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The Chlorella vulgaris harvesting was explored by magnetic separation using FeO particles coated with the plant polyphenol chemically modified by a Mannich reaction followed by quaternization (FeO@Q-PP). The -N(R) and Cl-N-C perssad of the Q-PP were linked to the FeO particles by N-O bonds, as suggested by the X-ray photoelectron spectroscopy spectra. The thermogravimetric analysis displayed the mass percentage of the Q-PP coated on the FeO surface was close to ~ 5%. Compared with the naked FeO particles, zeta potentials of the FeO@Q-PP particles were improved from the range of - 17.5~- 25.6 mV to 1.9~36.3 mV at pH 2.1~13.1. A 70.2 G coercive force was obtained for the FeO@Q-PP composite, which demonstrated its ferromagnetic behavior. The use of FeO@Q-PP resulted in a harvesting efficiency of 90.9% of C. vulgaris cells (3.06 g/L). The FeO particles could be detached from the cell flocs by ultrasonication leading to a recovery efficiency of 96.1% after 10 cycles. The recovered FeO could be re-coated with Q-PP and led to a harvesting efficiency of 80.2% after 10 cycles. The magnetic separation using FeO@Q-PP included charge neutralization followed by bridging and then colloid entrapment.

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