONS are fitted with molecules of opposite electrical charge. “While most plastic nanoparticle species have slightly negative surface charge, the SPIONs are tuned positive,” said Halik. This allows the iron nanoparticles to act as a sort of glue between plastic particles, which form electrostatic clumps that can also be collected using a normal magnet.

Capture demonstrations

In their paper, the team demonstrated the effectiveness of their magnetic water cleanup technology by successfully removing various organic pollutants from diverse water samples, including oils and microplastic particles.

The fact that the bonds the iron particles form with the hydrocarbons are non-covalent and no real chemical bond has formed means that these bonds are reversible and the iron particles can be recycled.

Initial tests revealed that the SPIONs could be recycled multiple times both for liquid and solid hydrocarbons. Remarkably, after one cycle of remediation, the mixture of iron particles and plastic waste still displayed oil-absorbing properties, outperforming clean SPIONs alone.

“This gives the waste a second life in wastewater treatment,” said Halik. “Afterwards, the oil-loaded plastic waste can still be recycled to the initial components and used again.”

There are still kinks that need to be ironed out. For example, recycling SPIONS requires the use of solvents and elevated temperatures, and though not on the team’s agenda, what to do with the collected hydrocarbons post-cleanup.

“So far, the focus is on the recycling the SPIONs and to generate clean water as ‘product’ rather than to reuse the hydrocarbons,” added Halik. “It is probable that novel recycling concepts (not within the focus of our current research) will make the reuse of the HCs (liquid and solid) more attractive and ecologically reasonable.”

The team is currently working on expanding the range of pollutants that the iron particles can target, including “forever chemicals” and inorganic pollutants like nitrates and metal ions. However, Halik notes that further development of a prototype magnetic separator and financial support are needed before these advancements can be realized.

Nonetheless, Halik remains optimistic about the future and believes that the work his team has undertaken will make a positive contribution to our planet.

“Cleaning up all our oceans in the near future is, unfortunately, an illusion,” said Halik. “Nobody and no one technique will be able to clean a volume of approximately 1.3 billion km3 of water. But we can contribute to the prevention of new microplastics and organic pollutants to the oceans by removing them before they reach rivers and other water sources.”

Reference: Marcus Halik, et al., A Sustainable Method for Removal of the Full Range of Liquid and Solid Hydrocarbons from Water Including Up- and Recycling, Advanced Science (2023). DOI: 10.1002/advs.202302495

Feature image credit: Naja Bertolt Jensen on Unsplash

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