Influence of Different Coagulants on Microplastics Removal

Microplastics (MPs) have become a topic of concern for human health over the past several years and drinking water has been identified as an important pathway for MP exposure. Coagulation processes in conventional drinking water treatment plants (DWTPs) have been found to significantly reduce MPs in treated water. Most research to date on the effects of coagulation and MP removal has been conducted using pristine plastic particles or lab-aged model particles. These pristine and model plastics lack many important surface characteristics of the environmental MPs found in the raw water treated by DWTPs. In this project, I analyzed the effectiveness of different coagulants, doses, and coagulant aids to remove environmental MPs under typical conditions used in DWTPs. This project utilized raw water from the Olentangy River to analyze MP concentration and removal via coagulation. Three different chemical coagulants commonly used in DWTPs (ferric chloride, aluminum sulfate, and aluminum chlorohydrate) were compared for MP removal using jar testing methods. Microplastics were isolated from the raw and treated water and imaged using a scanning electron microscope (SEM) to quantify MP content and determine removal efficiencies. The samples were also analyzed using optical photothermal infrared (OPTIR) spectroscopy to characterize polymer types present in raw and treated water samples. The results showed the dose of aluminum sulfate (alum), ferric chloride (FeCl3), and aluminum chlorohydrate (ACH) that provides optimal removal of MPs was 30 mg/L. Across all doses, coagulation provides significant removal of MPs of 69.1-93.7% as particles/L and 57.6-95.7% as µg/L), with observed removal for the tests conducted with ACH being the largest (86.9-93.7%). Turbidity was used to assess its relationship to MP removal. Alum had the largest reduction in turbidity compared to FeCl3 and ACH (79.5-92.9% across all three coagulants). This shows that turbidity is not necessarily an indicator of MP removal in drinking water, while it is an important water quality parameter in drinking water treatment. Removal efficiencies achieved in this study demonstrated greater MP removal during coagulation when compared to some studies that used model plastics. However, some studies using extensive aging methods for model plastics showed removal efficiencies consistent with the results of this test. Overall, this research demonstrated that lab-aged MPs can act as a model for environmental MPs when sufficiently weathered with treatments that mimic natural weathering processes.