Photochemical degradation of perfluoroalkyl substances (PFAS) using vacuum-UV (VUV) has been investigated in recent years. While efficient degradation of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) was observed, the application of the technology is limited by the need of prolonged UV treatment time (up to 72 h) and high energy consumption. Under VUV irradiation, the water molecules typically generate oxidative hydroxyl radicals (·OH) and reductive hydrated electrons (e_aq^-). Understanding the mechanism of the photochemical degradation pathway of PFAS is fundamental to improve the technology efficiency. In this study, PFAS degradation by VUV irradiation under oxidation and reduction conditions was compared by using selected oxidant persulfate (S₂O₈^2-) and reductant sulfite (SO₃^2-). Direct VUV irradiation for 4 h without any oxidant or reductant was able to degrade up to 80% of PFOA and perfluorohexanoic acid (PFHxA), but only 10–18% degradation for PFOS and perfluorohexane sulfonic acid (PFHxS). When reductant sulfite was added, the degradation was significantly improved to 80–100% for PFOS and PFHxS, and slightly increased the degradation of PFOA and PFHxA to 90-100%. In contrast, the addition of oxidant persulfate decreased the degradation of PFOA and PFHxA to 58-62%, and displayed poor degradation of PFOS and PFHxS similar to direct VUV irradiation. Our results revealed that the reducing process is the dominant pathway for PFAS degradation under VUV irradiation. Ongoing work will examine the impacts of pH, dissolved oxygen concentration, and combination of reductants using a lab-designed VUV photoreactor to improve the degradation efficiency further.

Granular activated carbon (GAC) is commonly used for removing per- and polyfluoroalkyl substances (PFAS) from impacted water. However, the impact of inorganic anions on PFAS removal by GAC adsorbers is poorly understood. Rapid small-scale column tests (RSSCTs) were conducted to study the effects of five anions (nitrate, sulfate, chloride, bicarbonate, and perchlorate - 3 meq/L per anion) on GAC use rate for PFAS removal. RSSCT data were scaled up using an approach developed in our lab to reflect full-scale GAC adsorber performance.

The impact of inorganic anions on PFAS removal by GAC depends on both anion and PFAS characteristics. Poorly hydrated anions, such as nitrate and perchlorate, negatively affected removal of short-chain PFAS. For example, nitrate reduced the volume of water that could be treated to 10% PFAS breakthrough (BV₁₀) by 78% and 37% for PFBA and PFPeA, respectively, and perchlorate decreased BV₁₀ by 99.5% and 95% for PFBA and PFPeA, respectively. In contrast, sulfate, chloride, and bicarbonate improved PFAS removal, with the improvement becoming more pronounced for shorter-chain PFAS. For instance, the addition of sulfate increased BV₁₀ for PFBA and PFOS by factors of 1.8 and 1.3, respectively. Our results suggest surface complexation contributes to the removal of short-chain PFAS, while increasing ionic strength shields repulsive electrostatic interactions between anionic PFAS and GAC surfaces that are negatively charged. Additional experiments are ongoing to develop a framework for mathematically describing the effects of inorganic ions on PFAS adsorption.

As with other experimental water treatment technologies capable of degrading and mineralizing poly-/perfluoroalkyl substances, research on semiconductor photocatalytic methods has revealed both advantageous attributes and critical weaknesses. For real water treatment using leading catalysts like Bi₃O(OH)(PO₄)₂, key concerns include interference by inorganic anions, acidic pH requirement, and inactivity toward perfluorosulfonate degradation. Herein, we present performance data and advantages of hexagonal boron nitride microparticles (h-BN) as a photocatalyst. Unlike previous studies of PFAS degradation by h-BN, which utilized ppm-range concentrations, results herein indicated that suspensions irradiated with UVC degrade perfluorooctanoic acid (PFOA) at much greater efficiency when lower, more environmentally relevant initial concentrations (ppb-range) were tested. Furthermore, higher solution pH and the presence of inorganic anions had less impact on efficiency, and degradation of short-chain PFAS compounds was more effective, compared to other catalysts. The use of vacuum-UV (VUV) emitting mercury lamps (254/185 nm) was found to additionally enable a VUV-driven photocatalytic mechanism that achieved degradation of perfluorooctane sulfonate (PFOS); however, degradation of h-BN and nitrate formation were observed. Pilot study results testing treatment of contaminated groundwater yielded electrical energy per order destruction (EE/O) values of 2.7 and 50 kWh∙m^-3order^-1 for PFOA and PFOS, respectively, by the UVC/VUV h-BN process.

Current PFAS treatment strategies generally employ adsorbents (e.g., GAC) or ion exchange to sequester these compounds from water. Once a sorbent’s capacity is reached, the sorbent is taken off-site for thermal regeneration or disposal; however, PFAS desorption kinetics are slow and their destruction is often incomplete, resulting in re-release of perfluorinated compounds into the environment (e.g., in landfill leachate). Recent oxidative and reductive technologies have demonstrated near complete destruction of PFAS, including the development of Ti₄O₇ reactive electrochemical membranes (REMs). However, oxidative processes frequently form undesired byproducts, while currently employed reductive processes utilize hydrated electrons (e.g., via UV/sulfite), which require excessive energy. Our work instead applies electrochemical reduction using electrocatalytic REMs to selectively degrade PFAS while decreasing energy consumption. Recent work by collaborators suggests that nanoscale zero-valent nickel and iron coatings on activated carbon can achieve 94% defluorination of PFOS in anaerobic batch reactors over 3 days. Our aim is to host these catalysts on Ti₄O₇ REMs to increase catalyst activity and lifetime. Preliminary stages of the project include developing a feedback loop for catalyst development. First, density functional theory calculations are used to screen potential catalysts by calculating adsorption energies of PFAS on different catalyst surfaces. Second, catalysts with favorable adsorption energies are synthesized via atomic layer deposition. Finally, the activity of each catalytic REM is determined via electrochemical voltammetry analyses prior to larger scale batch tests. This project addresses the critical need to develop more selective and complete PFAS treatment strategies that can be implemented by water utilities.

With the growing concerns surrounding PFAS exposure, treatment technology selection is vital to protect the health of the population. While several technologies have proven effective for treating PFAS-contaminated water, decision makers rarely utilize the power of life cycle assessment (LCA), ignoring important social and environmental considerations. We have developed a decision support tool that combines treatment performance, life cycle costing (LCC), and LCA to comprehensively compare granular activated carbon (GAC), anion exchange resin (IX), reverse osmosis (RO), and nanofiltration (NF) for PFAS treatment. The user inputs their process design, and the tool calculates inventory data for each process to evaluate economic, environmental, and social decision criteria. These decision criteria are combined into a single, overall score (i.e., recommendation) based on the user’s priorities and goals.

Several scenarios were analyzed to distinguish the tradeoffs of treatment with different technologies. GAC and IX processes were compared over a range of media usage rates to determine how several variables, including flowrate and treatment objective, impact costs and environmental impacts. While the selected variables had a significant impact, GAC gravity basins tended to have lower lifetime costs and IX filters lower environmental impacts. All technologies were then compared to determine the impact of the same variables, as well as decision criteria weighting, on the overall score. GAC and IX processes tended to have the lowest overall scores, but the selected weighting scheme could shift the recommendation. This tool will be highly beneficial to decision makers who want to comprehensively compare treatment processes for PFAS.