Photocatalytic performance of biochar-modified TiO₂ (C/TiO₂) for ammonia-nitrogen removal

Biochar-modified TiO₂ (C/TiO₂) was prepared by a sol-gel method in this study to improve the photocatalytic capacity for ammonia-nitrogen (NH₃-N) removal from aqueous solutions. The results showed that biochar was successfully modified on TiO₂ and helped improve its photocatalytic performance for pollutant degradation. The removal capacity of ammonia-nitrogen on the synthesized photocatalyst performed well at pH 10 with 1 g L^-1 C/TiO₂ under both 60 (12.25 mg g^-1) and 120 min (16.31 mg g^-1) irradiation (xenon lamp, AM1.5, 25 A). Characterization of C/TiO₂ through scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectrometry (FT-IR) analyses showed the successful introduction of biochar on TiO₂. SEM-EDS and BET analyses displayed that C/TiO₂ had a larger surface area and more pores than the raw materials. XRD spectroscopy illustrated that C/TiO₂ had typical characteristic peaks of anatase-TiO₂ and presented a good photocatalytic degradation performance. It was confirmed from XPS and FT-IR analyses that -COOH groups were present in C/TiO₂ and originated from biochar modification, and these enhanced the photocatalytic performance. Through radical quenching experiments, it was found that superoxide radicals (˙O₂^-) played a dominant role in NH₃-N photocatalytic reactions with hydroxyl radicals (˙OH) and valence band holes (h⁺) playing a synergistic role. N₂ was the main degradation product after 6 h NH₃-N photocatalytic degradation, which was much larger than NO₃^-/NO₂^- (both almost undetected) and NH₃ (ca. 2 times lower than N₂). The new composite C/TiO₂ has potential for ammonia-nitrogen degradation in wastewater treatment and favorable for treating sewage sludge.