Pipeline-pulse electrocoagulation coupled with gravity-driven ceramic membrane for microplastic-laden rainwater purification: Floc structure evolution and interface interaction mechanism

Gravity-driven ceramic membrane (GDCM) bioreactor effectively treats microplastics (MPs)-laden rainwater in-situ, yet membrane fouling restricts its application. Employing pipeline-pulsed electrocoagulation (PPEC) pretreatment, we elucidated the membrane fouling alleviating mechanisms through purification efficiency, membrane flux, and floc characteristics. The PPEC/GDCM system demonstrated effective purification of MPs-laden simulated or actual roofing rainwater, achieving complete MPs retention (100 %) and reducing effluent turbidity from 15.19 ± 1.2 NTU to below 0.2 NTU. Till Day 6, a functional Nitrospira-dominated nitrifying community formed, enabling complete NO₂⁻-N to NO₃⁻-N transformation. In addition, the flocculent structure evolution confirmed the composition dominated by γ-FeOOH and FeO, with progressive Fe³⁺ reduction to Fe²⁺. Moreover, interfacial energy analysis revealed that floc-floc and floc-membrane interactions sequentially exhibited an irreversible primary minimum and a repulsive energy barrier as separation distance increased. The floc-floc repulsive barrier is crucial for maintaining steady-state flux, while the floc-membrane primary minimum governed initial flux decline. This attraction zone of the primary potential minimal between floc and membrane mitigated the initial fouling by slowing flux reduction. During PPEC floc deposition, interfacial energies drove the formation of a hierarchical primary-secondary filter cake layer on the membrane surface, regulating filtration behavior in the PPEC/GDCM system. This study investigates floc formation and structural evolution in PPEC/GDCM system during MPs-laden rainwater treatment, further analyzing floc characteristics and interface behavior to elucidate membrane fouling mitigation mechanisms.