Adsorption to granular activated carbon (GAC) has been frequently used at pilot- and full-scale operations for treatment of water impacted by per- and polyfluoroalkyl substances (PFAS). The predominant residual from GAC systems is spent or exhausted carbon that contains elevated levels of PFAS. In this proof-of-concept project, the behavior and decomposition mechanisms of PFAS laden on GAC in various thermal processes will be investigated. Building on preliminary data already obtained by the project team, this collaborative project has four specific objectives:
- Investigate the thermal stability of PFAS from room temperature (non-heating control) to 1000℃;
- Determine the thermal decomposition mechanisms (temperature and time requirements, thermodynamics, kinetics) of PFAS on spent GAC and develop a full mass balance of PFAS thermal destruction;
- Tailor GAC to facilitate the adsorption of weakly hydrophobic PFAS anions, especially short-chain PFAS, from water and investigate the long-term performance of raw and tailored GAC after multiple cycles of adsorption‒thermal reactivation; and
- Remove PFAS from brine of nanofiltration (NF) and reverse osmosis (RO) by raw/tailored GAC with or without pre-ozonation treatment of the brine.
During this project, the project team will employ the following methodologies:
- Thermogravimetric analysis to study the thermal stability of PFAS chemicals in both oxidative (O2, CO2, air) and inert (N2) atmospheres.
- Temperature-programmable two-zone tube furnace to simulate the commercial thermal reactivation process of spent GAC,
- Ultrahigh pressure liquid chromatography coupled with waters quadrupole time-of-flight mass spectrometry (quadrupole time-of-flight mass spectrometry/ mass spectrometry [MS]) to identify ionizable decomposition products of PFAS on spent GAC heated at different temperatures and durations.
- Gas chromatography (GC) coupled with ion trap MS/MS and a thermal desorption‒pyrolysis device connected to a GC‒MS system to identify the gaseous thermal decomposition products of PFAS.
- Ion chromatography to determine the yield of fluoride ions from PFAS during thermal treatment.
Thermal decomposition kinetics (e.g., half-lives), thermodynamics, and pathways of PFAS with different chain lengths and functionalities will be determined. Also, the modification of GAC with ammoxidation and impregnation approaches will be investigated and the performance of raw/tailored GAC for PFAS removal from natural waters and NF/RO brine with multiple cycles of adsorption‒reactivation‒reuse will be evaluated. Preliminary results show great promise for these strategies.
It is envisioned that the results of this study will help to:
- elucidate the fundamental decomposition mechanisms of PFAS on spent GAC during thermal reactivation in N2 or CO2 and during combustion in O2 or air;
- determine the thermodynamics and kinetics of thermal PFAS decomposition in these atmospheres;
- understand how properties (e.g., chain length and functionality) of PFAS affect their thermal stability and decomposition;
- identify the thermal decomposition products of PFAS and develop sustainable thermal decomposition technologies for PFAS-laden GAC and other materials;
- improve field remediation of PFAS-impacted soils/water by thermal approaches; and
- evaluate how GAC properties affect its adsorption/removal of PFAS from water and waste streams of NF and RO and develop innovative GAC products suitable for long-term PFAS removal.
The results of this project will benefit the Department of Defense (DoD) as well as many other end-users and stakeholders. (Anticipated Project Completion - 2022)