A novel co-graft tannin-based flocculant for the mitigation of harmful algal blooms (HABs): The effect of charge density and molecular weight.

In this study, for the first time, we developed a series of co-graft tannin-based flocculants, TA-g-P(AM-DMDAAC), with different charge densities (CDs) and molecular weights (MWs) and evaluated their algal-removal performances. The effects of TA-g-P(AM-DMDAAC) on the cell integrity of Microcystis aeruginosa and release of extracellular organic matter (EOM) and microcystin-leucine-arginine (MC-LR) in flocculation and floc storage were also studied. Results suggested that TA-g-P(AM-DMDAAC) could not only efficiently remove algal cells over a wide pH range (pH 3-11) but also EOM. CD and MW significantly affected flocculation performance and floc characteristics of TA-g-P(AM-DMDAAC). A higher CD helped achieve a higher removal efficiency of algal cells and EOM, whereas a higher MW resulted in the formation of larger and more compact flocs. Furthermore, the larger and denser flocs could better protect algal cells and reduce the release of EOM during floc storage. Notably, algal cells in the TA-g-P(AM-DMDAAC) flocs did not appear to show signs of massive rupture nor did they release EOM and MC-LR extensively for at least 20 days of storage. The abundance and easy availability of tannin resources effectively reduce the cost of preparing tannin-based flocculants. Therefore, TA-g-P(AM-DMDAAC) can have broad application prospects in the management of cyanobacteria bloom.