Innovative CuBTC/g-C3N4 materials for tetracycline mitigation: adsorption, photocatalysis, and mechanistic perspectives†

The widespread accumulation of antibiotic pollutants in water sources calls for advanced and efficient remediation strategies to curb environmental contamination. In this study, a CuBTC (copper benzene-1,3,5-tricarboxylate) with g-C₃N₄ heterojunction photocatalyst was synthesized via a hydrothermal approach in varying ratios (1 : 1, 1 : 3, and 3 : 1) and comprehensively characterized using XRD, FESEM, EDS, HRTEM, EIS, UV-DRS, PL, TGA, FTIR, XPS, and BET measurements, confirming the composite's crystallinity, morphology, elemental composition, charge transport properties, optical behavior, stability, and porosity. Among the tested compositions, the 3 : 1 CuBTC/g-C₃N₄ composite exhibited the highest efficiency, achieving an impressive 97.4% degradation of 25 ppm tetracycline (TC) within just 60 minutes under UV illumination, with a remarkable rate constant of 0.02098 min⁻¹. Stability assessments confirmed its excellent reusability over six consecutive cycles, with only a slight decline in performance to 82.7%. The adsorption behaviour of the composite was analyzed using six isotherm models—Langmuir, Freundlich, Halsey, Harkins–Jura, Temkin, and Dubinin–Radushkevich—along with five kinetic models, including pseudo-first-order, pseudo-second-order, intraparticle diffusion, Elovich, and liquid film models. Adsorption followed the Langmuir isotherm (R² = 0.992) and pseudo-second-order kinetics (R² = 0.968), while photocatalytic degradation aligned with pseudo-second-order kinetics (R² = 0.993). Mechanistic studies identified superoxide radicals as the primary reactive species, supported by hydroxyl radicals, electrons, and holes in the degradation pathway. Mineralization studies revealed significant reductions in TOC (67.8%) and COD (68.6%), while LC-MS analysis provided a comprehensive degradation pathway, illustrating the breakdown of TC into intermediates through ring-opening and oxidative transformations. Thermodynamic assessments indicated that the degradation process was exothermic and spontaneous. ΔG, ΔH and ΔS values were found to be 92.7 J mol⁻¹, −63.84 kJ mol⁻¹, and −0.214 kJ mol⁻¹ K⁻¹ respectively.