Enhanced dual-functional performance on the tetracycline degradation and chromium detoxification via Z-scheme COF/TiO₂-NH₂ heterojunctions

The coexistence of emerging contaminants and heavy metals in wastewater poses significant environmental risks, necessitating the development of efficient and sustainable remediation strategies. In this study, a covalently integrated Z-scheme heterojunction photocatalyst (TrPa/TiO₂-NH₂) is synthesized for the simultaneous photodegradation of tetracycline (TC) and photoreduction of hexavalent chromium (Cr(VI)) under simulated sunlight irradiation. The composite exhibits enhanced charge separation efficiency, as demonstrated by photoluminescence, electrochemical impedance spectroscopy, and transient photocurrent measurements. Under optimized conditions, TrPa/TiO₂-NH₂ achieves 93.7 % of TC degradation (k_obs = 0.018 min⁻¹) and nearly complete Cr(VI) reduction within 120 min (k_obs = 0.025 min⁻¹) and also maintains excellent stability during multiple cycles in real water matrices. Mechanistic analysis and density functional theory (DFT) calculations reveal that photogenerated electrons predominantly reduce Cr(VI), a process that is thermodynamically more favorable than O₂ reduction. This sequential reduction pathway explains why the presence of ^•O₂⁻ slightly affects Cr(VI) removal but plays a vital role in TC oxidation. Similarly, reactive species scavenging and electron paramagnetic resonance analyses confirm that ^•O₂⁻, h⁺ and ^•OH are responsible for the oxidation of TC, while photogenerated electrons primarily mediate the reduction of Cr(VI). This study provides mechanistic insights into COF-based Z-scheme photocatalysts and presents a promising strategy for addressing complex wastewater contamination.