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

Abstract

• Harvesting efficiency of over 97% was achieved by using Fe 3 O 4 -chitosan composites. • The maximum adsorption capacity of Fe 3 O 4 -chitosan composites was 75.43 g/g. • Charge neutralization, double layer compression and bridging were the mechanisms. • The supernatant after harvesting could be reused to re-cultivate microalgal cells. 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, Fe 3 O 4 -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/Fe 3 O 4 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 Fe 3 O 4 -chitosan to harvest microalgae, indicating the microalgae harvesting by Fe 3 O 4 -chitosan composites was a heterogeneous multilayer process. The highest adsorption ability of the Fe 3 O 4 -chitosan on Chlorella vulgaris could be reached 75.43 g/g. Double-layer compression, charge neutralization and adsorption bridging are primarily harvesting mechanisms between Fe 3 O 4 -chitosan composites and microalgae cells. The second-order polynomial model well simulates the experimental data, establishing the relationship between Fe 3 O 4 -chitosan and algae slurry pH, concentration of microalgae cells and settling time. The supernatant after Fe 3 O 4 -chitosan harvesting could be reused, saving water resources and cost, showing its biocompatibility. The Fe 3 O 4 -chitosan composites had excellent reusability, and the harvesting efficiency could reach 85.10% after four cycles, which improved the economic feasibility of magnetic harvesting. Fe 3 O 4 -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 Fe 3 O 4 -chitosan for microalgae harvesting on a large scale.