Comparison of nano-TiO2 immobilization approaches onto biochar: Superiorities of click chemistry strategy and self-acceleration of pollutant degradation

To overcome the recovery challenge of nanoscale photocatalysts, great endeavor has been paid to explore effective nano-TiO 2 immobilization approaches, but comprehensive comparison between these methods is still lacking. Herein, four approaches, namely impregnation-calcination, sol-gel, solvothermal route and click chemistry strategy were fully compared to immobilize nano-TiO 2 onto biochar from the viewpoint of properties characterization, photocatalytic activity, structure stability and pollutant degradation pathway. It was found that, compared to other three methods, product obtained by click chemistry (CC-P25/BC) showed superior adsorption-photocatalysis synergy and persistent long-term stability. These superiorities were not attributed to pore structure, crystallinity or light absorption , but predominated by the robust chemical bonds between TiO 2 and biochar, which served as an effective internal channel for electrons transportation. Besides, with CC-P25/BC as photocatalyst, a unique self-acceleration period was observed during methylene orange (MO) degradation. Via investigating MO degradation pathway and solution environment, we found that it was the strong photocatalytic activity of CC-P25/BC that selectively accelerated MO demethylation, thus leading to solution pH drop and subsequent MO adsorption enhancement and then degradation acceleration. Thus, this study confirmed the superiority of click chemistry in nanoscale photocatalyst immobilization, and affirmed the importance of pollutant degradation process in governing its degradation rate.