Interplay of surface oxygen content and pore water during thermal regeneration of granular activated carbons

This study aims at unraveling the interplay between thermal regeneration and granular activated carbon (GAC) properties. First, twelve commercially available, unused, and thoroughly characterized GACs were pyrolyzed without per- and polyfluoroalkyl substances (PFAS) using a thermogravimetric analyzer at a 25 ºC/min heating rate up to 750 ºC under nitrogen. GACs with elevated oxygen content showed substantial weight loss due to the decomposition of acidic functional groups, leading to the formation of larger pores but a decrease in specific surface area and physical hardness. Additionally, pore water capacity of GAC influenced the regeneration, as water molecules sorbed to surface oxygenated groups via hydrogen bonding contributed to the formation of carboxylic acids and subsequent decomposition. Next, PFAS destruction mechanism during regeneration was demonstrated by loading GAC with perfluorooctane sulfonic acid (PFOS). Gaseous product analysis showed a catalytic effect for PFOS-laden GAC, where hydrogen fluoride formation occurred at 50 ºC lower temperatures than in pure PFOS, indicating improved PFOS thermolysis when adsorbed by GAC. This catalytic effect is likely due to chemical interactions between PFOS and the delocalized electrons on the GAC surface, warranting further investigation. The diverse GAC responses during regeneration underscore the importance of understanding regeneration conditions to maintain GAC functionality while enhancing PFAS treatment sustainability.