Synergistic adsorption and photocatalytic reduction of Cr(VI) using Zn-Al-layered double hydroxide and TiO2 composites

Three TiO2 and Zn-Al-layered double hydroxide composites, denoted LDH-TiO2 composites, were prepared using the sol-gel method and characterized utilizing scanning and transmission electron microscopy with energy dispersive spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, X-ray diffraction, N-2 adsorption-desorption, X-ray photoelectron spectroscopy, photocurrent, photoluminescence and electrochemical impedance spectroscopy techniques. The characterization results illustrated that TiO2 was attached onto the surface of LDH and the sizes of both TiO2 and LDH particles were in the nanoscale range. The combination of LDH and TiO2 promoted the photogenerated electron-hole transfer and separation. The removal abilities of the LDH-TiO2 composites for Cr(VI) were evaluated using the synergistic adsorption and photocatalytic method. The adsorption percentages of the three LDH-TiO2 composites for 20.0 mg/L Cr(VI) solutions ranged between 26% and 75%, and upon ultraviolet irradiation, the total removal percentages rapidly increased to approximately 100%. The removal efficiency for Cr(VI) depended on the TiO2 contents of the LDH-TiO2 composites. Increasing the TiO2 percentage resulted in decrease in the adsorption capacities and increase in the photocatalytic removal ratios. The kinetic and isothermal data well fitted the pseudo-second-order and Langmuir equations, respectively. The high removal efficiencies suggested that the composites were suitable for the treatment of Cr(VI)-containing wastewater.