Online Quantification of O2•- and H2O2 and Their Formation Kinetics in Ultraviolet (UV)-Irradiated Nano-TiO2 Suspensions by Continuous Flow Chemiluminescence

O-2(center dot-) and H2O2, the two reactive oxygen species (ROS) produced in TiO2 photocatalytic reactions, play important roles in the photodegradation of pollutants. In this work, according to the different lifetime of O-2(center dot-) and H2O2, online quantification of O-2(center dot-) and H2O2 was successfully achieved based on the two established continuous flow chemiluminescence (CFCL) methods using luminol as the chemiluminescence probe. For O-2(center dot-), the CL intensity was detected by mixing the irradiated TiO2 suspension containing O-2(center dot-) with luminol within 10 s due to its short lifetime and poor stability, and thus the concentration of O-2(center dot-) was indirectly quantified according to the linear relationship between the CL intensity and the luminol concentration. For H2O2 detection with the luminol/K3Fe(CN)(6) system, the CL intensity was detected after the irradiated TiO2 suspension was placed in darkness for 30 min to ensure the complete disappearance of O-2(center dot-), and the concentration of H2O2 produced was directly quantified by the calibration curve between the CL intensity and H2O2 concentration. The concentration of O-2(center dot-) and H2O2 was determined approximately in the range of 7.5 similar to 30 nmol/L and 0.60 similar to 3.0 mu mol/L, and the detection limit was 1.95 nmol/L and 18.0 nmol/L respectively. Based on the proposed models of O-2(center dot-) and H2O2 formation, the kinetic process of O-2(center dot-) and H2O2 formation was studied, and both of them followed the increase of exponential decay function. By simulating the time-dependent concentration change of O-2(center dot-) and H2O2 with their kinetic formulas, the formation rate constants (k(f)) of O-2(center dot-) and H2O2 were calculated to be 0.0653 nmol.s(-1), and 15.0 nmol.s(-1) respectively. The formation rate of H2O2 was higher than that of O-2(center dot-) in the TiO2 photocatalytic reactions. This work provides us more insight into the formation process of ROS, and may help to improve the photocatalytic efficiency of TiO2 photocatalysis.