Matrix Effects on Electrochemical Oxidation of Per- and Polyfluoroalkyl Substances in Sludge Centrate

Abstract

This study investigated the electrochemical oxidation of per- and polyfluoroalkyl substances (PFAS) using a Ti₄O₇ anode in centrate from sludge dewatering. Synthetic solutions containing perfluorooctanoic acid (PFOA), other PFAS, and inorganic constituents (phosphate, ammonium, chloride, carbonate, and acetate salts) found in centrate were studied to assess their impact on the oxidation process. PFOA removal decreased from 95% in a stable electrolyte (NaClO₄) to 81% in a Na₂HPO₄ electrolyte and 30% in a solution mimicking concentrated centrate. X-ray photoelectron spectroscopy detected phosphate and nitrogen species on the electrode surface. At potentials required to oxidize PFAS (>3.0 V/SHE), phosphate and ammonium were oxidized to radicals that blocked electrode sites, inhibiting PFAS removal and shifting PFOA oxidation from first-order kinetics. The kinetics were accurately modeled using a Langmuir–Hinshelwood approach with a transient inhibition term. Results suggested that phosphate, ammonium, and bicarbonate ions reduced hydroxyl radical availability, thereby limiting PFOA defluorination. In concentrated centrate, 95% of the chemical oxygen demand and 93% of total PFAS were removed after 233 s of electrolysis at 30 mA cm^–2. However, partial degradation of perfluorohexanoic acid and accumulation of perfluoroheptanoic acid, attributed to inorganic electrode fouling, suggested the need for a multistage reactor system for more complete PFAS mineralization.