Spray pyrolysis synthesis of mesoporous γ-AlOOH and γ-Al 2 O 3 microspheres and their properties for Cr (VI) adsorption

In this work, mesoporous spherical γ-AlOOH and γ-Al2O3 particles were rapidly prepared by spray pyrolysis (SP) at different temperatures (400°C–700°C) without the use of template materials. A sol-gel-derived colloidal solution of boehmite was ultrasonicated to generate fine droplets, which were continuously and rapidly transferred into a quartz reactor heated at a specific temperature by a constant flow of N2 gas. Under ultrasonication-assisted spray conditions, a phase change from boehmite to γ-alumina occurred at a temperature of 500°C, leading to a decrease in the surface area and surface hydroxyl group density of the products. At 400°C, the produced γ-AlOOH microspheres exhibited the highest surface area values of ca. ~ 510 m2/g and a surface hydroxyl group density of ca. ~ 2.92 mmol/g, surpassing those of commercial boehmite. The Cr (VI) removal experiments revealed that γ-AlOOH and γ-Al2O3 microspheres have good adsorption performance towards Cr6+. The optimal γ-AlOOH spheres exhibited the highest Cr(VI) adsorption capacity of ca.~ 44.5 mg/g (~74%) at 27 °C and pH = 5.5. After five adsorption-desorption cycles, the synthesised boehmite powder remained a Cr6+ adsorbing capacity of ~ 78%. Notably, the fabricated SP-γ-AlOOH was capable of simultaneously removing Cr6+ and textile dye from the solution. This study provides a facile, rapid, and scalable approach for producing boehmite powder with improved properties for Cr (VI) adsorption.