Raiders of the lost plastic

As a process engineer, Patrik Stenner is trying to continuously improve existing processes. Stenner, Head of the Electrochemistry & Exploration Group at the Process Technology unit of the Evonik plant in Hanau, is annoyed that a small percentage of polymers is lost as microplastics in aqueous production residues during polymer production and processing. These microparticles don’t get released into the environment, because they are chemically bound, dried, and incinerated. “However, we lose valuable raw materials and CO₂ is unnecessarily emitted,” says Stenner. He wants to use electrochemistry to change that.

Stenner and his 14-member team are working on electrochemical processes that are currently experiencing a boom due to the increasing availability of green electricity. Stenner has been investigating microparticles in wastewater for more than five years. He’s noticed that the usually immobile particles, which are only nanometers in size, move quickly in water as soon as an electric field is generated within it. As a rule, the polymer particles are attracted by the positively charged anode.

In an initial series of tests, two small parallel metal plates were dipped into the wastewater and connected to an electric current. The polymers were deposited on the anode, from which they could then be easily removed. “However, you need a continuous process in production,” says Stenner. Now, a rotating metal cylinder serves as the anode, while the wastewater basin acts as the cathode. The polymers that adhere to the cylinder are continuously removed by a scraper and returned to the production process. The cycle is closed.

This in-house test is now part of an EU project known as LimnoPlast. In this project, Stenner and his colleagues are working together with other scientists to understand and prevent the problems that microplastics cause in water. Their activities range from the search for the causes of this water contamination and its effects on people and nature to the discovery of ways in which the water can be purified. “We’re taking a holistic approach,” says Stenner. The project, which will run until October 2023, is being managed by the University of Bayreuth.

The experimental system at Evonik in Hanau has a width of around one meter and can now separate particles measuring between less than ten micrometers to as little as about 100 nanometers (i.e. only around one hundredth of the width of a human hair). The aim now is to upscale the facility, which means that the scientists have to design a cylinder that can be used at an industrial scale. The German Chemical Industry Association (VCI) awarded the project first place in both the Hessian and the German Responsible Care Competition of 2020.

According to Stenner, the process could conceivably not only reclaim microplastics from the production wastewater of the Wolfgang Industrial Park, but also be used in public sewage treatment plants. “We’ve already received inquiries from manufacturers who are currently using filters and membranes but are looking for a solution for these very small particles, which can’t be retained yet,” he says.

Photos: Stefan Wildhirt / Evonik