Researchers at the University of Erlangen-Nürnberg are developing an oil magnet

Chemical dispersion, the breaking up of the oil slick into small droplets with the help of so-called dispersants, remained the method of choice. The finely-divided oil sinks below the surface of the water and is then - at least according to the theory - broken down by microorganisms faster than the dense oil film on the surface of the water. Critics complain, however, that the constituents of the dispersants - e.g. petroleum-like solvents and salts of organic sulfonic acids, such as those used in synthetic cleaning agents - are sometimes more toxic than the crude oil components themselves, and that the dispersion makes the oil less visible. In fact, research vessels later located huge plumes of oil in deep areas of the Gulf of Mexico that had hardly been attacked by microorganisms.

Almost 7 million liters of dispersants were used during the Deepwater Horizon oil spill. Experts estimate that a maximum of 20% of the leaked crude oil could still be removed. The rest sank to the sea floor or was distributed in large volumes in the deep sea. Newer methods for combating oil pollution such as oil-decomposing microbacteria and oil-absorbing adsorption sponges were not yet available in 2010 and are still predominantly at the testing stage. They are also controversial because of their toxic side effects on the marine ecosystem.
 

But new developments are bringing new hope: Under the direction of the materials scientist Prof. Marcus Halik, a process that a working group at the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) has been researching since 2015 is likely without any environmental hazard: collecting oil with the help of magnets. Halik is convinced: "With our process we extract substances such as crude oil, gasoline and diesel practically completely from water, without any auxiliary substances remaining in the water."

Of course, oil isn't magnetic. So the FAU researchers use a clever trick: They designed tiny magnetic particles to which oil adheres so tightly that an electromagnet with the particles also fishes the oil out of the water. Marco Sarcletti, who was doing his master's thesis on the subject, describes the process in more detail: “We use nanoparticles made of magnetite that are surrounded by self-assembling monolayers made of alkylphosphonic acids. The acid ends bind to the magnetite particles, while the alkyl chains point away from them. These chains are strongly oleophilic, which means that they bind oily substances with a high degree of effectiveness." The FAU researchers were able to show that each functionalized nanoparticle accumulates up to 14 times the volume of oil molecules.

The researchers optimized the process through computer simulations and various experiments. The mineral magnetite, an iron oxide with the composition Fe3O4, proved to be suitable as a magnetic material. It is non-toxic, relatively inexpensive and can be easily processed into nanoparticles. In this extremely fine distribution, it is particularly effective for two reasons, explains Sarcletti: “On the one hand, we use nano-scaling to increase the active surface enormously. On the other hand, in contrast to massive magnetite, the nanoparticles show a special magnetic behavior, becoming SPIONs (SuperParamagnetic Iron Oxide Nanoparticles)." 

As a bonding agent between magnetite and oil, the Halik team uses types of molecules that have a similar structure to dispersants: alkyl acids, which consist of a long hydrocarbon chain and an acid residue. In contrast to dispersion, the acids adhere so tightly to the nanoparticles that a magnet extracts the entire magnetite-alkyl acid-oil complex from the water. "We achieved the best results with alkylphosphonic acids," explains Prof. Halik, "because the phosphonic acid residue binds strongly covalently to magnetite, i.e. it forms a real chemical bond." 

Prof. Halik looks to the future: "Together with industrial partners, we now want to transfer the process to a technical scale as part of a start-up." For this purpose, for example, inexpensive methods for separating the oil have to be developed, for example by evaporating or burning the oil, by centrifuging or by using very strong magnetic fields. A modified form of the process could also be suitable for removing other environmental toxins from water. The FAU team has already shown that glyphosate-based herbicides or nanoplastic particles can be extracted magnetically from water. So, perhaps more than one disaster can be solved through the nanomagnets. 

Prof. Dr. Halik heads the OMD (Organic Materials & Devices) working group at the Institute for Polymer Materials at the Friedrich-Alexander University Erlangen-Nuremberg. The group’s research focuses on self-assembling monolayers (SAM, Self-Assembled Monolayer) that form organic molecules on surfaces. With such layers, the OMD researchers manufacture new types of flexible electronic components and develop innovative applications, for example in environmental technology, medicine and optoelectronics.

Text: Stine Behrens