Visible-light-driven CQDs/TiO2 photocatalytic simultaneous removal of Cr(VI) and organics: Cooperative reaction, kinetics and mechanism

CQDs/TiO2 was synthesized with a coprecipitation method and characterized by XRD, SEM, TEM/HRTEM, BET, XPS, UV–Vis DRS and I-t curve technologies. UV–Vis DRS displayed that absorption spectrum of CQDs/TiO2 enlarged to visible light zone, suggesting that CQDs/TiO2 can be irradiated by visible light. I-t curve showed that photocurrent of CQDs/TiO2 was higher than that of bare TiO2, revealing that the doping of CQDs accelerated the transfer of photoelectrons and restrained the recombination of photoinduced carriers. Simultaneous removal of Cr(VI) and organics with CQDs/TiO2 photocatalytic reaction was investigated and factors were optimized, and almost all Cr(VI) and organics were removed under the optimum conditions. Experimental results displayed that there was a distinct cooperation removal effect between Cr(VI) and organics in CQDs/TiO2 photocatalytic reaction. XPS analysis proved that Cr(VI) was reduced to Cr(III) in situ on CQDs/TiO2 surface. There were e−, h+,·OH and ·O2− active species which were detected with DMPO in ESR test during CQDs/TiO2 photocatalytic reaction, and scavenger experiment proved that e− and h+ were the substantial reactants for Cr(VI) and organics, respectively. The pathway of photocatalytic simultaneous removal of Cr(VI) and organics underwent four steps: adsorption of Cr(VI) and organics on CQDs/TiO2 surface; production of photo electrons and holes in visible light; reduction of Cr(VI) and oxidation of organics; desorption of Cr(III) and intermediates. Photocatalytic reaction kinetics of Cr(VI) and organics were both confirmed to pseudo first-order reaction. Life span and small scale real application tests both demonstrated that CQDs/TiO2 had a potential application to wastewater containing Cr(VI) and organics.