Experimental and Theoretical Validation of the Chemical Kinetics for the Thermal Destruction of Perfluoroalkyl Alkyl Substances
C. Franklin Goldsmith | Brown University
Per- and polyfluoroalkyl substances (PFAS) are commonly found in groundwater through the United States. The U.S. Environmental Protection Agency has issued drinking water health advisories for two common PFAS, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), and thus removal of PFAS from impacted sites is now a critical issue for the Department of Defense (DoD) . Remediation of PFAS presents a difficult challenge. Source drinking water must be treated to remove PFAS, usually by adsorption onto granular activated carbon (GAC), which leads to the problem of how to dispose or treat PFAS once it is captured. The most successful and widely used approach is incineration of PFAS; however, the exact chemistry that occurs during the incineration process remains poorly understood. A detailed understanding of the thermal degradation kinetics would greatly benefit PFAS incineration processes. The primary objectives of this integrated experimental and theoretical project are:
- Develop a computational tool that can automatically provide an elementary kinetic mechanism for incineration of PFAS mixtures,
- Assess the potential for release of residual PFAS and reaction byproducts generated during thermal regeneration of GAC, and
- Examine how the nature of the commonly used activated carbons affects the regeneration process for these materials.
The research plan is organized around three technical tasks, encompassing the following:
- Development of accurate models for the gas-phase pyrolysis and combustion of PFAS
- Quantification of reaction byproducts obtained from the thermal degradation of specific PFAS-impacted sites
- Examination of how the nature of the widely used carbon adsorbents can impact the conditions needed for their regeneration
The gas-phase kinetics portion will include (i) high-accuracy computational chemistry for select PFAS and their reactive intermediates, and (ii) shock tube experiments to quantify the rate of thermal decomposition and identify decomposition byproducts. These data will then be used to refine software that can automatically generate kinetic models for different PFAS. These models will be capable of predicting the decomposition kinetics for other PFAS, including those identified in Task 2.
This project will make several unique contributions that will aid the DoD in management of PFAS-impacted sites: (1) a detailed understanding of the governing processes in the thermal destruction of PFAS, including (i) determination of the key decomposition reactions and their products, and (ii) identification of the rate determining steps in the degradation process; (2) development of a clearer understanding of where the key steps in destruction take place, when dealing with the adsorbents used to remove PFAS from water and waste streams. These new insights into PFAS reaction chemistry will facilitate the translation of laboratory results to field-applicable tools such as computational models that will provide science-based guidance for temperature, residence time, and oxidizer concentration to yield a desired PFAS conversion for a given set of operational conditions. (Anticipated project completion - 2024)