Investigation of the Transformation Products Formed During Thermal Desorption of PFAS

Per- and polyfluoroalkyl substances (PFAS), “forever chemicals”, 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 transformation mechanisms and products created during thermal desorption have not been fully assessed yet. Precursor substances, which transform to persistent PFAS substances in the environment, are of particular interest.

This study investigates the thermal desorption and transformation of PFAS. We conducted multiple thermal desorption experiments with artificially contaminated PFAS-sand in a stainless-steel column, which was heated by a heating rod and mantle. The maximum temperature reached in the column is 500 °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 lengths. 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 and evaluating the fluorine mass balance.

Our experiments showed that thermal desorption of PFAS is taking place in the regions of the column where the boiling temperatures of the individual compounds were exceeded. Depending on the substance and temperature setting used, complete removal of the spiked PFAS from the sand was achieved. By using LC-MS/MS target analysis we found multiple PFAS with shorter chain-length than the spiked substance after heating. In rare cases longer chain lengths were observed. These transformation products were mainly found in the samples taken from the gas stream. Based on these results we conclude that thermal desorption can be used as a treatment method to remove PFAS from contaminated material ­­– however, it is essential to keep in mind that PFAS transformation products will exist in the gas phase and therefore adequate treatment of the exhaust gas is necessary. Further studies will be conducted with artificial PFAS-soil as well as with additives. By using additives, we hope to improve the mineralization rate, suppress the formation of undesired transformation products, and minimize the energy demand. With our experiments we expect to enhance the chemical process understanding of thermal desorption of PFAS, which will lead to an improved application design of this thermal treatment method.