C,N-Codoped TiO₂ Nanoparticles Immobilized on Floating Alginate Beads for Diazinon Removal under Solar Light Irradiation

Filtration is a critical step during wastewater treatment, in which the catalyst must be separated from the aqueous system for the next treatment. For effective separation of the catalyst, a proper strategy is demanded to overcome the filtration issue in the suspended catalyst system. Therefore, an innovative photocatalytic system with C,N-TiO₂ NPs immobilized on floating alginate beads was designed via the ionotropic gelation method to treat diazinon as a model pollutant under solar light irradiation. The synthesized C,N-TiO₂ NPs showed unique features including an anatase phase with a crystallite size of 11 ± 2.1 nm, active under the visible light region with a lower bandgap energy of 2.94 eV than 3.2 eV of bulk anatase TiO₂, and has slower recombination of electron-hole pairs than the bulk anatase TiO₂. The XPS analysis also confirmed that C and N were codoped into TiO₂ from the shift of binding energy for Ti 2p. SEM analysis provided evidence that many accumulated powders of the synthesized C,N-TiO₂ NPs were dispersed onto the alginate bead surface. The designed C,N-TiO₂ NP/alginate bead floating photocatalyst achieved the excellent removal of diazinon of 80.61% and a mineralization degree of 60.08% with the presence of dominant ROS (·O₂^- radicals) under solar light for 8 h. The degradation kinetics followed first-order kinetics, whereas the Langmuir isotherm model was fitted for the adsorption isotherm. It could be reused for five cycles with 76.84% diazinon removal, reflecting the great reusability of the floating photocatalyst during photodegradation. Thus, this work creates a novel designation of C,N-TiO₂ NP/alginate bead floating photocatalysts that have great potential for application in photocatalytic wastewater treatment.