Is ozone pretreatment in algal-laden water adequately evaluated, or does synchronous O₃/Fe(II) offer a better...

Excessive algal blooms threaten drinking water safety by releasing toxins and algogenic organic matter (AOM) during cell lysis. Conventional ozone pretreatment is widely applied to enhance coagulation, yet it often ruptures algal cells and increases microcystin release, raising the question of whether its role in algae-rich waters has been adequately evaluated. In this study, a novel synchronous oxidation-coagulation (SOC) pretreatment strategy using ozone and ferrous (O₃/Fe(II)) was proposed to enhance pollutant removal and preserve algal cell integrity. The performance of SOC was systematically compared with pre-oxidation coagulation (POC), ozonation alone, and coagulation alone. Mechanistic analyses revealed that Fe(II) catalysis accelerated ozone decomposition, producing •OH to selectively degrade extracellular organics, while in situ Fe(III) flocs complexed and protected intact cells. Compared with single ozonation or POC, SOC achieved superior performance, removing up to 93 % of algal cells while reducing microcystin-LR release by ∼65 %. Under optimized conditions, 52-66 % of protein-like fluorophores and 37.71 % of macromolecular biopolymers were removed. Fourier transform ion cyclotron resonance mass spectrometry analysis (FT-ICR MS) further confirmed that SOC pretreatment selectively targets CHNO- and CHOS-type compounds associated with CRAM-like matter, polypeptides, and humic substances. Partial oxidation of high-molecular-weight biopolymers into low-molecular-weight neutral compounds contributed to floc formation. This synergistic strategy broadens the operable ozone dosage range and mitigates water quality risks, providing a novel and sustainable pretreatment option for algae-rich waters.