Oxidation pathways of Chlorella vulgaris: The role of commonly used oxidants in transforming cell structure and organic matter

The growing prevalence of nuisance and harmful algal blooms has increased studies on their treatment in the context of water supply. However, the majority of attention has been paid to cyanobacteria rather than more resistant phyla, such as green algae, which can accumulate downstream, impacting water quality. This study systematically investigates the oxidation mechanisms, structural changes and consequent algal organic matter (AOM) release of the green algal species, Chlorella vulgaris, when using three conventional oxidants-free chlorine (Cl₂), potassium permanganate (KMnO₄) and hydrogen peroxide (H₂O₂). Cells were oxidised using a wide range of doses, from 10 min to 168 h, with cell characteristics assessed using flow cytometry coupled with a dual-staining method, and concentration and character of AOM evaluated using total organic carbon analyser (TOC) and liquid chromatography-organic carbon detection (LCOCD). By monitoring the change in characteristics over time and with increasing oxidant dose, the oxidation pathways of C. vulgaris were elucidated. Four stages were identified: 1) cell membrane damage and inactivation; 2) pigment oxidation; 3) internal structure degradation; and 4) complete cell rupture. Cl₂ was the most effective, with 0.1 mg·L^-1 inactivating ∼95 % of the cells within minutes and causing minimal AOM release. KMnO₄ and H₂O₂ required longer contact times and higher doses to achieve similar effects. KMnO₄ oxidation caused substantial AOM release (up to 2.09 mg·L^-1C) and AOM shifts towards lower molecular weight organics (<500 Da), indicating degradation. In contrast, Cl₂ and H₂O₂ oxidation had little effect on AOM concentration or composition. It is proposed that Cl₂ is well suited for pretreatment, where only low doses would promptly inactivate cells without leading to significant AOM release, while H₂O₂, requiring longer contact times, would be better suited for managing downstream cell accumulation, e.g. in sludge lagoons. KMnO₄ is not recommended for pretreatment due to its high cell-disruptive potential and AOM release.