Insights into the application of the anodic oxidation process for the removal of per- and polyfluoroalkyl substances (PFAS) in water matrices

The current study aimed to investigate major knowledge gaps regarding the application of the anodic oxidation (AO) process with boron-doped diamond (BDD) anodes for the remediation of water matrices contaminated with per- and polyfluoroalkyl substances (PFAS). This included: (i) the degradation of ultrashort-chain (C1-C3) and long-chain (C9-C13) PFAS in addition to short- and medium-chain (C4-C8) PFAS, (ii) the application of multi-solute systems with different PFAS content (0.2 µg L-1 versus 2.0 µg L-1) and diversity (24 C1-C13 versus 8 C1-C8) in addition to single-solute systems, (iii) the use of real water matrices in addition to pure water, and (iv) the application of current densities (j) up to 250 mA cm-2 in addition to usual j (≤20 mA cm-2). C1-C4 PFAS with a sulfonated headgroup were the most recalcitrant compounds. By contrast, PFAS ≥C9 with a sulfonated headgroup and PFAS ≥C12 with a carboxylic headgroup were potentially instantaneously degraded. The content and diversity of PFAS mainly affected the degradation kinetics of PFEtS (C2), PFPrA (C3), and PFBA (C4). Four real water matrices were under focus: drinking water (DW), urban wastewater after secondary treatment (UWW), and nanofiltration concentrate (NF) and reverse osmosis concentrate (RO) from urban wastewater polishing step. PFAS degradation typically benefited from using real matrices primarily due to the presence of chloride ions and consequent electrogeneration of active chlorine species. However, for waters with a high organic content, namely a chemical oxygen demand (COD) of 319 mg O2 L-1, PFAS degradation was hindered. Furthermore, the removal of most PFAS benefited from the application of j >20 mA cm-2, and some specific PFAS required the use of j ≥250 mA cm-2 to have maximized removal rates.