Probing the multi-scale mechanisms of fine bubble-enhanced polyferric chloride coagulation for NOM removal.

Amidst frequent extreme weather events, the increasing concentration and structural complexity of natural organic matter (NOM) in surface waters exacerbate the generation potential of disinfection byproducts (DBPs), rendering conventional coagulation inefficient for NOM removal. To address this environmental health hazard, this study introduced fine bubbles (FBs) into the polyferric chloride (PFC) coagulation system, proposing a cleaner and efficient FBs-PFC enhanced coagulation process for NOM removal. The effects of operational factors (PFC dosages, gas flow rate of the fine bubble generator, and pH) on coagulation performance and anti-interference ability were systematically evaluated. Validated in natural surface water, the FB-enhanced system increased UV254, DOC, and total nitrogen (TN) removal efficiencies by 17.4%, 20.0%, and 21.1%, respectively. Mechanistically, multi-scale characterizations revealed that FBs induced a forced hydrolysis process, driving the transformation of low-activity iron monomers (Fea) into highly polymerized species (Feb and Fec), and reconstructed the three-dimensional floc skeleton. Furthermore, FBs enriched surface active hydroxyls, promoting chemical coordination with organic functional groups. This research provided a microscopic mechanistic basis for understanding FB-enhanced iron salt coagulation, offering theoretical guidance and engineering references for the efficient removal of NOM in low-turbidity complex water environment and the development of green water treatment processes.