2023-08-19 11:08:30
Clean water is a precious commodity that deserves protection. Due to carelessness and a lack of cleaning, too many pollutants still get into our waters and ultimately into drinking water. Problems are caused by oils, dyes and solvents, especially microplastics, herbicides and hormones. Even in low concentrations, these contaminants can have serious effects on aquatic flora and fauna, but also on human health. Processes are therefore being sought that specifically remove those pollutants that sewage treatment plants cannot filter out of the wastewater, or only with difficulty. Researchers from the Friedrich-Alexander University of Erlangen-Nuremberg have developed a promising process that reliably cleans contaminated water and promises easy handling.
The researchers use nanoparticles that can bind various pollutants to their surface. Since the particles have a core of iron oxide, i.e. rust, they can be fished out of the contaminated sewage with a magnet, along with the impurities that are stuck to them. The process already works for oil, microplastics and the herbicide glyphosate. With a special chemical modification of the iron oxide particles, the researchers led by Marcus Halik can now also remove hormones, as they reported at a conference of the American Chemical Society in San Francisco.
For many years, Halik and his colleagues have been researching how various pollutants can be removed from wastewater with superparamagnetic rust particles as residue-free and as environmentally friendly as possible. Commonly used as catalysts, pigments, or in medical diagnostics, these particles are non-toxic, have a high surface area, and can be efficiently attracted to a permanent magnet. However, they do not become magnetic themselves, so they cannot clump together.
Microplastics interact with iron oxide particles (SPION) and clump together. The aggregates can be removed from the water with a magnet. : Image: Marcus Halik et al. MatToday
To make the iron oxide particles “smart”, as the researchers call them, they cover them with a thin layer of phosphonic acid. Different molecules can now be attached to it, which have specific chemical and physical properties. With alkyl chains, for example, the surface becomes water-repellent. As a result, hydrophobic hydrocarbons such as crude oil, petrol or diesel also accumulate, which float in the water, attach to and are bound to the alkyl radicals. They wrap themselves around the fine rust particles like a coat.
microplastics, glyphosate and estrogens
If, on the other hand, the surface of the iron oxide particles is positively or negatively charged, for example by providing them with longer phosphonic acid molecule chains, they have an attractive effect on microplastic particles, for example, whose surface is charged in the opposite direction. The researchers tested the attractive effect on finely distributed polystyrene, PMMA and silicone particles. The rust particles clumped the microplastics, which could be fished almost completely out of the water by applying a magnetic field.
The experiments with water were similarly successful, contaminated with the herbicide glyphosate. In laboratory tests, Halik and his colleagues succeeded in using their process to fall below the European limit value of the drinking water ordinance (0.1 micrograms per liter). And the herbicide, which is controversial because of possible health risks, was also efficiently filtered out of water samples in field trials.
Estrogen molecules become entangled in the hydrocarbon chains on the surface of a rust particle. : Image: Dustin Vivod, Dirk Zahn, Friedrich-Alexander University of Erlangen-Nuremberg
So far, the researchers from Nuremberg have primarily targeted pollutants that are usually present in large quantities. Lukas Müller, a doctoral student in the working group who presented the current research work at the conference in San Francisco, investigated whether the rust nanoparticles can be chemically modified in such a way that they can also attract traces of hormones such as oestrogens. The levels of these sex hormones in the environment are low, making them difficult to remove using conventional methods. However, even low concentrations have been shown to affect the metabolism and reproduction of some plants and animals.
Molecular pockets trap hormones
Because of their nature, estrogen molecules are difficult to capture. They have a bulky steroid body and negatively charged ends. Lukas Müller coated the iron oxide nanoparticles with two different organic compounds: a longer hydrophobic hydrocarbon chain and a shorter one that was positively charged. The two molecules organized themselves on the surface of the nanoparticles in such a way that they formed innumerable small pockets. Electrostatically attracted, the estrogen molecules became trapped in the pockets.
Müller tested his method with the estrogen estradiol, which he added to normal drinking water in small doses. Now he wants to move on to real water samples, which he wants to take from various bodies of water. He also wants to get hold of different estrogen molecules at the same time. He is also interested in whether other pollutants end up in the nanopockets. At the same time, he wants to test how often the nanoparticles can be recycled and reused.
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Much of the research by Marcus Halik and his colleagues is still in the laboratory. But the researchers are already working on scaling the magnetic water purification process in such a way that the smart grate can one day be used in sewage treatment plants to clean wastewater from pollutants that are difficult to remove. A larger purification plant would already be under construction.
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