Visible light-initiated synthesis of a branched amphoteric flocculant for efficient perfluorooctanoic acid removal from wastewater: Synthesis mechanism and removal mechanism

Perfluorooctanoic acid (PFOA), a representative perfluorinated compound, was widely distributed in the environment and posed significant risks to human health and the ecosystem due to its extreme chemical stability, persistence and bioaccumulative potential. Conventional coagulation-flocculation was ineffective for removing dissolved PFOA from wastewater. Herein, a branched amphoteric flocculant (BAF) was synthesized via synergistic visible light initiation using indium zinc sulfide (ZnIn 2 S 4 ) as an initiator and self-assembled tetra (4-Carboxyphenyl) porphyrin (SA-TCPP) as a photosensitizer. Its synthesis mechanisms, molecular structure, characteristics and removal mechanisms of PFOA were comprehensively investigated via characterization and molecular dynamics simulation. The ZnIn 2 S 4 and SA-TCPP synergistic initiation system generated substantial •O 2 - and •OH radicals under visible light, efficiently triggering BAF synthesis. BAF exhibited a branched structure with amide, sulfonic acid and quaternary ammonium functional groups and good thermal stability. At optimal conditions, BAF achieved 70.2% PFOA removal from simulated industrial wastewater, significantly outperforming commercial PAM. Charge neutralization, electrostatic attraction, hydrophobic interaction, hydrogen bonding and adsorption bridging collectively contributed to PFOA removal. BAF possessed the highest binding energy with PFOA. It generated larger and more settleable flocs. This study provided a promising strategy for synthesizing high-performance flocculants in an efficient way and advancing PFOA wastewater treatment.