Fate of Perfluoroalkyl Acids during Thermal Regeneration of Hydrotalcite

Calcined hydrotalcite (cgHT) is a promising PFAS adsorbent because it may enable PFAS destruction and sorbent regeneration at reduced temperatures. This study evaluated the thermal behavior of PFOS and PFOA adsorbed on cgHT at temperatures ranging from 125 to 550 °C and compared it with their behavior on sand (a reference carrier). Targeted PFAS, total fluorine, and fluoride analyses were used to quantify fluorine-containing products and establish fluorine mass balances across the solid, liquid, and gas phases. For both PFOS and PFOA, more than 75% of the initial fluorine remained associated with cgHT in an unextractable, likely inorganic form. In contrast, less than 40% of the initial fluorine was recovered as fluoride overall from the sand experiments, with less than 6% retained as solid-phase fluoride. Fewer types of volatile organic fluorine byproducts were detected in the cgHT experiments than in the sand experiments. Enhanced PFOS and PFOA degradation on cgHT may involve reactions with the bentonite binder, or the magnesium oxide and water generated from cgHT at elevated temperature. Adsorption and thermal regeneration performance did not change significantly over three adsorption–regeneration cycles. The results support further evaluation of cgHT as a thermally regenerable PFAS sorbent.