Per- and polyfluoroalkyl substances (PFASs) continue to pose significant threats to the environment and human health, making effective remediation strategies a top priority. Destruction of PFAS molecules has proven to be quite challenging, and most large-scale treatment is focused on removal using membrane, ion exchange, and activated carbon. While this treatment results in a removal of PFAS from the polluted system, the PFAS molecules still need to be handled as hazardous waste, along with the medium used for removal. Thus, activated carbon or ion exchange membranes are typically disposed of after use, with a substantial cost of replacement. Recent work has shown the effectiveness of a hydrothermal aqueous environment, especially in the presence of strongly basic water at temperatures and pressures approaching the critical point, hasto break the C-F bonds in PFAS in the presence of catalyst. This paper reports on an experimental study that looks into the hydrothermal defluorination of PFAS that has been collected on- activated carbon with the simultaneous goal of fully regenerating the activated carbon for reuse. . Thus, the goals of this study are (1) to evaluate the efficiency of hydrothermal defluorination of model PFAS molecules loaded on activated carbon such as perfluorooctanoic acid (PFOA) and perfluoro-octane-sulfonic acid (PFOS) under varying hydrothermal conditions (300-350 oC for 0.5 -2 hours); (2), to regenerate the activated carbon while maintaining its properties and adsorption capacity to be applied for multiple uses; (3) to further test the application of hydrothermal liquefaction for degradation of PFAS in biosolids such as sludge.
Experiments were done with batch hydrothermal treatment of PFAS-laden activated carbon (55.4±0.2 mg PFOA per g of activated carbon). The carbon was subjected to hydrothermal treatment (350 oC, 1 M NaOH for 90 min), and the concentration of fluoride ion concentration released to the solution was measured with an ion-selective electrode (IES) after cooling. The result demonstrated that hydrothermal treatment achieves up to 96.8±4.8 % defluorination of PFOA. Characterization of the regenerated activated carbon will be presented, including BET surface area, elemental analysis, mass loss, and adsorption isotherm.
Future work includes hydrothermal destruction at different NaOH concentration and other catalysts, surface characterization of activated carbon before and after treatment using BET surface area analysis, SEM, and elemental analysis to evaluate changes in porosity, surface functional groups, and elemental composition. Furthermore, our work extends to exploring the potential of hydrothermal treatment applied to biosolids both with and without PFAS spiking. These experiments will show a full fluorine mass balance in the products.
This study shows the possibility of a sustainable and regenerative treatment technique by offering a thorough framework for the degradation of PFAS utilizing hydrothermal defluorination in conjunction with regeneration of activated carbon. This strategy lowers material costs and waste production by allowing the regeneration of activated carbon through ecologically friendly means, opening the door for continuous or semi-continuous operation in practical applications like groundwater remediation and industrial effluent treatment.