Breakthrough and desorption of PFAS in aged GAC filters during drinking water treatment - a question of DOM...

Shifts in dissolved organic matter (DOM) composition, caused by seasonal or episodic events, can significantly alter the removal of per- and polyfluoroalkyl substances (PFAS) by granular activated carbon (GAC). To investigate how changes in feedwater composition impact PFAS removal in aged GAC, a 20-day experiment was conducted in eight parallel small-scale columns. Columns were filled with aged GAC from a full-scale drinking water treatment plant that still retained >80% efficiency for removing the sum of PFAS21 from groundwater. The GAC was exposed to feedwaters with varying DOM composition and aromaticity. The removal performance was evaluated using fluorescence spectroscopy combined with measurements of ¹⁴C-labelled PFOA and unlabelled PFAS. This combination facilitated an assessment of changes in both DOM composition and PFAS removals. The results showed that a change in DOM composition of the feedwater resulted in the desorption of shorter-chained PFAS from the GAC and a simultaneous increase in the adsorption of lower molecular weight DOM, resulting in higher aromaticity in the outgoing water. These findings suggest a dynamic sorption process in which newly introduced DOM in the GAC feedwater competes for adsorption sites, potentially displacing previously adsorbed PFAS. Experiments with carbon-14 labelled perfluorooctanoic acid (¹⁴C-labelled PFOA) indicated that an increase in feedwater DOM aromaticity, while maintaining a constant dissolved organic carbon (DOC) concentration, resulted in earlier breakthrough. This highlights the need to monitor and evaluate not only the quantity of DOM in feedwater, but also its composition.