Biocompatible magnetic flocculant for efficient harvesting of microalgal cells: Isotherms, mechanisms and water recycling

Abstract As an environmentally friendly bioflocculant, chitosan has a great application potential for microalgae harvesting due to its biocompatibility. The rapid development of magnetic separation technology has attracted extensive attention because of its reusability, energy and time saving. In this work, Fe3O4-chitosan composites flocculant with high magnetic harvesting ability and fast separation was successfully prepared with sodium tripolyphosphate as crosslinking agent, and applied to the harvesting and separation of microalgae cells. The harvesting efficiency of 97.63% for Chlorella vulgaris was achieved within 3 min using chitosan/Fe3O4 with the weight ratio of 3:1 and the dosage of 0.5 g/L under pH 3.0. Freundlich isotherm was more suitable for Fe3O4-chitosan to harvest microalgae, indicating the microalgae harvesting by Fe3O4-chitosan composites was a heterogeneous multilayer process. The highest adsorption ability of the Fe3O4-chitosan on Chlorella vulgaris could be reached 75.43 g/g. Double-layer compression, charge neutralization and adsorption bridging are primarily harvesting mechanisms between Fe3O4-chitosan composites and microalgae cells. The second-order polynomial model well simulates the experimental data, establishing the relationship between Fe3O4-chitosan and algae slurry pH, concentration of microalgae cells and settling time. The supernatant after Fe3O4-chitosan harvesting could be reused, saving water resources and cost, showing its biocompatibility. The Fe3O4-chitosan composites had excellent reusability, and the harvesting efficiency could reach 85.10% after four cycles, which improved the economic feasibility of magnetic harvesting. Fe3O4-chitosan has the advantages of high efficiency, fast separation, low economic cost, and low energy consumption. This work provides basic guidance and data analysis in the study of Fe3O4-chitosan for microalgae harvesting on a large scale.