Novel application of red mud: Facile hydrothermal-thermal conversion synthesis of hierarchical porous AlOOH and Al 2 O 3 microspheres as adsorbents for dye removal

Abstract Originated from the by-product of alumina production, red mud has caused severe impact on environment. To cope with the increasingly urgent environmental issue, red mud has now been utilized for the first time to obtain the hierarchical porous γ-AlOOH/γ-Al 2 O 3 microspheres as high efficient adsorbents for dye removal. The NaAlO 2 leached from the red mud is hydrothermally treated at 150 °C for 6.0 h in the presence of urea, leading to hierarchical porous γ-AlOOH microspheres, with a specific surface area of 76.8 m 2  g −1 , 93% of which have a diameter within the range of 3.5–7.5 µm. The subsequent mild thermal conversion at 600 °C for 2.0 h results in the hierarchical porous γ-Al 2 O 3 microspheres with a specific surface area of 158.6 m 2 g −1 , and ca. 92% of which bear a size of 3.0–7.0 µm. The porous γ-AlOOH and γ-Al 2 O 3 microspheres exhibit excellent adsorption for MB with the maximum adsorption capabilities of 953.0 mg g −1 and 1587.6 mg g −1 , respectively, and the latter is distinctly higher than most of the reported results. The possible formation is proposed, and the adsorption mechanism is discussed. The present work provides a new insight into the comprehensive utilization of the red mud as great potential adsorbents for removal of anionic dyes from mimetic waste water, and also is helpful for the future potential applications of the unique hierarchical porous structures in catalysis and other related fields.