Visible-light simultaneous photocatalysis of methyl orange and Cr(VI) in water by rare earth modified titanium dioxide

A series of TiO₂ doped with rare earth elements La and Nd were synthesized via sol-gel technique. The as-prepared catalysts were characterized by powder X-ray diffraction (XRD), nitrogen adsorption-desorption analysis and scanning electron microscopy (SEM). Analytical results demonstrate that doping of La and Nd could inhibit the agglomeration of TiO₂ particles, increase the surface area and decrease the crystallite size of TiO₂ with mesoporous structures. By using the materials, visible-light (λ ≥ nm) photocatalysis activities for the reduction of hexavalent chromium Cr(VI) and degradation of methyl orange MO in composite system were investigated. The results show that the catalysts order by the photocatalytic efficiency in Cr(VI)-MO composite system is 1% La/TiO₂ > 1% Nd/TiO₂ > TiO₂. Methyl orange and Cr(VI) were totally eliminated by 1% La/TiO₂ in 30 min under the conditions: visible-light (λ ≥ nm), a 50ml mixture of 40 mg/l MO and 20 mg/l Cr(VI), catalyst of 0.08 g dose and pH 1.0. Kinetic studies indicate that the reduction of Cr(VI) and degradation of MO in composite system were not in agreement with first-order kinetics very well due to synergistic effect in the simultaneous photocatalytic process.