Photocatalysis offers a promising avenue for completely mutilate harmful algal blooms (HABs), a significant threat to global freshwater reserves. In this study, a series of BiOBrxI1-x photocatalysts were synthesized and the most optimal catalyst was integrated with pristine g-C3N4 and pre-synthesized CoFe2O4/g-C3N4 and NiFe2O4/g-C3N4 to form binary and ternary composite heterojunction photocatalysts (BiOBr0.95I0.05/g-C3N4 - BG, CoFe2O4/BiOBr0.95I0.05/g-C3N4 - CBG, and NiFe2O4/BiOBr0.95I0.05/g-C3N4 - NBG). Synthesized photocatalysts were thoroughly characterized and their performance was evaluated through the visible light driven photocatalytic degradation of both Microcystis aeruginosa (prokaryotic) and Scenedesmus acuminatus (eukaryotic) algal cells sourced directly from ponds. Exceptional photocatalytic efficiency of CBG evidenced through the variation in chlorophyll-a content, malondialdehyde, and electrolytic leakage confirmed the complete rupture of the algal cells after 3 h of light exposure. This was further reconfirmed through fluorescent microscopy analysis and interestingly, both HABs failed to re-grow even after 10 days. Enhanced performance of CBG was attributed to the boosted generation of charge carriers facilitated by its extended visible light absorption, which in-turn produced reactive oxygen species (•O2- and •OH radicals) that caused irreparable oxidative damage to algal cells, while effectively suppressing the exciton pair recombination supported by its double Z-scheme heterojunction. Furthermore, magnetic recyclability feature of CBG facilitated their easy removal from treated water for avoiding secondary pollution. Design of magnetically recyclable photocatalysts for degrading both prokaryotic and eukaryotic HABs demonstrated here is anticipated to inspire the development of efficient photocatalysts and design cost-effective solutions required for treating ponds and lakes infected with HABs.

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