Magnetic flocculation of microalgae using magnetite nanoparticles: Progress and challenges

Microalgae present significant advantages as renewable feedstocks with applications spanning food and nutraceuticals, cosmetics, and pharmaceuticals. Microalgae can be used for food supplements, animal feed, fertilizers, biofuels, and bioplastics. However, large-scale utilization is constrained by the lack of sustainable harvesting systems, as harvesting remains the primary bottleneck in terms of both energy demand and economic sustainability. Conventional harvesting methods face limitations in terms of cost, scalability, and applicability. Thus, there is a growing need for low-cost, robust, eco-friendly, and highly efficient dewatering strategies. Magnetite nanoparticles (MNPs) have recently gained considerable attention for microalgal harvesting owing to their large surface-to-volume ratio, high recovery efficiency, time-saving separation, and potential for reuse. MNPs remain stable in suspension and can be easily recovered by applying an external magnetic field, making them a promising option for enhancing both harvesting and dewatering processes. Although magnetic harvesting and dewatering have emerged as promising alternatives to conventional methods, several challenges remain, particularly concerning the recovery, reuse, and overall cost of magnetite nanoparticles (MNPs). Among these, the most critical hurdle is achieving efficient detachment and repeated reuse of MNPs. This review critically evaluates the potential of MNPs for microalgae harvesting, with emphasis on strategies to enhance their stability, harvesting efficiency, economic sustainability, recoverability, and reusability. Current advancements and future perspectives of this technology are also discussed. Furthermore, various approaches for recovering MNPs are examined, focusing on their effectiveness, limitations, and long-term sustainability for industrial scale microalgal harvesting.