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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 BiOBrxI1−xphotocatalysts were synthesized and the most optimal catalyst was integrated with pristine g-C3N4and pre-synthesized CoFe2O4/g-C3N4and NiFe2O4/g-C3N4to 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). 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.

Conventional wastewater treatment plants (WWTPs) suffer from high carbon emissions and are inefficient in removing emerging organic pollutants (EOPs). Consequently, we have developed a low operational carbon emissions multifunctional photoelectrochemical (PEC) system for saline sewage treatment to simultaneously remove organic pollutants, ammonia, and bacteria, coupled with H2evolution. A reduced BiVO4(r-BiVO4) photoanode with enhanced PEC properties, ascribed to constructing sufficient oxygen vacancies and V4+species, was synthesized for the aforementioned technique. The PEC/r-BiVO4process could treat saline sewage to meet local WWTPs’ discharge standard in 40 min at 2.0 V vsAg/AgCl and completely degrade carbamazepine (one of EOPs), coupled with 633 μmol of H2production; 93.29% reduction in operational carbon emissions and 77.82% decrease in direct emissions were achieved by the PEC/r-BiVO4process compared with large-scale WWTPs, attributed to the restrained generation of CH4and N2O. The PEC system activated chloride ions in sewage to generate numerous reactive chlorine species and facilitate •OH production, promoting contaminants removal. The PEC system exhibited operational feasibility at varying pH and total suspended solids concentrations and has outstanding reusability and stability, confirming its promising practical potential. This study proposed a novel PEC reaction for reducing operational carbon emissions from saline sewage treatment.