Magnetite nanoparticles for microalgal biomass dewatering: A study on Chlorella sp.

Magnetic flocculation using magnetite nanoparticles (MNPs) has emerged as a promising technique for microalgae harvesting and dewatering, offering advantages over conventional methods. This study evaluated the key factors influencing the efficacy of magnetic harvesting of Chlorella sp. using commercial MNPs in two comparisons with pH-adjustment-sonication-extraction and pH-adjustment-sonication recovery methods for testing the recyclability of MNPs. Results showed that magnetic harvesting efficiencies exceeding 99 % were achieved within the pH range of 7–9 when MNPs were applied at concentrations between 5 and 10 g/L. From pH 7–9, there was no statistically significant variability in performance. In contrast, MNP concentration had a more pronounced effect: less than 50 % harvesting efficiencywas observed at 0.1–0.3 g/L, whereas increasing the concentration to 0.5–1 g/L resulted in 70–90 % efficiency. To enable MNP recycling, pH adjustment—with and without ultrasonication—and solvent extraction were tested and compared. The pH-only method enabled up to two reuse cycles with less than a 20 % reduction in efficiency, whereas the combined method extended reuse to three cycles but involved higher chemical usage and a greater risk of cell damage. These findings suggest that the pH-only detachment approach can be a more cost-effective and sustainable method for achieving efficient (>99 %) harvesting of Chlorella sp., particularly at neutral to alkaline pH levels. Additionally, this approach enables the repurposing of spent MNPs in applications such as soil and environmental remediation, offering potential to add value to waste materials and contribute to a more sustainable future. • Magnetite nanoparticles simultaneously harvest and dewater Chlorella . sp. effectively. • High harvesting and dewatering performance maintained across the pH 7–9. • The effect of MNPs concentration on biocompatibility and harvesting efficiency was synergistically observed. • Three MNPs reuse and recycling were demonstrated.