In Germany, the water quality of rivers and lakes has improved significantly in recent decades. Many other countries around the world, however, are far from meeting this standard. Environmental and natural catastrophes can also severely pollute bodies of water. “In such cases, we often see quite a number of possible contaminants, from organic and inorganic substances to bacteria”, explains Professor Carsten Streb, head of the Institute of Inorganic Chemistry I at Ulm University. And then, of course, the virtually omnipresent microplastic particles also play a role.

Filtration is a tried and tested procedure for purifying contaminated water, but it has its disadvantages as well: it is time intensive and not particularly suitable for large quantities of water, due to the fact that different classes of pollutants generally require different filters. Researchers from Ulm thus aspired to develop a type of material that would absorb various classes of pollutants. “We originally aimed for a highly-porous material, which we then enhanced with additional bonding properties on its surface”, explains Archismita Misra. The PhD student from Ulm is the primary author of a study relating to the development of this special water filter that has recently been published in the journal “Angewandte Chemie”.

The key to the research team’s success was an ionic liquid developed specifically for this purpose. “It is basically a liquid salt”, explains Streb. Whereas table salt has a melting point of several hundred degrees, this compound is liquid at room temperature. Like any other salt, this ionic liquid consists of positively charged cations and negatively charged anions. The cations have an especially antimicrobial effect and the anions bind heavy metals. The liquid has a honey-like consistency, making it sticky and enabling it to bind microplastic particles in the water.

A dirt magnet, in the truest sense of the word

The powder was already displaying suitable cleaning properties in a lab setting, but Streb’s team then achieved a breakthrough for potential large-scale applications based on an idea from Professor Robert Güttel. The head of the Institute of Chemical Engineering at Ulm University suggested equipping the coated filter particles with a magnetically active iron oxide core. This has the effect that the contaminant-laden particles can be extracted from the water using magnets. “In this way, it becomes possible to filter much larger amounts of water in a shorter amount of time”, explains Professor Streb.

The potential areas of application for these newly developed cleaning particles are manifold. They could be used in sewage treatment plants to filter microplastic out of the water, for instance. De-centralised uses, such as purifying contaminated bodies of water, could also be an option. “In the early stages of development, we already put a great deal of thought into the issue of scalability”, says Streb. A further consideration for the researchers is that the filtration particles should be interesting from an economic standpoint as well.

The material is simple and inexpensive to recycle

Recycling also plays a key role for the researchers. According to Professor Streb, “We can use certain solvents to wash the particles after the cleaning step, thereby separating out the contaminants”. The idea of recycling the particles has been at the core of the development since the early stages. Professors Streb and Güttel led the study in cooperation with Dr Scott G. Mitchell from Zaragoza University. Mitchell, a biochemist, contributed his expertise on the antimicrobial effect of ionic liquids.

As Streb sees it, their success in the lab is an important intermediate step: “This is just the beginning. In the next step, we plan to test the particles in real life”. The particles could be tested in a contaminated pond, for example, which could also give scientists the opportunity to find out how the filter particles are affected by the presence of algae or other natural floating particles.

Text: Jens Eber

Publication:
Archismita Misra, Christian Zambrzycki, Gabriele Kloker, Anika Kotyrba, Montaha H. Anjass, Isabel Franco Castillo, Scott G. Mitchell, Robert Güttel and Carsten Streb: Water Purification and Microplastics Removal using Magnetic Polyoxometalate Supported-Ionic Liquid Phases (magPOM-SILPs), Angewandte Chemie, doi.org/10.1002/anie.201912111