Supercritical carbon dioxide fluid assisted synthesis of hierarchical AlOOH@reduced graphene oxide hybrids for efficient removal of fluoride ions

• Supercritical CO2 fluid is used to synthesize AlOOH@reduced graphene oxide hybrids.
• The hierarchical hybrid exhibits excellent defluoridation capacity at low concentrations of fluoride ions.
• The hybrid exhibits a large surface area of 513 m2 g−1 and a high adsorption capacity of 118.7 mg g−1.
• The covalently bonded structure of the hybrid affords prominent separability.

Hierarchical AlOOH@reduced graphene oxide (RGO) hybrids are synthesized by an anti-solvent process with the assistance of supercritical carbon dioxide (scCO2) fluid due to its high diffusivity and low viscosity. The presence of RGO allows the uniform and vertical assembling of AlOOH nanosheets to form hierarchically nanostructured hybrids with a large specific surface area of 513 m2 g−1. The favorable structure with numerous mesopores provides sufficient active sites for the adsorption of fluoride species, leading to a maximum adsorption capacity of 118.7 mg g−1, which is among the best performances of aluminum based adsorbents. Compared to neat AlOOH, it is interesting that AlOOH@RGO hybrid exhibits a much better defluoridation capacity at lower initial concentrations of fluoride ions, which is quite valuable when considering the low fluoride ion concentrations in contaminated groundwater. The stable and robust hierarchical structure afforded by covalent bonding between AlOOH and RGO further endows the hybrid as a promising adsorbent for practical treatment of wastewater due to its excellent fluoride adsorption capacity and ease of separation from aqueous solutions.

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