Coagulation of polyvinyl chloride microplastics by ferric and aluminium sulphate: Optimisation of reaction conditions and removal mechanisms

Abstract

The occurrence of microplastics (MPs) in water intended for human consumption, including water supplied by drinking water treatment plants (DWTPs), has raised considerable attention. DWTPs appear to be capable of removing some of the MPs present in raw water; however, little is known about the efficiency of distinct treatment processes for MP elimination. This study investigated the coagulation of model MPs, i.e., polyvinyl chloride particles < 50 µm, using ferric and aluminium sulphate. The effects of the coagulant type, dose, and coagulation pH value were described, and different methods of aggregate separation were applied. It has shown that certain proportion of MPs could be removed even in the absence of coagulant via sedimentation or deep-bed filtration, herein simulated by centrifugation. However, when either Fe- or Al-based coagulation was conducted under suitable conditions (ferric sulphate at 20 mg L−1 and coagulation pH 7–8, or 40–60 mg L−1 and pH 5–8; aluminium sulphate at 20–60 mg L−1 and pH 7–8), the removal efficiency of MPs by coagulation-sedimentation significantly increased. The residual MP concentrations then approached those obtained after centrifugation and corresponded to approximately 80% removal. However, when focusing on Fe and Al residuals, a narrower coagulation optimum was defined. Under the optimised coagulation conditions, MPs ≥ 15 µm were completely removed. Nevertheless, a certain fraction of MPs (approximately 20% of the initial concentration, vast majority < 5 µm) was not removable despite the wide range of coagulation conditions tested, which points out the limits of coagulation for MP removal.