Flocculation of Microcystis aeruginosa Using Modified Larch Tannin

To flocculate the cyanobacterium Microcystis aeruginosa from water, larch tannin, a natural polymer, was modified by Mannich reaction to obtain a flocculant, named A-TN, which was then quaternized to yield another flocculant, named Q-TN. A-TN and Q-TN were characterized by Fourier transform infrared spectra (FTIR) and zeta potential analysis. The effects of the flocculation parameters, e.g., dosage, pH, cell density, culture time, and extracellular organic materials, were studied. The results showed that Q-TN was effective under a wider range of pH values than A-TN and could work under a pH of 9.0, whereas A-TN could work only under a pH of 7.0. For algal samples with densities from 1 × 10⁸ to 5 × 10⁹ cells/L, the optimum dosages of Q-TN to achieve more than 90% removal efficiency ranged from 0.5 to 20 mg/L, and the optimum dosages had a good linear relationship with cell density. Furthermore, the required dosage of Q-TN clearly increased along with the algae culture time, most of which was consumed by the extracellular organic materials (EOM) excreted from the cells. The spectra of the three-dimensional excitation–emission matrix showed that 100% of simple aromatic proteins and 78.8% of protein-like substances in the EOM could be removed by Q-TN. However, Q-TN was less effective in humic/fulvic-like substance flocculation. Q-TN functioned to settle the algae cells and a large amount of their metabolites effectively.