Effect of Bi5O7I/calcined ZnAlBi-LDHs composites on Cr(VI) removal via adsorption and photocatalytic reduction

Highlights: • Bi₅O₇I/ZnAlBi-CLDHs composites were first prepared and used to remove Cr(VI) • Bi³⁺ was doped in ZnAl-LDHs to improve its adsorption ability for Cr(VI) • Bi₅O₇I/ZnAlBi-CLDHs has a high removal rate of 98% via adsorption and photocatalysis. • Excellent Bi₅O₇I/ZnAlBi-CLDHs composite can efficiently remove heavy metal pollutants for environmental protection. Bi₅O₇I/calcined Bi-doped ZnAl layered double hydroxides (Bi₅O₇I/ZnAlBi-CLDHs) composites were prepared using the ultrasonic-assisted wet-chemical method, and the removal efficiency of Cr(VI) in an aqueous solution—through the adsorption and photocatalytic reduction of Bi₅O₇I/ZnAlBi-CLDHs—was investigated. The Bi doping concentration and composition ratio of Bi₅O₇I/ZnAlBi-CLDHs influence the removal rate of Cr(VI), and the optimal removal ability of Bi₅O₇I/ZnAlBi-LDHs is attained with 3% Bi doping and a Bi₅O₇I/ZnAlBi-CLDHs ratio of 1/10. The adsorption rate of Cr(VI) reaches 84% when infiltrating Bi₅O₇I/ZnAlBi-CLDHs for 60 min, while a higher removal rate of 98%, is attained through photocatalysis with visible light irradiation. The experimental data fit well with the pseudo second-order kinetic model, Langmuir adsorption isotherm model and the first-order reaction equation, which explains the possible Cr(VI) removal mechanism based on the fitting data. XPS results prove that about 61% of the Cr(VI) is gradually reduced to safe Cr(III) through the photocatalysis of Bi₅O₇I/ZnAlBi-CLDHs by the light irradiation duration 6 h. These results indicated that harmful Cr(VI) can be effectively removed by as-prepared Bi₅O₇I/ZnAlBi-CLDHs for water security, and that Bi₅O₇I/ZnAlBi-CLDHs provide a new prospect for the removal of Cr(VI) through adsorption and photocatalysis.