Enhanced removal of extracellular microcystin-LR using chitosan coagulation-ultrafiltration: Performance and mechanisms

Although many studies have proposed different drinking water treatment strategies to cope with harmful algal blooms (HABs), the removal of dissolved cyanotoxins remains a technical challenge. In this study, well-controlled laboratory experiments were performed to investigate the performance of chitosan as an alternative to traditional metallic coagulants in a hybrid coagulation-ultrafiltration (UF) process for addressing multiple HABs impacts in drinking water treatment. Compared to aluminum chlorohydrate (ACH), chitosan induced the formation of much larger and compact flocs, resulting in about 50% of M. aeruginosa cells removal after simple straining. While coagulation with ACH slightly reduced the rate and extent of UF permeate flux decline, chitosan coagulation at low doses (up to 0.25 mg/L) significantly reduced membrane fouling by 33%. At optimal dose (0.25 mg/L), chitosan coagulation showed a remarkable enhancement effect on the extracellular microcystin-LR (MC-LR) removal by UF membrane, up to 50% higher than those achieved with ACH (20%) coagulation and control condition (15%). However, excessive chitosan dosing would lead to severe membrane fouling. Molecular dynamic simulation results confirmed that chitosan exhibited affinity towards MC-LR via electrostatic interactions and weaker yet prevalent hydrogen bonding. The binding between chitosan and MC-LR would lead to toxin encapsulation within the flocs structure and thus an increased level of toxin retention by UF membrane based on the mechanism of size exclusion.