The effect of erbium on the adsorption and photodegradation of orange I in aqueous Er3+-TiO2 suspension

Pure TiO2 and erbium ion-doped TiO2 (Er3+-TiO2) catalysts prepared by the sol-gel method were characterized by means of XRD and diffusive reflectance spectra (DRS). The XRD results showed that erbium ion doping could enhance the thermal stability of TiO2 and inhibit the increase of the crystallite size, and the DRS results showed that the optical absorption edge slightly shifted to red direction owing to erbium ion doping and the Er3+-TiO2 catalysts had three typical absorption peaks located at 490, 523 and 654 nm owing to the transition of 4f electron from 4I15/2 to 4F7/2, 2H11/2 and 4F9/2. With a purpose of azo dyes degradation, orange I was used as a model chemical. And the adsorption isotherm, degradation and mineralization of orange I were investigated in aqueous suspension of pure TiO2 or Er3+-TiO2 catalysts. The results showed that Er3+-TiO2 catalysts had higher adsorption equilibrium constants and better adsorption capacity than pure TiO2. The adsorption equilibrium constants (Ka) of Er3+-TiO2 catalysts were about twice of that of pure TiO2. The maximum adsorption capacity (Qmax) of 2.0% Er3+-TiO2 catalyst was 13.08 × 10−5 mol/g, which was much higher than that of pure TiO2 with 9.03 × 10−5 mol/g. Among Er3+-TiO2 catalysts, 2.0% Er3+-TiO2 catalyst achieved the highest Qmax and Ka values. The kinetics of the orange I degradation using different Er3+-TiO2 catalysts were also studied. The results demonstrated that the degradation and mineralization of orange I under both UV radiation and visible light were more efficient with Er3+-TiO2 catalyst than with pure TiO2, and an optimal dosage of erbium ion at 1.5% achieved the highest degradation rate. The higher photoactivity under visible light might be attributable to the transitions of 4f electrons of Er3+ and red shifts of the optical absorption edge of TiO2 by erbium ion doping.