The role of biochar in the photocatalytic treatment of a mixture of Cr(VI) and phenol pollutants: Biochar as a carrier for transferring and storing electrons.

Biochar (BC) is widely used to remove environmental pollutants due to its photocatalytic activity. However, the mechanism of BC in photocatalysis remains unclear. In this study, soybean straw biochar (D500), dewatered sludge biochar (S500) and TiO2/BC composite catalysts were prepared to test their photocatalytic activity in the photocatalysis-dark reaction using phenol and Cr(VI) as the representative pollutants. D500 had a good graphitized structure, layered structure and more active sites, which led to good photocatalytic activity. Compared with D500, S500 did not have a similar structure, resulting in a lack of photocatalytic activity. In addition, the efficiency of Cr(VI) and phenol removal using D500/TiO2 as a catalyst was higher than that obtained using D500 and TiO2, respectively. TiO2 coupled with D500 increased the generation of photoexcited electrons and reduced the recombination of e--h+ pairs. The removal efficiency of TiO2/D500 for Cr(VI) (80.4 %) and phenol (77.7 %) in the hybrid systems was higher than that of Cr(VI) and phenol in unitary systems. This difference was mainly attributed to the inhibition of e--h+ pair recombination by phenol and Cr(VI), which function as electron quenchers and hole quenchers, respectively. Furthermore, D500 stored electrons under light and released these electrons under dark conditions. When D500 was combined with TiO2, the electrons on the biochar activated the catalytic redox activity of TiO2, thereby removing pollutants under dark conditions. Meanwhile, TiO2/D500 also exhibited good reusability and stability. In summary, this study provides new insight into the role of biochar in photocatalysis.