Harvesting of Microcystis flos-aquae using chitosan coagulation: Influence of proton-active functional groups originating from extracellular and intracellular organic matter.

Algogenic organic matter (AOM) produced by Microcystis cells inhibits coagulation harvesting; however, the harvesting inhibitory mechanisms at the functional groups level remain to be determined. This study fractionated extracellular organic matter (EOM) and intercellular organic matter (IOM) from Microcystis flos-aquae into five different hydrophilic and hydrophobic fractions and investigated their inhibition of chitosan coagulation harvesting. The proton-active functional groups in the inhibitory fractions were further analysed by potentiometric titration, and the interaction between these functional groups and chitosan was elucidated. The results showed that the harvesting inhibition of M. flos-aquae cells was dominated by HPI in AOM due to its high charge density, which resulted in greater consumption of coagulant. Potentiometric titration results suggested that the proton-active functional groups of both HPIEOM and HPIIOM consist mainly of phosphodiester, carboxylic, phosphoryl and amine/hydroxyl functional groups, and the harvesting inhibition of HPI on M. flos-aquae cells at pH 6.5 was mainly due to the deprotonation of phosphodiester and carboxylic functional groups. Moreover, carboxylic functional groups with stronger polarity could enhance the intermolecular interaction between HPI and chitosan more effectively than phosphodiester at pH 6.5. Preventing the deprotonation of carboxylic functional groups by adjusting the pH to 4.3 could effectively alleviate the harvesting inhibition caused by HPI. These findings revealed the inhibition mechanism of AOM on the coagulation harvesting of M. flos-aquae cells from the perspective of deprotonation of proton-active functional groups, which may provide important insights for assessing the role of AOM in the coagulation harvesting of Microcystis cells.