Floatable Chitosan Composite Spheres Based on Nanoscale Donor–Acceptor Type g-C3N4 with Enhanced Photogenerated Carriers and ROS Generation for Photocatalytic Degradation of Malachite Green in Aquaculture

Removal of malachite green from aquaculture water is an urgent issue. Photocatalytic technology presents an expansive perspective due to its efficiency, economic, and environmental friendliness. In this work, donor–acceptor (D–A) type conjugated g-C3N4 nanocomposites (TAPCN) were successfully prepared by a one-step thermal polymerization using dicyandiamide and 2,4,6-triaminopyrimidine as precursors. The morphology and structure of the materials were thoroughly characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), the Brunauer–Emmett–Teller (BET) theory, and X-ray photoelectron spectroscopy (XPS). The results indicated that the pyrimidine ring serves as an electron acceptor group to promote the photogenerated charge separation and transfer of TAPCN-70. Theoretical calculations showed that the construction of the D–A structure is more favorable for oxygen adsorption, which creates conditions for ROS generation. Under visible light, MG degradation by TAPCN-70 showed the highest kinetic constant, which was 5.76 times higher than that of pure BCN. In addition, to facilitate large-scale applications, floatable photocatalysts are constructed by immobilizing D–A type g-C3N4 on hollow chitosan spheres. The prepared floating photocatalysts improve the light absorption and utilization, enhance the adsorption of pollutants, and enable the sustainable supply of O2. With these synergistic effects, the production of ROS (•O2– and •OH) is significantly promoted, which causes the enhanced photocatalytic degradation of malachite green in aquaculture water. This study provides a design concept for the efficient degradation of malachite green in water and aquaculture water by constructing g-C3N4-based donor–acceptor conjugated polymers.