PFAS–CTAB Complexation and Its Role on the Removal of PFAS from a Lab-Prepared Water and a Reverse Osmosis Reject Water Using a Plasma Reactor

Abstract

Electrical discharge plasma reactors with argon bubbling can effectively treat long-chain perfluoroalkyl acids (PFAAs) in contaminated water, and the addition of a cationic surfactant cetrimonium bromide (CTAB) is known to enhance the removal of short-chain PFAAs. However, the roles of PFAA chain length, functional group, and water matrix properties on PFAA–CTAB complexation are largely unknown. This work investigated the bulk liquid removal of different PFAAs by CTAB in the absence of plasma. Stepwise addition of CTAB was subsequently used to efficiently treat PFAAs in a lab-prepared water and a reverse osmosis (RO) reject water using an enhanced contact plasma reactor. The results show that CTAB inhibited the bulk liquid removal of long-chain PFAAs in the absence of plasma likely due to the formation of hydrophilic CTAB-PFAA mixed micelles and competition for interfacial access between long-chain PFAAs and CTAB. On the contrary, CTAB enhanced the removal of short- and ultrashort-chain PFAAs by forming hydrophobic complexes. After 6 h of treatment in the plasma reactor with CTAB, PFAAs were 86 to >99% removed from the lab-prepared water and 29 to >99% removed from the RO reject water. This study provides important insights for overcoming mass transfer limitations for PFAA treatment technologies.

Available and emerging liquid treatment technologies for PFASs

“…In one type of configuration, an inert gas (typically argon) is introduced at the bottom of the reactor to generate bubbles for concentrating PFASs at the plasma–water interface where PFAS molecules are exposed to chemically reactive species in the plasma. In some cases, cationic surfactants can be added to the reactor to enhance transport of short‐chain PFASs ( <C5) to the plasma–water interface and improve overall treatment efficiency (Li, Isowamwen, et al, 2023; Nau‐Hix et al, 2021).…”

“…Non‐thermal plasma applications have been used to treat a variety of liquid PFAS wastes including groundwater (Nau‐Hix et al, 2021; Palma et al, 2021), surface water (Richardson et al, 2023), ion exchange regenerant still bottoms (Singh et al, 2020), landfill leachate (Singh et al, 2021), AFFF rinsate (ESTCP, 2023a), and membrane concentrate (Li, Isowamwen, et al, 2023). The treatment efficiency of the plasma technology depends on (1) source water properties, including PFAS concentration and composition, electrical conductivity, pH, and dissolved organic matter, and (2) operating conditions, including applied voltage, energy input, gas input, surfactant addition, and liquid flow rate (residence time) (Meegoda et al, 2022).…”

“…bottom of the reactor to generate bubbles for concentrating PFASs at the plasma-water interface where PFAS molecules are exposed to chemically reactive species in the plasma. In some cases, cationic surfactants can be added to the reactor to enhance transport of shortchain PFASs ( <C5) to the plasma-water interface and improve overall treatment efficiency(Li, Isowamwen, et al, 2023;Nau-Hix et al, 2021).Non-thermal plasma applications have been used to treat a variety of liquid PFAS wastes including groundwater(Nau-Hix et al, 2021;Palma et al, 2021), surface water(Richardson et al, 2023), ion exchange regenerant still bottoms(Singh et al, 2020), landfill leachate(Singh et al, 2021), AFFF rinsate (ESTCP, 2023a), and membrane concentrate(Li, Isowamwen, et al, 2023). The treatment efficiency of the plasma technology depends on (1) source water properties, including PFAS concentration and composition, electrical conductivity, pH, and dissolved organic matter, and (2) operating conditions, including applied voltage, energy input, gas input, surfactant addition, and liquid flow rate (residence time)(Meegoda et al, 2022).In 2019, the first pilot-scale 4 gpm mobile plasma reactor unit was field-tested for treatment of PFAS-impacted groundwater (Nau-Hix et al, 2021) (Figure 2) and achieved a ≥90% decrease in long-chain PFAAs and PFAS precursors, versus an average of an 88% decrease in short-chain PFAAs.…”