Optimization of cyanobacterial harvesting and extracellular organic matter removal utilizing magnetic nanoparticles and response surface methodology: A comparative study

Abstract

Improvements in the microalgal harvesting efficiency and separation processes will accelerate cost-effective biomass utilization. Magnetic harvesting is known as a low-cost and environmentally friendly downstream processing technology, but information on optimization of the harvesting procedures is rare. Here, we present optimized harvesting factors for Microcystis aeruginosa 1343 (M1) and 905 (M9) based on polyethylenimine (PEI)-coated iron oxide nanoparticles (IONPs) and response surface methodology. Four important factors including mass ratio (PEI-coated IONPs to dried cell biomass, g/g), stirring speed (rpm), stirring time (s), and adsorption time (min) were evaluated to obtain the optimal operational parameters for different types of cyanobacteria under natural environmental conditions. The maximum harvesting efficiencies for M1 and M9 were 93.3% and 97.5%, respectively; the optimal mass ratio, stirring speed, stirring time, and adsorption time were 0.14–0.18, 85–120 rpm, 70–95 s, and 5.5–7 min, respectively. Our results indicated that the mass ratio was the leading factor in algal harvesting. Changes in the number of cells bound with PEI-coated IONPs were closely related to the mass ratio, and this was confirmed via scanning electron microscopy results. Moreover, the PEI-coated IONPs were able to remove extracellular organic matter (EOM) synchronously via charge neutralization. Variations in fluorescence excitation emission spectra demonstrated an effective EOM removal. Quantitatively, over 34.97% and 45.35% of EOM in M1 and M9 were reduced, respectively, based on total organic carbon analysis. This study provides new insights into algal harvesting operations using magnetic separation technologies and provides practical guidance for performing magnetic separation under environmental conditions.