Development and study of a plasma-based process for the degradative desorption of perfluorooctanoic Acid (PFOA) from granular activated carbon (GAC)

Perfluorooctanoic acid (PFOA) belongs to the group of perfluoroalkyl substances (PFAS), a class of anthropogenic chemicals that has recently attracted significant global attention due to its classification as high-risk environmental contaminants. Perfluorooctanoic acid (PFOA) has been classified by the International Agency for Research on Cancer (IARC) as carcinogenic to humans (Group 1). Conventional water treatment technologies have generally proven ineffective in removing PFAS from contaminated water streams. Currently, the most widely applied removal method involves adsorption using granular activated carbon (GAC). However, this process only transfers the contaminants from the aqueous phase to the adsorbent surface without achieving their actual degradation. As a result, PFAS-saturated activated carbon forms a secondary waste stream that requires further treatment. In practice, saturated GAC is typically regenerated through thermal treatment, incinerated or disposed of into landfill. Nevertheless, thermal treatment approach leads to significant carbon loss and alterations in the porous structure of the material, thereby reducing its adsorption efficiency. Moreover, thermal regeneration raises additional environmental concerns due to the potential emission of fluorinated compounds into the atmosphere. The application of non-thermal atmospheric dual frequency (LF+RF) dielectric barrier discharge (DBD) and low-pressure RF inductively coupled plasma was investigated as an alternative approach for the degradative desorption of PFOA from GAC. GAC samples of atmospheric pressure plasma treatment were characterized by thermogravimetric analysis (TGA), ESI/MS and ion selective electrode before and after plasma treatment. The percentages of degradation/desorption determined by TGA and percentage of defluorination by ion-selective electrode were ~40%, 29%, and 47%, and 18%, 8%, and 29% for argon, humid argon, and argon containing 1% air, respectively. With detection of trace amounts of degradation products followed by ESI/MS analysis. Addition of trace amounts air n noble gas plasma resulted in higher degradation due to formation of reactive nitrogen species (RNS) and reactive oxygen species (ROS) GAC with adsorbed PFOA were treated in low pressure hydrogen plasma, and the effects of different power profiles and treatment times were investigated. High power outputs of 500 W, 600 W, and 700 W for 20 minutes of active plasma time resulted in approximately 91–92% degradation/desorption through TGA. The percentage recovery of surface area and total pore volume of samples was approximately 97% and 96–100%, respectively, indicating preservation of the original surface area and porosity after successful regeneration. Furthermore, adsorption kinetics of PFOA on pristine and plasma-regenerated GAC were compared, showing approximately 99% adsorption within the same adsorption time, thereby indicating restoration of the adsorption functionality of the carbon. However, samples treated at 500 W for only 7 minutes of active plasma time showed approximately 49% degradation/desorption, indicating that insufficient treatment time can result in partial degradation. The percentage of defluorination was below 4% for all treatments lasting 20 minutes, while samples treated with shorter plasma exposure displayed approximately 5.5% defluorination. It is hypothesized that the fluoride released from PFOA degradation rapidly reacts with atomic hydrogen species and evaporates as hydrogen fluoride (HF) due to the high temperature of the system, therefore, resulting in low percentage of defluorination. This work demonstrates the potential of non-thermal plasma reactors as a scalable and environmentally friendly solution for the degradation and regeneration of PFOA-contaminated GAC. In particular, the employed low-pressure plasma system showed a high percentage of removal and regeneration of spent GAC due to their ability to operate at higher power outputs.