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Towards increasingly severe worldwide pollution of industrial solid waste red mud (RM) released from aluminum industry, constitutional valuable element Al has been successfully separated for a novel mild rotating hydrothermal synthesis (150 °C, 12 h, 5 Hz) of the uniform hierarchical porous flowerlike boehmite (γ-AlOOH) microspheres in the presence of appropriate urea, which exhibit distinctly small average diameter (1.52 μm) and narrow particle size distribution (PSD: 1.12-1.97 μm), as well as high specific surface area (129.37 m2g-1). On the one hand, the rotating hydrothermal synthesis promotes the mass and heat transfer, enabling γ-AlOOH microspheres at a lower temperature within a shorter time. On the other hand, moderate rotation provides predominant shear force, rendering the uniform γ-AlOOH microspheres with small average diameter and narrow PSD. The optimal AlOOH-U2M-R5Hz microspheres demonstrate satisfactory adsorption performance for Congo Red (CR) and Methyl Blue (MB), with the maximum adsorption capacities of 602.4 and 1208.7 mg g-1, respectively. Various isotherm models of Langmuir, Freundlich, Temkin and Dubinin-Radushkevich are utilized, adsorption kinetics are analyzed, adsorption mechanism is uncovered based on hydrogen bonding and electrostatic attraction. The increase in the temperature or the presence of coexisting cations facilitates the adsorption of CR, whereas coexisting anions weaken the adsorption of CR on the AlOOH-U2M-R5Hz microspheres. Furthermore, the excellent recycling performances and especially dynamic adsorption (retainment of removal efficiency of approx. 99.0% within 1000 min) as well as authentic water bodies (e.g. tap water and river water) simulated wastewater treatment undoubtedly indicate great practical applications of the AlOOH-U2M-R5Hz microspheres, towards cleaner aluminum production and cost-effective sustainable solution to anionic dye-bearing wastewater.

Toward the imperative treatment of the industrial wastewater containing 4-nitrophenol (4-NP) and industrial solid waste red mud (RM), an innovative approach of “Using waste to treat waste” is developed. Valuable element Al is leached from the RM first, the resultant NaAlO2solution is hydrothermally converted to γ-AlOOH hierarchical porous microspheres (RM γ-AlOOH HPMSs, average diameter: 2.0 μm, SBET: 77.81 m2 g−1, pore volume: 0.38 cm3 g−1) in the presence of urea. The subsequent mild thermal conversion results in γ-Al2O3hierarchical porous microspheres (RM γ-Al2O3HPMSs). Both of the RM γ-AlOOH and RM γ-Al2O3HPMSs are employed as the Pd catalyst support for the catalytic reduction of 4-NP. Particularly, the as-obtained composite Pd/RM γ-AlOOH and Pd/RM γ-Al2O3exhibit excellent catalytic activities with superior knoras 8204.5 and 4831.4 s−1 g−1, respectively, significantly higher than that of most Pd based catalysts. Moreover, the excellent catalytic stability and durability of the Pd/RM γ-AlOOH and Pd/RM γ-Al2O3within 10 successive cycles of reduction enable the present industrial solid waste RM induced γ-AlOOH and γ-Al2O3HPMSs as great promising Pd catalyst support for the reduction of the industrial wastewater containing 4-NP.