Analytical characterisation and thermal desorption of PFAS-contaminated activated carbon

Per- and polyfluoroalkyl substances (PFAS) are persistent contaminants of increasing environmental interest. Granular activated carbon (GAC) is widely used for its removal from water, but it transfers contaminants to a solid matrix requiring management and regeneration treatments. The present thesis focused on the analytical characterization and thermal desorption of PFAS from contaminated GAC. From an analytical point of view, a methodology for organic fluorine (OF) quantification based on Combustion–Ion Chromatography (CIC) was developed using a pressurized oxygen bomb. The optimization, carried out on PFBA, showed that increasing the fluorine content introduced into the system (0.25 mL of sample) improves efficiency and reproducibility (efficiency ≈ 100%, CV ≈ 4.4% at 50 mg/L). The method was then applied to liquid and solid matrices contaminated with PFNA as a model compound. The tests showed that, in liquid phase at low concentrations, Liquid Chromatography (LC) analysis is more suitable due to CIC sensitivity limits, while in contaminated GAC samples the CIC method provides a more representative estimation of OF than extraction techniques, which tend to underestimate the contaminant. Solid analysis also highlighted a marked heterogeneity of GAC, managed through controlled drying and triplicate pellet analysis. Thermal desorption under inert atmosphere (N2) shows that the residual fluorine content decreases rapidly in the first hours and then more slowly, indicating that an initial fraction of more weakly bound species is rapidly removed, while the remaining fraction is more strongly retained and requires longer times, with a possible contribution from thermal transformation phenomena (e.g. fragmentation). At 400–600 °C after 12 h, normalized fluorine loadings are around 0.4–0.6, while more severe conditions lead to much higher removal (values close to blank GAC at 900 °C - 12 h and comparable at 700 °C - 24 h). To evaluate the fate of released species, a sampling system with a double liquid trap (decane and water) was introduced. Fluorine was detected in both phases, with prevalence in the aqueous phase, indicating that the process involves not only desorption but also chemical transformations such as fragmentation and partial mineralization to HF. Steam-assisted tests showed improved removal efficiency compared to N2. In particular, at 500 °C - 12 h, normalized values are significantly lower (≈0.2 vs ≈0.5), highlighting the positive role of steam, likely associated with enhanced mass and heat transfer due to higher gas velocities. Overall, thermal desorption confirms itself as a promising strategy for the treatment of PFAS-contaminated GAC, highlighting the importance of total fluorine quantification (CIC), since target-only techniques may significantly overestimate treatment efficiency. However, the identification of optimal operating conditions remains a key aspect for assessing process efficiency and sustainability.