Soybean Pulp Extract as a Natural Coagulant for Microplastic Removal in Domestic Wastewater

Microplastics (MP) have emerged as persistent pollutants in aquatic environments, often coexisting with other contaminants in domestic wastewater. Conventional coagulants such as alum are effective but present environmental drawbacks, including harmful sludge generation. This study investigates soybean pulp extract (SPE), a plant-based and protein-rich byproduct of soymilk production, as a natural coagulant for MP removal from municipal wastewater. SPE was prepared using ethanol extraction with and without sodium chloride (NaCl) supplementation, and its protein content, zeta potential, and coagulation performance were evaluated. Jar tests were conducted with varying SPE and alum dosages (10–50 mg/L) in wastewater spiked with 500 mg/L polyethylene (PE) microplastics. The concentration of microplastics used was higher than that normally found in domestic wastewater, and exposure to this concentration was lethal to microorganisms. Results showed that ethanol extraction alone was sufficient for protein recovery, while NaCl addition offered no significant advantage and introduced further negative charge to the extract. The optimum SPE dosage was identified at 30 mg/L, achieving 557.67± 50 mg/L of floc sediments, comparable to alum at the same dosage. FTIR confirmed the removal of PE microplastics, with peaks observed at 2915–2917 cm–¹. SPE achieved up to 96.22% turbidity removal and 59.58% total suspended solids (TSS) removal, which is nearly comparable to alum, although alum was more effective at lower dosages. These findings demonstrate the potential of SPE as an eco-friendly alternative coagulant, capable of microplastic and pollutant removal under real wastewater conditions.