(396c) Reversible Adsorption and Desorption of Pfas Via Switching the Electric Field on Carbonaceous Adsorbents

Per- and polyfluoroalkyl substances (PFAS) are a group of organofluorine compounds that possess a carboxylic or sulfonic acid attached at one end while a fluoroalkyl chain (CF₂ or CF₃) attached to the other end. The fluoroalkyl chain can provide excellent thermal stability and chemical durability that enable PFAS to be used in a wide range of practical applications.

Recent studies showed that PFAS was detected in groundwater and surface water near a PFAS manufacturing facility and military bases. More recently, PFAS was also detected in drinking water systems which have raised serious concerns. This is because PFAS is persistent and accumulative in both environment and the human body. Thus, the U.S. administrations established regulations on the manufacturing, use, and disposal of PFAS. These regulations were mainly targeted to PFAS with a long fluoroalkyl chain (i.e., (CF₂)₆ or longer). However, there is a sharp increase in the production and use of PFAS with a short or ultrashort (i.e., (CF₂)₃ or shorter) fluoroalkyl chain to replace those with a long fluoroalkyl chain. Thus, the total amount of PFAS discharged into the environment remains the same or even increased.

Conventional separation technologies including coagulation and flocculation showed the limited capability to remove PFAS from water. This can be attributed to extremely low concentration (i.e., parts per trillion) or the chemical durability of PFAS. While physio-chemical processes such as chemical oxidation and reduction, and plasma-based technology can decompose PFAS, this often results in fragmented PFAS which can cause secondary pollution.

Adsorption is perhaps the most prevailing technology to remove PFAS from the water. Carbonaceous materials (e.g., activated carbon, graphite, graphene) based adsorbents are common because of their excellent chemical durability and thermal stability. Also, they can provide a large specific surface area. One of the primary challenges is that these adsorbents often suffer from a relatively low adsorption capacity (i.e., the amount of PFAS taken up by the adsorbent per unit mass) for PFAS with a short fluoroalkyl chain. Also, they exhibit a decrease in the PFAS removal efficiency upon repeated adsorption-desorption processes.

To improve the adsorption capacity and reusability of the adsorbents, applying an electric field can be a viable option. Electric-field aided sorption (i.e., electrosorption) has demonstrated that it can remove the ionized contaminants from water. When an external electric field is applied, ionized substances can get attracted and adsorb to an electrode surface. Given that PFAS is dissociated in water, anionic PFAS and proton will be attracted and adsorbed to an anode and a cathode, respectively. Also, PFAS with a higher polarity (i.e., short fluoroalkyl chain) can be subjected to a stronger coulombic force resulting in a higher adsorption capacity. Furthermore, the electrostatic repulsive force produced by reversing the voltage across the electrodes will enable the desorption of anionic PFAS from the electrode surface.

Herein, we demonstrate reversible adsorption and desorption of PFAS with varied fluoroalkyl chains by applying an electric field across inexpensive graphite electrodes. The results show that it can exhibit very high adsorption capacity for both long and short fluoroalkyl chain PFAS (perfluorooctanoic acid and perfluoropentanoic acid, respectively). We also demonstrate that adsorbed PFAS can be released to water upon reversing the voltage which makes the adsorbent reusable. Finally, we develop a mathematical relation that can describe the kinetics of adsorption and desorption of PFAS by utilizing a pseudo-first-order kinetic model.