Magnetic recovery of microplastics in dynamic flow systems: progress toward continuous separation

• Flow-through magnetic separation as a step forward in microplastic remediation • Evaluation of magnetic field distribution and magnitude in magnetic separators • Investigation of magnetic-to-drag force balance acting on magnetized microplastics • Analysis using a dimensionless parameter coupling force balance and system dimensions • Outline force balance and system dimensions for enhancing overall process efficiency Magnetic separation offers promise for microplastic removal from water bodies. Since microplastics lack magnetic properties, their magnetization is required prior to magnetic recovery. The practical implementation of magnetic separation for microplastic removal and water remediation relies on the availability and rigorous design of systems to retrieve magnetized microplastics operating under flow conditions; however, this aspect remains scarcely investigated. Herein, we advance the design of flow-through magnetic separators (FMSs) for microplastics retrieval from water by combining experiments and magnetic field modeling. Using ∼35µm polystyrene beads as model microplastic, the magnetite-to-polystyrene ratio and contact time in the magnetization stage leading to high recovery yield are identified. Then, magnetic recovery of microplastics is assessed considering three FMSs comprising channels with different lengths and magnets with varying total magnetic energy. Results are analyzed in terms of residence time in the FMSs and the dimensionless parameter θ (product of the magnetic/fluidic drag forces balance and the channel’s length/width-depth ratio). We find that for residence times below ∼0.5s, microplastic recovery increases exponentially with residence time, and there is a notable influence of θ (at 0.25s residence time, the recovery yield is reduced by half when θ varies from 0.62 to 1). This effect becomes less pronounced and the trend smooths out for residence times above ∼0.5s. As high θ is preferred (it entails higher flow rates for similar retrieval), the relevance of coupling the geometrical features of the separator and the force balance is highlighted. Collectively, this work provides guidelines for designing FMSs for microplastics recovery.