Hierarchically porous calcined lithium/aluminum layered double hydroxides: Facile synthesis and enhanced adsorption towards fluoride in water

Lithium/aluminum layered double hydroxides (Li/Al-LDHs) were synthesized via a facile hydrothermal route, using lithium and aluminum chloride mixed solutions with various molar ratios (Li+/Al3+ = 2, 3, 4, 5) as precursors and urea as a precipitating agent. The structure, morphology, and textural properties of the calcined Li/Al-LDHs (Li/Al-CLDHs) were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and nitrogen adsorption-desorption. It was found that the three-dimensional petal-like Li/Al-CLDHs assemblies were constructed from hexagonal nanosheets with different sizes. The Li/Al-CLDHs contain three types of hierarchical porous organization such as small mesopores (ca. 4.5-10 nm), large mesopores (ca. 40-50 nm) and macropores (ca. 200-500 nm). The as-prepared Li/Al-CLDHs samples exhibit excellent adsorption capacity of 158.7 mg g(-1) towards fluoride species in water. Thermodynamic and kinetic studies revealed that the adsorption process was spontaneous and endothermic in nature. Analyses by X-ray photoelectron spectroscopy confirmed that fluoride is distributed in the layer channel by ion exchange, physical adsorption as well as insertion into the host layer lattice during the rehydration process. The superior sorption capacity of Li/Al-CLDHs is attributed to the unique hierarchically porous structures and high specific surface areas.