(186e) Phase Behavior of PFAS Following Hydrothermal Liquefaction of Sewage Sludge

Hydrothermal liquefaction (HTL) has proven successful at destruction of PFAS. However, less is known about the distribution of PFAS in the different HTL product phases. In this work, we treated PFAS contaminated sewage sludge under different HTL conditions, measuring the PFAS contained in the sludge, aqueous phase, and biocrude to quantify fate. A suite of 12 important PFAS compounds were measured. In all cases, except for PFBS (perfluorobutane sulfonic acid), water phase PFAS concentrations are less than the instrumental detection limits, with reductions relative to the sludge feed exceeding 99%. Figure 1 provides representative data. PFBS is a short-chain PFAS with a polar head group, consistent with its partitioning into the water phase. Based on these data, we forwarded a second hypothesis: partitioning of PFAS between the biocrude and aqueous phases can be modeled using the octanol-water partition coefficient, sometimes termed logP. Accordingly, we constructed a machine learning regression model based on the random forest algorithm to predict the logP values of PFAS substances. Comparison of the new model against values measured for >10,000 compounds and reported in the literature resulted in a root mean square error of approximately 1.0. We then applied this model to the data shown in Figure 1, finding good agreement between observations and predicted values of logP. These insights help explain the fate of PFAS following HTL.