Optimizing Microplastic Removal Through Coagulation-Sedimentation with Permanganate Pre-oxidation and Pre-chlorination

Improving the removal efficiency of microplastics (MPs) in water treatment plants is important to reduce their threats to the environment and human beings. In this study, the removal performance and mechanisms of MPs by pre-treatment-enhanced coagulation-flocculation-sedimentation (CFS) were evaluated. The sinking ratio of MPs was employed to quantify their removal efficiency in CFS, while zeta potentials and floc morphology were analyzed to understand the underlying mechanisms. Two key mechanisms for MP removal were identified: charge neutralization and sweep flocculation. As KMnO4 pre-treatment was conducted, oxidation and MnO2 attachment made the MP surface tend to interact with coagulants to form compact flocs. The removal efficiencies of 200-μm polyethylene terephthalate, polystyrene, and polyvinyl chloride increased 24%, 22%, and 17%, respectively, after KMnO4 pre-treatment was performed. The increases were 38%, 40%, and 41%, respectively, for 6.5-μm ones. Notably, 6.5-μm polythene MPs seemed persistent in floating on the surface. Yet, chlorination pre-treatment contributed slight improvements. The intrinsic features of MPs, including density, size, hydrophobicity, and roughness, affected the basic removal in CFS. KMnO4 oxidation and chlorination changed these features, promoting their interactions with coagulants and removal efficiencies. KMnO4-enhanced CFS can be a potential treatment method for MP removal in water treatment plants.