Removal of Pristine and UV-Weathered Microplastics from Water: <i>Moringa oleifera</i> Seed Protein as a Natural Coagulant

The prevalence of microplastics (MPs) poses significant threats to freshwater ecosystem and human health worldwide. Although synthetic coagulation and flocculation chemicals have demonstrated initial success in removing certain MPs, naturally abundant and nontoxic coagulants and flocculants can potentially reduce the carbon emission of centralized water treatment plants and could be better candidates for the removal of MPs in open natural water systems without adverse ecological impacts. In this study, Moringa oleifera-derived protein-based coagulant and flocculant, particularly the Moringa oleifera cationic protein (MOCP) and protein-coated sand (f-sand) were developed to remove pristine and photo-weathered polyethylene (PE) MPs, which was compared to conventional synthetic alum and polyacrylamide (PAM) chemicals. Through lab-scale ballasted flocculation jar tests, we found the MOCP performed comparably to conventional alum in removing pristine and photo-weathered MPs in distilled water when using PAM as the flocculant (70% removal). The f-sand alone was less effective, but still achieved 60% removal for photo-weathered MPs. The MOCP and f-sand combined were the least effective, likely due to the charge reversal and less incorporation of MPs into flocs. MOCP+PAM and f-sand alone also were similarly effective in removing pristine and photo-weathered MPs suspended in Mississippi River water, compared to alum+PAM. Preliminary results also demonstrated the potential to treat MPs using MOCP in real river water. Our findings suggest that MOCP and f-sand can be effective, sustainable, eco-friendly, and affordable alternatives for conventional synthetic alum/PAM chemicals for mitigating MPs pollution in drinking water and in open surface water where the use of synthetic chemicals might be prohibited.