Abstract:
Per- and polyfluoroalkyl substances (PFAS) are a large group of anthropogenic chemicals, produced since the 1940s, which are characterized by unique properties such as high stability, hydrophobicity as well as oleophobicity and surface activity, that make them attractive for a wide range of industrial applications. Their high stability results from the strength of the C-F bond which makes them also very persistent in environmental matrices. For some PFAS, such as perfluoroalkyl acids (PFAAs), toxic effects have been reported, leading to their ban and restriction. Although PFAAs are no longer used in industry, they can still be formed as terminal products of biotransformation of polyfluorinated, non-restricted PFAS precursor compounds.
Extensive use of PFAS led to their world-wide distribution in all environmental compartments, and through their use in aqueous film forming foams (AFFF) they can be discharged directly into the environment, resulting in heavily contaminated hot-spots. PFAS used in AFFF do not have to be disclosed by the manufacturers, can show diverse chemical structures, be of anionic, zwitterionic and cationic nature and may be also transformed to PFAAs under environmental conditions.
To prevent and manage threats for drinking water resources, AFFF contaminated sites should be efficiently remediated. A prerequisite for effective remediation is a well-characterized extent of the contamination. Since analytical standards are often unavailable for many PFAS, non-target screening (NTS) by high-resolution mass spectrometry (HRMS) can reveal the contamination, and, through semi-quantification (quantitative NTS, qNTS), even estimate its magnitude.
Thermal remediation techniques have been discussed as viable options for PFAS remediation, however little is known about thermal behaviour of PFAS and formation of potentially toxic products of incomplete destruction (PIDs), especially for AFFF PFAS.
Therefore, in the first part of this dissertation, soil and groundwater from an AFFF contaminated field site in Germany were subjected to a newly developed qNTS workflow, revealing the presence of 124 individual PFAS from 42 subclasses in the soil. The analysis showed that the contamination was diverse and from potentially different AFFF formulations due to fire-fighting acitvities. Semi-quantification revealed that more than 50% of the identified contamination would have been missed without the qNTS approach. Also, most of the contamination was still present in the top meter of the soil, which was attributed to the presence of zwitter- and cationic- precursors which have strong retention in soil due to electrostatic effects. Nevertheless, mostly short-chained, mobile PFAAs were identified in deeper soil layers and groundwater, highlighting the threat to drinking water resources.
In the second part of this work, thermal transformation and desorption behaviour of 6:2 FTSAm-Pr-B, the most abundant compound in the soil from the Reilingen field site, was investigated to evaluate the potential of thermal remediation methods to AFFF contaminated soil. Results demonstrated, that 6:2 FTSAm-Pr-B can produce a variety of PIDs in both a closed and open system-setup and that its transformation can be initiated at temperatures ≥ 150 °C. However, a mass balance (MB) approach revealed a gap in the mass balance at temperatures > 150 °C, indicating the presence of additional PIDs that were not detectable with the applied methods.
In an open system, no thermal desorption (TD) of 6:2 FTSAm-Pr-B was observed. Instead, transformation was the dominant process. Temperatures, at which transformation was initiated as well as reaction kinetics were found to be strongly dependent on the sorbent material. Reaction kinetics were fastest with granular activated carbon (GAC) and slowest with organic carbon (OC) rich soil, indicating that GAC may facilitate and OC may hinder the reaction. It was further shown that some PIDs of FTSAm-Pr-B were more stable than their precursors and may also be able to thermally desorb.
In conclusion, this work demonstrates the importance of quantitative NTS workflows for the characterization of complex contaminations, such as the Reilingen field site. Furthermore, the evaluation of the thermal transformation of 6:2 FTSAm-Pr-B revealed that the application of thermal remediation strategies to contaminated soils is possible, but not straightforward, as a variety of PIDs must be considered and monitored, even in low-temperature applications, aiming for thermal desorption.
PFAS