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

  • 0School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia.
Water research +

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June 14, 2025

Abstract

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.

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