Investigation of the transformation products formed during thermal desorption of PFAS

Per- and polyfluoroalkyl substances (PFAS) are contaminants of emerging concern, as they are persistent, ubiquitous, and toxic. They pose a threat to both human health and the environment, therefore efficient remediation strategies are urgently needed. One possible remediation technology to treat contaminated soil is thermal desorption. However, the chemical processes and potential transformation products created during thermal desorption have not been fully assessed. Especially precursor substances, that transform to persistent PFAS substances in the environment, are of interest.

This study investigates the thermal desorption and transformation of PFAS. We developed an experimental stand, where sand and soil, artificially contaminated with various PFAS substances, is heated by a heating rod in a stainless-steel column.  The maximum temperature reached in the column is 450 °C. We hypothesize that during this experiment the PFAS will desorb from the sand and enter the gas phase. Further, we assume that chemical transformation processes will occur, leading to products with shorter chain lenghts. To understand the fate of the PFAS substances, we analyze the gas phase and the concentration of PFAS in the sand before and after the heat application. We use target and non-target approaches to identify transformed products. Furthermore, the decomposition of PFAS is examined by measuring the produced fluoride ions.

Initial experiments with short-chain (PFBA) and long-chain (PFOA, PFOS) PFAS showed that thermal desorption of the substances is taking place in the regions of the column where the boiling temperatures of the individual compounds were exceeded. No transformation products have been found using target analysis to date, however we expect more transformation processes with the next round of experiments, where two precursor substances will be tested. Based on these first results and the coming experiments we expect to enhance our understanding of the chemical processes taking place during thermal desorption. With this knowledge, it will be possible to make well informed decisions and improve the application of thermal desorption remediation strategies for PFAS contaminated soils.