Synthesized cationic starch grafted tannin as a novel flocculant for efficient microalgae harvesting

Owing to low cost and excellent biodegradability, cationic starch-based flocculants (CCs) have been researched in the field of harvesting microalgae, however lacking of targeted affinity to microalgae cells limited their flocculation performance. In present study, tannin was used in the synthesis of a novel CCs grafted tannin-based flocculant (TCCs). SEM, XRD, FTIR, H-1-NMR, zeta potential and UV-Vis analysis proved the successful graft of tannin on CCs skeleton. TCCs exhibited outstanding harvesting performance at 4.0 mg/L dosage, much better than CCs (20.0 mg/L) and flocs formed by TCCs possessed higher reflocculation ability. The flocs size, flocs shape and zeta potential changing regulation indicated that both CCs and TCCs can flocculate microalgae cells through charge neutralization and bridging flocculation. To further explore the flocculation mechanisms, effects of flocculants on microalgal dissolved and/or bound extracellular organic matter were systematically tested. Results suggested that the introduction of tannin make TCCs targeted to combine with proteins in the bound extracellular organic matter and then form compact flocs through hydrogen bonds and hydrophobic bonds, thus saving the flocculant dosage and improving the harvesting efficiency. Additionally, harvesting per ton microalgae by TCCs costs only 19.4 USD, which is more economic saving than CCs and some traditional flocculants.