Adsorption to granular activated carbon (GAC) has been frequently used at pilot- and full-scale operations to treat water impacted by per- and polyfluoroalkyl substances (PFAS). The predominant residual from GAC systems is spent or exhausted carbon containing elevated levels of PFAS. In this proof-of-concept project, the project team symmetrically investigated the decomposition mechanisms of PFAS laden on GAC in various thermal processes. The primary objectives were focused on analyzing the thermal stability of PFAS, uncovering their decomposition pathways on spent GAC, tailoring adsorbents for enhanced PFAS adsorption in water, and removing PFAS in nanofiltration (NF) and reverse osmosis (RO) brine.

The project team employed the following techniques:

• Thermogravimetric analysis to study the thermal stability of PFAS in both oxidative (O₂, CO₂, air) and inert (N₂) atmospheres.
• A temperature-programmable two-zone tube furnace to simulate the commercial thermal reactivation process of spent GAC.
• Ultra-performance liquid chromatograph coupled with a quadrupole time-of-flight mass spectrometer (MS) to identify ionizable decomposition products of PFAS on spent GAC heated at different temperatures and durations.
• Thermal desorption‒pyrolysis device connected to a gas chromatograph MS system to identify the gaseous thermal decomposition products of PFAS.

The project team also investigated the modification of GAC and evaluated the performance of raw/tailored GAC for PFAS removal from natural waters and NF/RO brine with multiple adsorption‒reactivation‒reuse cycles.

In this proof-of-concept project, the team yielded critical insights into PFAS thermal degradation mechanisms. The project team has detailed the necessary thermal conditions and elucidated the mechanisms of PFAS decomposition in both the thermal regeneration and thermal reactivation processes of GAC. For example, it was found that 30-minute thermal treatment of spent GAC at ≥300 °C achieved ≥99% removal of perfluorooctanoate (PFOA). It was discovered that the thermal stability of PFAS varies with its structure, particularly impacted by the number of perfluorinated carbons and the presence of ether groups in GenX. Various strategies for extracting PFAS from spent GAC were evaluated and discovered that methanol, when amended with salt, yielded promising results in the efficient extraction of PFAS. This research uncovered novel PFAS thermal degradation mechanisms, including random C–C scission. Many polar and nonpolar thermal degradation products of PFAS were detected and identified for the first time. GAC significantly influenced the decomposition kinetics and pathways of perfluoroalkyl carboxylates, such as PFOA, leading to markedly higher yields of shorter-chained intermediates at lower temperatures. Another breakthrough was the creation of modified sorbents with increased affinity for weakly hydrophobic PFAS anions, marking a substantial advancement in PFAS treatment methodologies. Finally, the removal of PFAS using NF/RO methods and the treatment of PFAS-laden NF/RO brine was explored.

This proof-of-concept project has yielded numerous innovative and important findings, greatly enhancing the understanding of the thermal stability and degradation mechanisms of PFAS. Understanding PFAS degradation during the thermal regeneration and reactivation of GAC will enable more efficient and cost-effective chemical removal methods. Identifying PFAS decomposition products will help assess potential environmental risks. This project’s insights into PFAS thermal degradation mechanisms, particularly regarding the influence of structural variables and the efficacy of GAC, contribute to the scientific knowledge base. This groundwork is crucial for future explorations into low-temperature PFAS mineralization and the creation of novel treatment approaches. Therefore, the outcomes of this project hold significant real-world implications for addressing PFAS impact in the environment, offering substantial benefits. (Project Completion - 2024)

Alinezhad, A., P. Challa Sasi, P. Zhang, B. Yao, S. Golovko, M. Golovko, A. Kubátová, and F. Xiao. 2022. An Investigation of Thermal Air Degradation and Pyrolysis of PFAS and PFAS Alternatives in Soil. ACS ES&T Engineering, 2(2):198-209. doi.org/10.1021/acsestengg.1c00335.

Alinezhad, A., H. Shao, K. Litvanova, R. Sun, A. Kubatova, W. Zhang, Y. Li, and F. Xiao. 2023. Mechanistic Investigations of Thermal Decomposition of Perfluoroalkyl Ether Carboxylic Acids and Short-Chain Perfluoroalkyl Carboxylic Acids. Environmental Science & Technology, 57(23):8796-8807. doi.org/10.1021/acs.est.3c00294.

Challa Sasi, P., A. Alinezhad, B. Yao, A. Kubátová, S. Golovko, M, Golovko, and F. Xiao. 2021. Effect of Granular Activated Carbon and Other Porous Materials on Thermal Decomposition of Per- and Polyfluoroalkyl Substances: Mechanisms and Implications for Water Purification. Water Research, 200:117271. doi.org/10.1016/j.watres.2021.117271.

Wang, Z., A. Alinezhad, R. Sun, F. Xiao, and J.J. Pignatello. 2023. Pre- and Postapplication Thermal Treatment Strategies for Sorption Enhancement and Reactivation of Biochars for Removal of Per- and Polyfluoroalkyl Substances from Water. ACS ES&T Engineering, 3(2):193‒200. doi.org/10.1021/acsestengg.2c00271.

Wang, Z., A. Alinezhad, S. Nason, F. Xiao, and J.J. Pignatello. 2023. Enhancement of Per- and Polyfluoroalkyl Substances Removal from Water by Pyrogenic Carbons: Tailoring Carbon Surface Chemistry and Pore Properties. Water Research, 29:119467. doi.org/10.1016/j.watres.2022.119467.

Xiao, F. 2022. A Review of Biochar Functionalized by Thermal Air Oxidation. Environmental Functional Materials, 1(2):187-195. doi.org/10.1016/j.efmat.2022.03.001.