Granular activated carbon (GAC) adsorption is widely used to remediate sites impacted by per- and polyfluoroalkyl substances (PFAS). While GAC can effectively remove PFAS from water, PFAS are concentrated on the GAC, and strategies for managing spent GAC need to be developed. One management option is to thermally reactivate spent GAC and reuse the reactivated product. Little is currently known about factors that control the fate of PFAS during thermal reactivation of GAC. Strategies need to be developed that effectively mineralize adsorbed PFAS, prevent the release of products of incomplete destruction (PID) and hydrogen fluoride (HF) into the air, and prevent the leaching of PID and fluoride from reactivated GAC.

This work is being conducted in two phases. The overarching objective of Phase I work was to identify thermal reactivation conditions that effectively mineralize PFAS adsorbed to GAC and permit effective reuse of thermally reactivated GAC.

Specific technical objectives associated with this work included:

1. Evaluate the effects of PFAS chain length, head group, and salt/acid form on thermal behavior
2. Evaluate the effects of calcium and base on thermal behavior of PFAS
3. Evaluate the effects of natural organic matter (NOM) on thermal behavior of PFAS

Results from Phase I are summarized in the Phase I Results section below.

In Phase II of this project, sample collection and analysis workflows will be further developed with the goal of closing the fluorine mass balance and evaluating the effects of heating rate and hydrogen sources on PFAS mineralization. Specific technical objectives are as follows:

1. Confirm and quantify suspected PID identified by gas chromatography-high resolution mass spectrometry (GC-HRMS) through inclusion of authentic standards into a targeted GC-MSMS method
2. Evaluate the effect of heating rate on the thermolysis and mineralization of PFAS adsorbed to GAC
3. Evaluate the effect of different sources of hydrogen (OH-, H₂O, fulvic acid, H₂ in a N₂/H₂ 99/1% mixture) on PFAS mineralization

In Phase I, thermogravimetric analyses (TGA) were conducted with PFAS, PFAS/hydroxide mixtures, and PFAS/NOM mixtures. These analyses were performed in the absence and presence of GAC to determine the thermal stability of nine PFAS in their sodium or potassium salt forms. Additional experiments were carried out with perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in their acid forms.

Exhaust from the TGA experiments was collected using a sampling train. This train included impingers to trap gaseous compounds soluble in water amended with 0.1 M NaOH (e.g., hydrogen fluoride, perfluoroalkyl acids). Additionally, a SUMMA canister was used to capture volatile fluorinated compounds that passed through the impingers.

Impinger solutions and TGA pan residues were analyzed for fluoride and other anions and cations using ion chromatography. Targeted PFAS analysis of the transfer line rinses and impinger solutions was performed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Moreover, targeted and non-targeted analyses was conducted of volatile fluorinated compounds with low boiling points that were captured in the SUMMA canister using GC-HRMS.

In Phase II of this project, the off-gas sampling train will be optimized for capturing semi-volatile PID, which are compounds with higher boiling points. An XAD resin trap will be incorporated and XAD resin extraction protocols will be implemented for both LC-MS/MS analysis and GC-HRMS analysis. Additionally, a systematic evaluation of the effects of heating rate and reactive gases on PFAS mineralization will be conducted. This will involve TGA and fluidized bed furnace experiments. In the event that suspect screening analyses reveal additional PID, the project team will confirm tentatively identified PID and expand the targeted GC-HRMS method to quantify their formation.

Phase I Results

Results to date showed that thermolysis of all tested perfluoroalkyl acids (PFAA), PFAA salts, and PFAA salt/hydroxide mixtures in the absence of GAC was complete at temperatures used to reactivate GAC (PFOA and PFOS in their acid form at <200°C; perfluoroalkyl carboxylic acid [PFCA] salts at ~ 300°C; perfluoroalkyl sulfonic acid [PFSA] salts at ~700°C). In contrast, thermolysis of adsorbed PFOA/PFOA-K and PFOS/PFOS-K was not complete at 800°C. In both the absence and presence of Ca(OH)₂, NaOH, CaCl₂, and NaCl, salt forms of PFAA were thermally more stable than acid forms, and PFSA salts were more stable than PFCA salts. Results from fluoride, LC-MS, and GC-MS analyses accounted for 11-107 % of the fluorine content of the initially added PFAS. The degree of mineralization to fluoride ions was 1-77 % for non-adsorbed PFAA and 46-107% for adsorbed PFAA when wet GAC was introduced into the TGA. Low levels of PFAA with 2-8 carbon atoms were detected in samples from the transfer line rinses and first impinger following thermolysis of PFBA, PFBS, PFOA, and PFOS salts. For non-adsorbed PFAS, fluorine recoveries as HF in the first impinger were low, both in the absence and presence of Ca(OH)₂ and NaOH but increased to 10% (PFOA potassium salt [PFOA-K]) and 45% (PFOS potassium salt [PFOS-K]) in the presence of NOM. For adsorbed PFAS on wet GAC with and without pretreatment with Ca(OH)₂, NaOH, CaCl₂, and NaCl, HF production ranged from 9-44% for adsorbed PFOA-K and 26-58% for adsorbed PFOS-K. In the presence of Ca(OH)₂ and NaOH, fluorine recoveries as CaF₂ and NaF in the residue of the TGA pan were >50% for PFOS-K but <10% for PFOA-K for non-adsorbed PFAS. For adsorbed PFAS, fluorine recoveries as CaF₂ and NaF ranged between 7-72% and were greatest for adsorbed PFOS-K on wet GAC pretreated with Ca(OH)₂ and NaOH. Contributions of volatile organic fluorine (VOF) compounds, e.g. fluoroalkanes and fluoroalkenes, to the fluorine mass balance were determined by quantitative analysis as well as a semi-quantitative approach. For non-adsorbed PFBA sodium salt (PFBA-Na) in the absence and presence of Ca(OH)₂ and NaOH, ~60% of fluorine was recovered as VOF. For non-adsorbed PFBS potassium salt (PFBS-K), fluorine recoveries in the form of VOF decreased from 70% in the absence of base to 49 and 27% in the presence of Ca(OH)₂ and NaOH, respectively, due to the formation of CaF₂ and NaF in the TGA pan residue.

Based on the maximum contaminant levels that the Environmental Protection Agency has proposed for PFOA and PFOS as well as for four additional PFAS (PFBS, perfluorohexanesulphonic acid, perfluorononanoic acid, and GenX), it is anticipated that GAC adsorption processes will have to be installed at many sites. Remediation site managers need information for the appropriate management of spent GAC once GAC has lost its ability to meet treatment criteria. In the proposed research, the project team is studying one management option for spent GAC: thermal reactivation. It is expected the information generated in the proposed research will support the development of thermal reactivation protocols that (1) effectively mineralize PFAS adsorbed to GAC and (2) permit safe and effective reuse of thermally reactivated GAC. Reuse of reactivated GAC is expected to lower remediation costs at PFAS-impacted sites. Information generated will support the development of thermal reactivation protocols and life-cycle inventory analyses for GAC treatment in the context of PFAS remediation. Such analyses are needed in decision support systems for selecting effective PFAS remediation strategies. (Anticipated Phase II Completion - December 2024)