Electron-bridge mediated Z-scheme heterojunction in a CS-GO/CuO/g-C₃N₄ hybrid for synergistic adsorption-photocatalytic removal of tetracycline.

The limited charge separation and slow reaction kinetics of conventional photocatalysts restrict their practical application. This work introduces a strategic "adsorbent-electron bridge" concept for the effective adsorption-photocatalysis removal of tetracycline (TC) by integrating a chitosan-graphene oxide (CSGO) biopolymer matrix into a 10%-CuO/g-C₃N₄ (GCuO-10) heterostructure via ultrasound-assisted hydrothermal synthesis. Characterization techniques (XRD, FTIR, TEM, XPS) confirmed the successful exfoliation of g-C₃N₄, the uniform dispersion of CuO, and the formation of a coherent interface through electrostatic interaction, H-bonding, coordination, and π-π interactions between the CSGO-GCuO composite components. TC adsorption kinetics followed the pseudo-second-order model, with intraparticle diffusion analysis confirming a multi-stage chemisorption process on the CSGO-GCuO composite. Subsequent photocatalytic degradation under visible light showed a significant enhancement in rate constant (k), which was 4.1 and 2.2 times faster than pristine g-C₃N₄ and the binary GCuO-10, respectively. EPR spectroscopy confirmed a high density of unpaired electrons (2.91 × 1022 spins/g) associated with oxygen vacancies, directly correlating with the dominant superoxide radical pathway observed in trapping experiments and verifying the prolonged electron lifetime achieved through suppressed charge recombination. LC-MS/MS analysis confirmed the complete degradation of the parent TC molecule (m/z 445) into smaller molecular weight fragments. Crucially, an Ecological Structure Activity Relationships (ECOSAR) toxicity assessment of these intermediates revealed a significant detoxification effect, converting the highly toxic parent compound into products classified as "practically non-toxic" or "less harmful" towards aquatic organisms. A detailed mechanistic investigation proposed that the integrated CSGO layer acts as a dual-function mediator: concentrating TC molecules near active sites via multifaceted adsorption, while shuttling photogenerated electrons from the Z-scheme GCuO interface, thereby drastically prolonging charge carrier lifetime.