Assessing strategies to measure hidden per- and polyfluoroalkyl substances (PFAS) in groundwater and to evaluate adsorption remediation efficiencies.

The widespread presence of per- and polyfluoroalkyl substances (PFAS) in the environment, driven by extensive industrial use, has raised global concerns due to their persistence and adverse health effects. Despite the increased regulatory focus on a sub-set of well-known PFAS, over 12,000 compounds exist, many poorly characterized. Our study assessed hidden PFAS concentrations, undetectable by standard LC-MS/MS analysis, in contaminated groundwater. We analyzed total oxidizable precursors (TOP) via TOP assay followed by LC-MS/MS, and total organic fluorine (TOF) via combustion ion chromatography (CIC). Results were compared with those from LC-MS/MS analysis of 25 individual PFAS (∑PFAS25), representing the non-hidden PFAS fraction. We also evaluated the removal of hidden PFAS employing conventional and novel adsorbents. Groundwater samples from drinking water sources and contaminated military sites in the USA showed varying PFAS contamination levels as indicated by TOF values ranging from non-detect (<0.7 μg L) to 40.2 μg L. ∑PFAS25 was a major fraction of the TOF (41.7 - 92.8%) in some samples, whereas in others it only accounted for 5.1 - 20.4% of the TOF. The remaining percentages consisted of hidden PFAS not detected by conventional LC-MS/MS, but detectable as TOF by CIC. Organic fluorine content of oxidizable precursors accounted for 0.0-39.0% of TOF content, depending on the sample. Selected samples underwent adsorption with activated carbon (AC), anion exchange resin (IX), polyaniline (PANI), and poly-o-toluidine (POT). All adsorbents removed the hidden PFAS less effectively than the PFAS quantified by direct LC-MS/MS techniques. This is likely because PFAS adsorbents investigated to date primarily target anionic per- and polyfluoroalkyl acids, not effectively removing cationic, neutral, or zwitterionic hidden PFAS. AC exhibited the best overall performance among the investigated adsorbents. The results demonstrate that measuring TOP and TOF concentrations is effective for evaluating the removal of hidden PFAS in groundwater remediation.