Professor Mayer, it's a pleasure to talk with you today about your research in the field of environmental chemistry. To start, I'd love to hear what initially drew you to this field. Was there a particular 'aha' moment that sparked your interest?

During the final project of my MSc studies in Environmental Engineering, I did not plan to go into research. However, that changed when doing experiments where even my supervisor did not know the outcome. In the following years, my research focused on the environmental exposure and effects of hydrophobic organics. An “aha” moment was when I realized that phase partitioning of organic contaminants - besides being an important environmental process – can also be utilized in many novel experimental and analytical approaches. Over the years, we developed “passive dosing” that employs phase partitioning from a loaded silicone donor for establishing and maintaining constant freely dissolved concentrations. We use this for controlling constant exposure concentrations in various aquatic and in vitro tests as well as in the analytical laboratory, for single chemicals and for mixtures. We also developed several analytical partitioning techniques, where analytes equilibrate from an environmental matrix and into micrometer thin silicone coatings. Such equilibrium sampling techniques can be tailored for measuring freely dissolved concentrations down to the pg/L and in some cases even fg/L range, or for measuring thermodynamic contaminant parameters such as fugacity and chemical activity. The critical parameter here is the silicone to water partition coefficient that can exceed 1 million for the most hydrophobic and lipophilic analytes, which can provide enormous enrichment and consequently very low quantification limits. Solid Phase Micro Extraction can for instance be operated as an equilibrium sampling technique.

At DTU, your research focuses on the fate of organic contaminants in the environment. For those unfamiliar, could you explain what this means and why it's so crucial to study?

Well, when I returned to the Technical University of Denmark about 10 years ago, we changed our focus to biodegradation and persistency research. Perhaps the most important property of many priority organic pollutants is their persistence, meaning their resistance against microbial degradation. In fact, the main underlying reason for the current PFAS crisis is the extreme persistence of PFAS chemicals rather than their specific toxicological profiles. Today, it is clear that these persistent substances should have been regulated if not banned 1-2 decades ago, and it is now important to be pro-active with regards to finding the next groups of persistent chemicals. For this, we need better methods for determining biodegradation kinetics and singling out the most environmentally persistent chemicals. Such biodegradation research is traditionally done in standardized tests of single substances at high concentrations, which takes too much time and resources when considering the substantial number of chemicals in production and use. My colleague Dr. Heidi Birch and I have spent the last 10 years building a new research platform for simultaneously determining the biodegradation of numerous chemicals in mixtures at low environmentally relevant concentrations. Our biodegradation experiments are conducted in gas tight vials and a PAL autosampler is then used for automated Solid Phase Micro Extraction coupled to GC/MS on unopened test systems. This minimizes test substance losses, blank levels and analytical errors, and importantly it allows us to work with highly volatile and hydrophobic test substances. We always measure pairs of biotic and abiotic test systems and can then use the biotic/abiotic peak area ratio for determining biodegradation kinetics for each chemical in the tested mixture. In other words we are closely aligning our experiments with the analytical instrument, which can be done in many different ways and for many different purposes.

While we develop and apply this approach within our research projects, we also envision that it can open new possibilities within regulatory biodegradation testing by increasing throughput, improving data quality and extending the applicability domain towards more hydrophobic, volatile and complex substances. In January, Heidi Birch and I attended an international workshop concerning the upcoming revision of the OECD 309 biodegradation testing guideline, which indeed confirmed the need for improving standard biodegradation methods for difficult to test substances.

Recently, you and Mette T. Møller published a fascinating study on chemicals discharged from offshore oil platforms. What prompted this research, and what were the most significant findings?"

Offshore oil production is associated with enormous marine discharges of produced water containing a complex mixture of petrogenic chemicals and production chemicals. To be honest, we did not intentionally develop our research platform for such complex discharges. Our approach rather developed from project to project until it suddenly appeared well suited for determining biodegradation kinetics and persistence of chemicals in complex mixtures. Looking back, it was the combination of (1) simple experiments in gas tight autosampler vials, (2) automated sampling and analysis on unopened test systems and (3) the application of biotic/abiotic peak area ratios that paved the way for novel complex mixture research.

In this research, we have learned that it is crucial to operate the biodegradation experiments at high dilutions and thus low concentrations in order to avoid that the chemicals are inhibiting the biodegradation process we are studying. Fortunately, the combination of SPME with GC/MS analysis provides sufficient analytical sensitivity to measure biodegradation even at high dilutions. This analytical method yields mainly hydrophobic, (semi)volatile and thus petrogenic chemicals, and the vast majority of these chemicals degraded well. Mette T. Møller is currently extending this work using LC/HRMS to determine the biodegradation and persistence of the more polar production chemicals in the mixtures. The approach of using biotic/abiotic peak area ratios is once again facilitating the determination of biodegradation kinetics for known and unknown chemicals, and can potentially also help to focus the data analysis and identification to the most persistent chemicals.

In this study, you employed advanced analytical techniques like SPME GC/MS and LC/HRMS. As someone with a background in analytical chemistry, I'm curious to hear your insights on these techniques and their application in identifying and quantifying chemicals within complex samples.

I am confident with our approach to designing simple, sound and powerful experiments and then aligning them to advanced analytical techniques, and we also have substantial expertise with regards to SPME. On these aspects, we constantly try to push the barriers. However, when it comes to the actual instrumental analysis by for instance LC-HRMS, we rely on the collaboration with other experienced analytical groups. It seems that LC/HRMS can provide the analytical sensitivity, selectivity and mass resolution for progressing our biodegradation research into very complex mixtures. In return, we hope that our approach to aligning experiments with analytical instruments can help focus non-targeted analytical work to for instance the most persistent chemicals and contribute to high quality data and results.

Looking ahead, what do you see as the most pressing research needs in environmental chemistry? How does your work contribute to addressing these challenges?

Facilitating the green transition is probably the most important task of our time. At the same time, we must maintain a constant focus on the potential pollution issues related to this green transition. Maybe the biggest challenge will be to recycle materials within a circular economy, without accumulating pollutants in the recycled materials. An example is the pyrolytic conversion of carbon rich waste streams into biochar which is then used for agricultural and environmental applications. In this context, we are currently investigating how the pyrolytic conversion affects the concentrations and leaching of a wide range of organic pollutants, metals and also nutrients. It is probably not a big surprise that we again try to align our experiments with various analytical techniques and then apply peak area ratios. Our first results suggest that pyrolysis is an effective treatment for removing or at least immobilizing most contaminants.

We are also continuing our biodegradation research and increasingly applying it to cases within the green transition, such as chemicals that are emitted from off-shore wind mills or essential oils that are plant based natural fragrances such as lavender oil. These essential oils are UVCBs (Substances of Unknown or Variable composition, Complex reaction products, and Biological materials), which are complex mixtures of hundreds of constituents. Fortunately, all known and unknown constituents that appeared in our SPME-GC-MS chromatograms degraded rapidly and were thus not found to be persistent.  

Finally, I would like to thank you for reaching out and CTC Analytics for making such a versatile and reliable autosampler which indeed has been instrumental for our research and particularly for aligning our experiments with SPME-GC-MS analysis. I would also like to thank DTU, Concawe, RIFM, CEFIC-LRI, GPC Europe, Unilever, Danish Offshore Technology Centre, Velux Foundation, German EPA and Danish EPA for their support.