Microplastic removal under different salinity levels using kaolin-assisted coagulation with modified Caryocar brasiliense coagulants

The increasing occurrence of microplastics in aquatic systems has intensified the search for efficient removal strategies, particularly for low-density polymers such as polyethylene (PE), whose limited settling capacity hinders separation from water. This study developed and compared two coagulants derived from pequi tree bark extract ( Caryocar brasiliense) for PE removal under different salinity conditions: a magnetic iron oxide nanoparticle-functionalized coagulant (E-PTB@NPsFe) and a Mannich-modified cationic coagulant (E-PTB/MR). Kaolin was used as a mineral ballasting agent to promote floc formation and sedimentation. The process was optimized by response surface methodology using a central composite rotatable design, considering coagulant dosage, ballast concentration, and salinity as independent variables. Under optimized conditions, E-PTB@NPsFe achieved predicted removals of 96.1% for turbidity, 90.9% for gravimetric PE removal, and 80.0% for microplastic removal, using 76 mg L⁻¹ coagulant, 0.19 g L⁻¹ kaolin, and 9 g L⁻¹ salinity. E-PTB/MR achieved 90.0%, 90.1%, and 72.1% removals, respectively, but required higher coagulant dosage and salinity; moreover, its application in real coastal waters was limited by decreased pH and increased organic load. Mechanistic analyses indicated that E-PTB@NPsFe formed larger, more heterogeneous flocs consistent with sweep flocculation and particle entrapment, whereas E-PTB/MR produced compact polymer-mediated aggregates. In a real saline coastal water matrix, E-PTB@NPsFe maintained effective removal with limited physicochemical disturbance and no increase in residual iron. The magnetic system also retained relevant PE removal capacity over successive reuse cycles. Model validation showed satisfactory agreement between predicted and observed values across evaluated responses, supporting the predictive adequacy of the models under the tested conditions. These results demonstrate that combining natural polyphenol-based coagulants, magnetic functionality, and mineral nucleation is a promising strategy for removing low-density microplastics from saline water matrices under controlled treatment conditions.