A ‘Concentrate-&-Degrade’ Technology for Cost-Effective Adsorption and Photodegradation of Per-and Polyfluoroalkyl Substances in Municipal Landfill Leachate

Abstract

Per-and polyfluoroalkyl substances (PFAS) are a group of man-made chemicals that have been widely used in industry and daily consumer products. Due to the wide application of these products and their recalcitrant nature to conventional degradation procedures, these chemicals have been widely found in the environment including water, wastewater, and landfill leachate, which poses various adverse health effects. Because municipal landfill has served as the endpoint of many PFAS-laden materials, high concentrations of PFAS have been detected in landfill leachate. As the PFAS-related regulations are rapidly evolving in recent years, it is critical to reduce the PFAS concentrations in landfill leachate before it is transferred to wastewater treatment plants. However, there has been no cost-effective destructive technology available for the degradation of PFAS in landfill leachate. In this regard, we have developed and tested a two-step “concentrate-&-destroy technology” for the treatment of PFAS. The key for the technology was a new type of adsorptive photocatalyst referred to as Bi/TNTs@AC. Using perfluorooctanoic acid (PFOA), one of the most widely detected PFAS in landfill leachate, as a model compound, we tested the material with respect to both adsorption and the subsequent photodegradation of PFAS under real-world landfill leachate matric conditions. The specific objectives were to: 1) Prepare the material based on commercially available activated carbon (AC) and TiO2 and under alkaline hydrothermal conditions, 2) Measure the adsorption capacity and kinetics of PFOA, 3) Test the subsequent solid-phase photodegradation of PFOA under UV irradiation (wavelength = 254 nm, intensity = 210w/m2), 4) Explore ways to enhance the photodegradation effectiveness (pH, temperature, oxidants, and of Fe3+), and 5) Examine the reusability of the materials without invoking chemical regeneration. Batch experimental data indicated that Bi/TNTs@AC at a dosage of 5 g/L was able to adsorb more than 75% of PFOA (initial concentration = 100.947 g/L) within 1 hour despite the strong matrix effect of landfill leachate. The adsorption equilibrium was achieved in 24 h, with >95 % of PFOA adsorbed. After the adsorption, the solid was subjected to UV irradiation (wavelength = 254 nm UV intensity = 210 W/m2 and irradiation time = 4 h) at a pH of 9.43. As a result, >46% of pre-sorbed PFOA was defluorinated (i.e., conversion of fluorine in PFOA into fluoride) in 4 h. The addition of Fe3+ (60 µM) during the photodegradation increased the defluorination rate by 58 %. Furthermore, elevating the reactor temperature from 38 C to 63 C and addition of persulfate (100 µM) increased the defluorination from 46% to 55.4%% and 54%, respectively. The technology appeared promising for cost-effective treatment of PFAS in landfill leachate.