Efficient photoreduction of Cr(VI) on TiO2/functionalized activated carbon (TiO2/AC-AEMP): improved adsorption of Cr(VI) and induced transfer of electrons.

Slow and random transfer of pollutants and photo-induced carriers on photocatalysts causes loss of efficiency in photodegradation of contaminants. Enhancing and directing mass transfer of them are considered as two major methods for improving the photodegradation of pollutants over photocatalysts. Here in this work, we focused on the design of a novel photocatalyst which not only accelerated the transfer rate of Cr(VI) and electrons but also provided specific transfer routes for them. By careful characterizations, it is indicated that 2-((2-(2-aminoethylamino)ethylimino)methyl)phenol (AEMP) was covalently attached onto activated carbon (AC), which enhanced Cr(VI) transfer from bulk solution to AC through electrostatic or coordinative interactions. The external mass transfer coefficient (Kf) of Cr(VI) over TiO2/AC-AEMP was estimated as 1.75 × 10-6 m s-1, which was ~ 12.79 and ~ 5.96 times that of TiO2 and TiO2/AC, respectively. Dense and homogeneous heterojunctions between AC and TiO2 were acquired synchronically by forming Ti-O-C linkages, which increased traveling of electrons from TiO2 to AC. Accordingly, Cr(VI) can capture photo-induced electrons on the surface of AC via concrete routes and then be reduced efficiently. The results showed that the photoreduction rate of Cr(VI) on TiO2/AC-AEMP reached to ~ 92.7%, and the overall photocatalytic activity of this well-designed TiO2/AC-AEMP has been enhanced significantly by 5.5 times compared to TiO2/AC. The enhanced photocatalytic activity of TiO2/AC-AEMP was mainly attributed to an improved synergetic process of mass transfer-induced adsorption-photoreduction by forming specific transfer routes for accelerative motion of Cr(VI) and electrons. This work provides a feasible strategy to improve the photoactivity of photocatalysts for the degradation of pollutants by effective mass transfer. Graphical abstract.