Preparation of composite coagulant for the removal of microplastics in water

In this work, a composite flocculant (polyferric titanium sulfate–polydimethyldiallylammonium chloride [PFTS–PDMDAAC]) with a rich spatial network structure was prepared for the treatment of simulated wastewater containing polystyrene (PS) micro–nanoparticles. Characterization results showed that the surface of the PFTS–PDMDAAC was a three-dimensional network polymer of chain molecules that exhibited good thermal stability and formed an amorphous polymer containing multiply hydroxyl-bridged titanium and iron. When n(OH⁻) : n(Fe) = 1:2, n(PO₄³⁻) : n(Fe) = 0.35, n(Ti) : n(Fe) = 1:8, n(DMDAAC) : n(Fe) = 5:100, and the polymerization temperature is 60°C, the prepared composite flocculant has the best effect. The effects of dosage, pH, and agitation intensity on the flocculation properties of PFTS–PDMDAAC were also studied. The optimal removal rates of PS-μm and haze by PFTS–PDMDAAC were 85.60% and 90.10%, respectively, at a stirring intensity of 200 rpm, a pH of 9.0, and a PFTS–PDMDAAC dosage of 20 mg/L. The flocs produced by the PFTS–PDMDAAC flocculation were large and compact in structure, and the flocculation mechanism was mainly based on adsorption bridging. Kaolin played a promoting role in the process of PS-μm removal by PFTS–PDMDAAC floc and accelerated the formation of large and dense flocs. This study provided a reference for the coagulation method to remove micro–nanopollutants in the actual water treatment process. Practitioner Points: A composite flocculant with rich spatial network structure (PFTS–PDMDAAC) was prepared. PFTS–PDMDAAC can effectively remove micro–nano polystyrene (PS) in wastewater. The floc produced by PFTS–PDMDAAC is large and compact in structure. The flocculation mechanism of PFTS–PDMDAAC is mainly adsorption bridging.