Enhanced removal characteristics of Raphidiopsis raciborskii and dissolved organic matter by a novel preozonation-coagulation/ozonation process using micro-nano bubbles

Unlike Microcystis, the worldwide spread and invasion of diffuse harmful Raphidiopsis raciborskii (R. raciborskii) along with excessive dissolved organic matter (DOM) put a new and critical challenge to lake ecology and human health. Though micro-nano bubbles (MNBs) were widely used for water cleaning process, the knowledge of the performance of MNBs on the removal of algal cells and DOM in diffuse R. raciborskii bloom water remains limited. Herein, a novel preozonation-coagulation/ozonation process using MNBs was proposed for the enhanced removal of algal cells and DOM, and the removal characteristics were extensively explored in this study. Overall, under the optimal conditions (preozonation and enhanced-ozonation by 1 mg/L ozone MNBs for 1 min separately), the removal efficiency of the whole process achieved 90.9 % for R. raciborskii and 60.9 % for DOM, with the front preozonation-coagulation accounting for 86 % and 50 % separately. In the preozonation-coagulation stage, MNBs promoted the compactness of flocs by improving the hydrolysis of coagulant polyaluminum chloride and the content of active component Al₁₃ (23 %), which strengthened the charge neutralization and net sweep characteristics based on ESI-MS analysis. XPS results revealed that the Al^IV of flocs reached a maximum content at 1 min of MNBs preoxidation, and the content of hydrophilic groups such as carboxyl groups within the flocs increased significantly. Hydrophobic humic substances (e.g. lignins) were much more removed in the MNBs preozonation-coagulation stage, while hydrophilic algal organic matter (e.g. proteins) were most favorably removed in the MNBs enhanced ozonation stage based on EEM and FT-ICR-MS analyses. Across the whole process, the cell membranes of R. raciborskii were not significantly disrupted and the release of algal organic matter was negligible. This study can offer an effective strategy and underlying insights for removing new harmful algae and DOM in natural water bodies.