Coupling harmful algae derived nitrogen and sulfur co-doped carbon nanosheets with CeO₂ to enhance the photocatalytic degradation of isothiazolinone biocide

Removing the algae from water bodies is an effective treatment toward the worldwide frequently occurred harmful algae blooms (HAB), but processing the salvaged algae waste without secondary pollution places another burden on the economy and environment. Herein, a green hydrothermal process without any chemical addition was developed to resource the HAB algae (Microcystis sp.) into autogenous nitrogen and sulfur co-doped carbon nanosheet materials C-CNS and W-CNS, whose alga precursors were collected from pure culture and a wild bloom pond, respectively. After coupling with CeO₂, the obtained optimal C-CNS/CeO₂ and W-CNS/CeO₂ composites photocatalytically degraded 95.4% and 88.2% of the marine pollutant 4,5-Dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) in 90 min, significantly higher than that of pure CeO₂ (63.15%). DCOIT degradation on CNS/CeO₂ was further conducted under different conditions, including pH value, coexisting cations and anions, and artificial seawater. Although different influences were observed, the removal efficiencies were all above 76%. Along with the ascertained good stability and reusability in five consecutive runs, the great potential of CNS/CeO₂ for practical application was validated. UV-vis DRS showed the increased light absorption of CNS/CeO₂ in comparison to pure CeO₂. PL spectra and photoelectrochemical measurements suggested the lowered charge transfer resistance and thereby inhibited charge recombination of CNS/CeO₂. Meanwhile, trapping experiments and electron paramagnetic resonance (EPR) detection verified the primary roles of hydroxyl radical (OH) and superoxide radical (O₂^-) in DCOIT degradation, as well as their notably augmented generation by CNS. Consequently, a mechanism of CNS enhanced photocatalytic degradation of DCOIT was proposed. The intermediates involved in the reaction were identified by LC-QTOF-MS, giving rise to a deduced degradation pathway for DCOIT. This study offers a new approach for resourceful utilization of the notorious HAB algae waste. Besides that, photocatalytic degradation has been explored as an effective measure to remove DCOIT from the ocean.