Matrix Effects on Electrochemical Oxidation of PFAS in Real Wastewater Systems

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants of increasing global concern due to their chemical stability, mobility, and resistance to degradation in conventional water treatment processes. Electrochemical oxidation has emerged as a promising technique for degrading PFAS, particularly in complex and high-strength wastewater systems. However, the efficiency of electrochemical oxidation is strongly influenced by the chemical composition of the water matrix, which can impact reaction kinetics, electrode performance, and byproduct formation. This thesis systematically investigates the effects of background matrix constituents on PFAS electrochemical oxidation in three distinct wastewater types: municipal sludge centrate, wastewater effluent reverse osmosis (RO) concentrate, and municipal landfill leachate, using tubular Ti4O7 reactive electrochemical membranes (REMs). The first study (Chapter III) focused on understanding how specific inorganic anions and model organic matter (humic acid) present in sludge centrate influence PFAS electrochemical oxidation kinetics. Experiments were conducted using synthetic matrices containing individual ions, including phosphate, bicarbonate, and ammonium. These ions are commonly found in municipal sludge dewatering streams and are known to interact with reactive oxygen species such as hydroxyl radicals. The findings demonstrated a significant decrease in perfluorooctanoic acid (PFOA) removal rates in the presence of these ions compared to an inert NaClO4 electrolyte. The modeled Langmuir-Hinshelwood apparent first rate constant dropped from (1.02 ± 0.090) × 10⁻⁴ m s⁻¹ in NaClO₄ to as low as (3.69 ± 1.4) × 10⁻⁵ m s⁻¹ in ion-rich matrices. X-ray photoelectron spectroscopy (XPS) analysis confirmed that phosphate and ammonium species adsorbed onto the electrode surface during treatment, suggesting these ions interfered with active site availability and contributed to lower PFAS removal. The study also explored energy consumption for centrate electrochemical oxidation, highlighting that despite these limitations, energy requirements per log removal remained relatively low (~1.24 kWh m⁻³ for 83% PFAS removal). Building on these results, Chapter IV explored the electrochemical oxidation of PFOA in RO concentrate obtained from an advanced potable reuse facility. This matrix presents a unique challenge due to its high concentrations of salts, organic compounds, and hardness ions such as calcium and magnesium. Electrochemical oxidation experiments were performed under varying applied current densities (16 - 60 mA cm-2), revealing several key findings. First, the presence of calcium and magnesium resulted in significant cathodic scaling due to the formation of CaCO3, Ca(OH)2, and Mg(OH)2, particularly under high local pH near the cathode. This scaling led to pore blockage, membrane fouling, and increased internal resistance. In parallel, high chloride levels promoted the formation of reactive chlorine species (Cl2, ClO2⁻, ClO3⁻, ClO4⁻), with total ClOx⁻ concentrations reaching 0.54 mM and a free chlorine as high as 3.0 mM. While the overall chlorine mass balance was nearly complete (97–105%), the formation of regulated byproducts such as chlorate and perchlorate poses a risk if the treated effluent is discharged directly to the environment. Despite these challenges, COD and ammonia were effectively removed at higher current densities (> 87.7% and > 87.9%, respectively), and PFOA removal exceeded 85%, indicating that electrochemical oxidation remains a viable option if matrix interferences are addressed. Chapter V shifted focus to municipal landfill leachate, a complex and variable matrix typically containing high levels of organic matter, heavy metals, and scaling ions ( calcium and magnesium). The study investigated a series of pretreatment approaches to reduce matrix complexity and improve the downstream electrochemical oxidation process. Coagulation and flocculation using FeCl3 were found to be highly effective in removing COD (up to 83.4% ± 4.3 %) and certain heavy metals, including As, Mo, Hg, with respective removal efficiencies of 94%, 89%, and 86% at the highest FeCl3 dosage and pH of 8.1. Electrochemical oxidation was further tested for PFBS, a short-chain PFAS known to be more recalcitrant than its long-chain counterparts. In the landfill leachate matrix, PFBS removal reached only 48% after 82 seconds of treatment at a current density of 30 mA cm-2, with minimal removal observed under open circuit potential (OCP) conditions. These results highlight the persistent challenge posed by short-chain PFAS, which are less susceptible to both adsorption and oxidative degradation and may require novel materials for effective treatment. Overall, this thesis comprehensively evaluates matrix effects on PFAS electrochemical oxidation and identifies key mechanisms, such as radical scavenging, electrode fouling, and cathodic scaling, that limit system performance. The results underscore the importance of pretreatment and system optimization, particularly when transitioning from laboratory-scale synthetic matrices to real wastewater applications. Based on these findings, a series of future research directions is proposed, including the development of multi-stage electrochemical systems and the incorporation of catalytic materials to enhance electrode reactivity.

Author dc:creator

• Shirin Saffar Avval (23291995)

• Engineering
• Chemical

Statement dc:rights

• In Copyright
• Open Access after 2028-01-01

DOI dc:identifier

https://doi.org/10.25417/uic.31451611.v1

OAI identifier oai:identifier

oai:figshare.com:article/31451611