Efficient Microcystis aeruginosa removal by moderate photocatalysis-enhanced coagulation with magnetic Zn-doped Fe3O4 particles.

Photocatalysis is becoming a popular method for the inactivation of algae cells. However, the previous research has mainly focused on the destruction of algae cells by photocatalysis for control of harmful blooms in natural waters. This study aims to investigate the effect of photocatalytic pretreatment on the coagulation process for Microcystis aeruginosa removal. Photocatalytic pretreatment by recyclable magnetic Zn-doped Fe3O4 particles under visible-light was indicated to enhance the algae removal efficiency from 10% to 96% with the catalyst dose of 0.05 g/L. The possible mechanism involved in the enhancement was explored by analyzing variations in the algal suspension from the aspects of cell integrity, superoxide dismutase (SOD) activity, cell morphology, and dissolved organic matter (DOM). The photocatalytic process was proved to realize moderate pretreatment of algae cells by destabilization of the algae cells without damaging cell integrity. The damaged cell ratios were all below 6% even after 360-min photocatalytic pretreatment, which could avoid the undesirable release of intracellular organic matter (IOM). The increase in SOD activity with prolonged photocatalytic time indicated that algae cells were stimulated to extensively activate SOD to resist the oxidative damage induced by the photocatalysis. Electron paramagnetic resonance (EPR) measurements further revealed that superoxide radicals were generated and involved in the photocatalytic pretreatment process. Additionally, increased DOC values in the algal suspension were induced by the desorption of mucilage from algae cells. The desorbed mucilage was proved to be mainly composed of large or medium MW rather than small MW compounds, which could further enhance the coagulation. Therefore, the efficient coagulation of algal suspensions can be realized by moderate pretreatment of M. aeruginosa via the magnetic Zn-doped Fe3O4 particle photocatalysis process.