Plastics have become an integral part of our everyday lives. This leaves its footprints in the environment. In addition to carelessly thrown away bags, cups and other plastic waste, pollution with microplastics is increasingly coming into focus. First, the small particles were discovered in oceans; then they were found in lakes and rivers, and eventually in soil. “Plastic particles are everywhere,” says Luisa Barkmann, a doctoral student in the field of wastewater technology at the TU Darmstadt. In her Master’s thesis, she dealt with microplastics in municipal sewage treatment plants. Now she is dedicated to industrial wastewater. She takes a close look, above all, at plastic-producing and processing companies in her doctoral thesis: At which stages of the value chain and through which channels do plastic particles enter the environment? And what is the share of industry in the overall burden?
As part of the joint project EmiStop, launched at the beginning of the year, the Darmstadt-based research teams are analysing industrial wastewater to calculate their share of microplastics. Not an easy task, because even sampling is a challenge, says Barkmann. For each company, she had to think of an individual strategy, since the companies differ not only in their production processes, but also in their channel systems and water management. Many do not even know how much wastewater is generated in a particular area. And not only in the production, but particularly also in the further processing, transfer and transport of plastics microplastics into the environment.
Many large chemical companies are located on rivers
The Darmstadt scientists recently took the first samples from a company producing plastic granulates. Like many large chemical companies, for logistical reasons it is located on a river. Such sites could contribute to the microplastic pollution of bodies of water, for example if surface water enters the river untreated, explains Barkmann’s supervisor Professor Markus Engelhart. Thus, in the worst case, the flood of plastic particles that land on the ground, for example during transfer into transport vehicles, ends up directly in the surrounding waters. The EmiStop team will also examine such entry routes.
Concrete recommendations for action can only be made if the burden is analytically determined, emphasizes Professor Susanne Lackner, Head of the Department of Wastewater Management and a partner of EmiStop. The detection of microplastic in water samples has long been a weak point, and the data accordingly weak. For several years, a complex spectroscopic method, Raman microspectroscopy, has been used here. This technique is also used by the Institute for Environmental and Process Engineering at RheinMain University of Applied Sciences in Rüsselsheim, which also participates in EmiStop.
Hajo Bitter, a PhD student in Lackner’s group, is refining a method called differential scanning calorimetry. It is well established in the quality assurance of pure plastics but must be adapted to the more complex wastewater samples. First, researchers sieve out particles larger than five millimetres – which are not microplastics – and then they are destroyed by chemical treatment with oxidants that do not attack plastics. Finally, mineral components such as sand are separated through a special suspension technique.
The plastic particles thus isolated are heated in a tiny crucible with a given heating rate up to almost 300 degrees Celsius. The device records the heat flows as a function of the temperature, with an empty crucible serving as a reference. Peaks in the diagram, which occur at the melting points of the contained plastics, provide information about the plastic types as well as their concentrations. So far, scientists have measured six common plastics in this way. However, they only use the method to capture plastic particles larger than ten microns. In addition, because of the tedious sample preparation, the procedure takes up to two weeks.
Significantly faster, but less accurate are procedures that Luisa Barkmann tests for a first estimate: It determines so-called sum parameters, which do not indicate the content of a single substance but a group of related compounds. In environmental analysis, for example, organic pollutants containing chlorine, bromine or iodine are recorded together as a sum parameter by standard.
Companies must measure this value as standard procedure before introducing wastewater into rivers. If it is elevated and if, additionally, there is a particle load, this indicates contamination with the chlorine-containing plastic PVC. “Although this is not an exact quantification, it does give companies a quick indication of a microplastic load,” says Engelhart.
In line with the title of EmiStop, researchers involved not only want to develop detection methods and record entries, but also stop the industrial emissions of microplastics. Project partner EnviroChemie from Roßdorf, less than ten minutes by car from TU Darmstadt, is already testing flocculants that remove microplastics from wastewater. But how effective is the separation process? Is it enough, or are filters more effective?
For the evaluation of deposition techniques, PhD student Bitter is developing a model particle test: Magnetic plastic particles are added to wastewater to be purified as a tracer. Their content before and after treatment can be measured with a magnetic scale – and that is more sensitive than with differential scanning calorimetry while, at the same time, being faster because there is no need for lengthy sample preparation. The model particles consist of plastic with enclosed Nano-iron oxide. Their density can be modified so that they can imitate different types of plastic. The Darmstadt-based scientists are developing the particles in cooperation with project partner BS-Partikel from Mainz.
In short: EmiStop travels a long path from micro-plastic analysis through mass balancing to emissions reduction measures. A clear interest can be identified within the industry, says Engelhart, and emphasizes: “Our aim is not to pillory companies, but provide them with tools to help them respond quickly to unwanted events.”