TiO₂/activated carbon synthesized by microwave-assisted heating for tetracycline photodegradation

A furfural residue-derived activated carbon (AC) supported black-TiO₂ photocatalyst was successfully prepared by ultrasonic-assisted sol-gel treatment (USG) and solvothermal treatment (ST) combined with microwave-assisted heating (MH). The prepared composites were characterized and evaluated based on the degradation of tetracycline hydrochloride (TC) under ultraviolet (UV) illumination. The average TiO₂ nanoparticle size of the as-synthesized catalysts was between 9 and 11 nm. The bandgap of TiO₂-USGM was 1.6 eV, much lower than that of other reference catalysts. Organic carbon and AC in the catalyst play positive roles in reducing the band gap (e.g. 1.6∼2.6 eV) and improving visible-light absorption. The oxygen vacancies are responsible for UV-visible absorption. Adding AC into black TiO₂ resulted in a lower degree of recombination of photogenerated electrons. Mott-Schottky plots showed that AC-containing TiO₂@AC-STM reduced the value of conduction band value from -0.59 eV to -0.24 eV, which is beneficial to photogenerated electrons. Compared with TiO₂, the Ti-O-C and Ti-C- in TiO₂@AC remarkably improved the adsorption and catalytic efficiency of TC. In a near-neutral pH environment, TiO₂@AC-STM and TiO₂@AC-USGM exhibited high removal efficiencies (88.0% and 75.7%, respectively) and degradation rates (0.0418 and 0.0302 μmol/g/s, respectively) at a catalyst load of 0.25 g/L. Notably, the catalyst can be effectively used over a wide range of pH (6-9). The solution pH after treatment was close to neutral, which is advantageous for wastewater treatment. The activation energies were found to be approximately 3.47 kJ/mol. The thermodynamic parameters showed that the photodegradation process was non-spontaneous and endothermic. Based on the trapping experiments, O₂⋅^- was mainly responsible for TC photodegradation over TiO₂@AC-STM, followed by h⁺. The TC degradation pathways and catalyst stability were also investigated. Biomass-derived carbon-supported catalysts have great potential for waste biomass utilization as green, and low-cost catalysts.