Synergistic Mn(II)/Fe(VI) oxidation-coagulation: A green strategy for cyanobacterial removal with moderate intracellular organic matter release

This study establishes a manganese-activated ferrate (Mn(II)/Fe(VI)) process for autonomous Microcystis aeruginosa removal, addressing the critical challenge of additive dependency and secondary pollution in conventional algal control strategies. System performance was critically governed by Mn/Fe stoichiometric ratios and total oxidant dosages, with near-complete algal elimination (>98 %) achieved at Mn/Fe = 1:1 (≥200 μM total dosage) while maintaining cell integrity. TOC analysis revealed dosage-dependent control over cellular lysis and intracellular organic matter (IOM) releasing. Spectroscopic characterization demonstrated process efficacy in degrading dissolved organics through decreased UV 254 absorbance (unsaturation reduction), declining SUVA (aromaticity loss), and elevated E2/E3 ratios (molecular weight reduction). Comparative analyses identified dissolved organic carbon as a more sensitive cellular stress indicator than K + release, attributed to MnO 2 adsorptive sequestration of released intracellular constituents. Mechanistic investigations revealed dual role of Mn(II) that rapid surface accumulation provided oxidative protection, while subsequent Fe(VI)-driven MnO x formation enabled floc growth through particle bridging. Water matrix effects followed valence-specific modulation, where divalent cations (Ca 2+ /Mg 2+ ) enhanced removal via charge screening, and excessive monovalent ions (Na + /NH 4 + ) impaired efficiency through ionic strength interference. This work pioneers a green oxidation-coagulation synergy that eliminates supplemental additives, offering a sustainable paradigm for mitigating cell rupture and IOM release in algae-laden water treatment.