Z-Scheme Heterojunction of Cu ₂ O/Ni-BiVO ₄ for Tetracycline Degradation Under Visible-Light: Mechanism, Pathway, and Toxicity Evaluation

Tetracycline (TTC), a commonly used antibiotic, has become an emerging contaminant in aquatic environments, posing chronic toxicity and ecological risks. To address this, we introduce a novel Ni-doped BiVO₄ (Ni-BVO) and Cu₂O/Ni-BiVO₄ (CNBVO) composites by doping nickel into bismuth vanadate (BVO) and subsequently decorating it with truncated octahedral Cu₂O nanoparticles. This combined strategy of doping and coupling for CNBVO aims to improve visible-light absorption, optimize the band gap, and enhance charge-separation efficiency. Their photocatalytic effectiveness was tested by decomposing TTC. Remarkably, the CNBVO photocatalyst achieved almost complete (98%) degradation of TTC within 180 min. This decomposition followed first-order reaction kinetics, and the reaction rate constant for CNBVO was significantly higher by 675%, 337%, and 225% compared to pure Cu₂O, BVO, and Ni-BVO, respectively. Optimal degradation conditions were determined as a catalyst dose of 0.2 g L^-1 at neutral pH for an initial TTC concentration of 10 mg L^-1, maintaining stability over five successive cycles. Toxicity tests performed on degradation intermediates demonstrated reduced toxicity to brine shrimp (in terms of hatchability and lethality) and earthworms (acute lethality, growth, and biochemical markers). Computational toxicity predictions further supported these findings. This study presents a unique photocatalyst synthesis integrating metal doping and semiconductor coupling, offering mechanistic insights into pollutant degradation and real-world applicability.