Greener Microplastics Removal: Progressive Replacement of Iron-Based Coagulants with Sodium Alginate and Chitosan to Enhance Sustainability

Wastewater treatment plants (WWTPs) currently represent one of the main sources for microplastics (MPs) and other emerging contaminants entering the environment. Coagulation is a longstanding and cost-effective process designed to enhance the removal of colloidal particles and proved to be efficient in the abatement of MPs. The present study investigates the feasibility of a progressive replacement of ferric chloride (FeCl₃) with chitosan (CT) and sodium alginate (SA), starting from their use as coagulant aids. Coagulations tests were carried out to assess the performance of FeCl₃-CT and FeCl₃-SA systems in the removal of polystyrene (PS) microbeads, polyethylene (PE) and polyethylene terephthalate (PET) fragments with sizes lower than 500 μm. Results from experiments have shown that both CT and SA are useful to enhance the removal performance of conventional coagulation by improving the settling characteristics of flocs. The use of CT allows a reduction of coagulant dosage for removing PS and PE particles, while it turned out to be detrimental for the removal of PET fragments. Instead, SA at a concentration of 0.2 mg L⁻¹ proved to be useful both to achieve higher removal rate at a medium dosage of coagulant and to improve the efficiency of the process at lower dosages.