Combating eukaryotic and prokaryotic harmful algal blooms with visible-light driven BiOBrxI1−x/MFe2O4/g-C3N4 (M = Co & Ni) recyclable photocatalysts

Abstract

Photocatalysis offers a promising avenue for completely eliminating harmful algal blooms (HABs), a significant threat to global freshwater reserves. In this study, a series of BiOBr_xI_1−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 (BiOBr_0.95I_0.05/g-C₃N₄ – BG, CoFe₂O₄/BiOBr_0.95I_0.05/g-C₃N₄ – CBG, and NiFe₂O₄/BiOBr_0.95I_0.05/g-C₃N₄ – NBG). The 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. The 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 fluorescence microscopy analysis and interestingly, both HABs failed to re-grow even after 10 days. The 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, the magnetic recyclability feature of CBG facilitated its easy removal from treated water for avoiding secondary pollution. The 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 of cost-effective solutions required for treating ponds and lakes infected with HABs.