Removal of micron-sized microplastic particles from simulated drinking water via alum coagulation

Microplastics are being recognized as an emerging pollutant entering wastewater and drinking water treatment plants. Microplastics are often defined as having a dimension less than 5 mm, however much smaller plastic particles have been identified in the environment. While a few studies have addressed the effectiveness of traditional water treatment processes for the removal of microplastics in the mm-size range, additional investigations that study the behavior of smaller plastic particles during water treatment are needed. The results of this work confirm that conditions which are suitable for the removal of a conventional oxide colloid, kaolin, are also effective for the removal of a model plastic sphere (density 1.3 g/cm3, 1–5 µm diameter). Coagulation using alum at concentrations between 5 and 10 mg/L Al produced water with turbidity less than 1.0 NTU from solutions containing 5 mg/L microspheres with an initial turbidity of 16 NTU. Although the presence of 20 mg/L of surfactant in solution shifted the microsphere zeta potential, it did not impair microsphere coagulation at low alum doses and all solutions showed a gradual trend toward higher residual NTU at higher alum dose. Evidence from floc photos and zeta potential measurements point to sweep flocculation as the dominant mechanism for microsphere removal for the conditions of this study. The dispersion of polyethylene microfibers (density 0.96 g/cm3, 5 µm diameter cut to 0.1 mm lengths) in water was strongly influenced by surfactants yet the fibers were still effectively removed via coagulation. Further experimentation with other plastics and solution conditions is needed to enhance understanding of the effectiveness of traditional water treatment processes for microplastic removal.