Electrochemical oxidation of PFOA and PFOS in landfill leachate using a novel molybdenum disulfide (MoS2) and biopolymer-coated electrode

In this study, an innovative catalytic electrode using molybdenum disulfide (MoS₂) coated with biopolymer-nanocarbon (MoS₂-B-NC) was developed for the electrochemical oxidation of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) in landfill leachate. It was assumed that the novel biopolymer composites (e.g., chitosan) with MoS₂ catalytic nanoparticles (NPs) can provide large surface areas and thus effectively destruct per- and polyfluoroalkyl substances (PFAS) through electrochemical oxidation (either direct or indirect) via NPs or adsorption of PFAS through the interactions between carboxylate groups and PFAS. The developed MoS₂ composite NC electrode with biopolymer showed 20 and 4 times lower charge transfer resistance compared to the NC electrode and MoS₂ coated NC electrode, respectively. With the initial concentration of 1, 3, and 5 µg/L for both PFOA and PFOS, it was found that a low current density of 10 mA/cm² showed a low degree of PFOA (22.7%) and PFOS (19.4%) removal for 150 min probably due to the complex mixture of landfill leachate. By increasing the current density to 100 mA/cm², the MoS₂-B-NC electrode showed the improved removal of PFOA (88%) and PFOS (81%) in 30 min and complete removal in 40 min for both PFOA and PFOS. The removal rate constant was 0.0903 min^-1 for PFOA and 0.065 min^-1 for PFOS at 100 mA/cm², which is 60 and 65 times higher compared to PFOA and PFOS removal at 10 mA/cm², respectively. A separate batch test showed that approximately 53% of fluoride ion was produced from the PFOA and PFOS electrochemical oxidation at 100 mA/cm². It seems that a higher current density is necessary to generate sufficient ●OH for PFAS removal in the complex matrix like landfill leachate. The effect of conductivity and the relationship between COD removal and PFAS removal were highlighted to discuss the competition of electrochemical oxidation in landfill leachate. Overall, the MoS₂-B-NC electrode showed successful electrochemical destruction of PFOA and PFOS in landfill leachate.