Chemical Coagulation Applied for the Removal of Polyethylene and Expanded Polystyrene Microplastics

Abstract

Microplastics are emerging pollutants, formed through weathering, with sizes equal to or smaller than 5 mm. They can reach surface and groundwater sources, as well as oceans and seas through natural pathways or from the discharge of liquid effluents, causing immeasurable effects on human beings. This study aimed to evaluate the optimal conditions for the removal of polyethylene (PE) and expanded polystyrene (EPS) microplastics through coagulation and flocculation processes using aluminum sulfate. To achieve this goal, two 22 full factorial designs were employed, including two replicates at the central points. The sizes of the microplastics were fixed at 0.6 mm and 0.9 mm for PE and EPS, respectively. The selected independent variables were Al2(SO4)3 and pH. The experiments were conducted considering rapid mixing parameters (400 rpm for 1 min), slow mixing (100 rpm for 15 min), and sedimentation (30 min), with a velocity of 0.1 cm·min–1 in the Jar Test. Turbidity determination was applied to quantify the remaining microplastics. Consequently, it was observed that the highest efficiency occurred for PE microplastics at 4.25 mg·L–1 of Al2(SO4)3 and pH 5, and for EPS microplastics at 6.00 mg·L–1 of Al2(SO4)3 and pH 4, resulting in removal rates of 96.81% and 96.30% and turbidity levels of 0.38 and 0.50 NTU, respectively. The removal efficiencies of microplastics were similar, with a decrease at pH 6 for both, as low ionic strength prevents the release of H+ ions.