Preparation Of Hierarchically Porous Layered Double Hydroxides And Their Adsorption Performance

Inorganic nanoporous materials have a wide range of applications, such as catalysts, filtration and segregation. The aim of this paper was to effectively remove various pollutants from aqueous solution by adsorption of layered double hydroxides (LDHs) via controlled synthesis route. Calcined lithium/aluminum layered double hydroxides, surfactant-modified zinc/aluminum layered double hydroxide and graphene/zinc/aluminum composite layered double hydroxide were prepared by simple methods, and their adsorption capacity for pollutants (fluoride, dimethyl phthalate, parachlorophenol) in water were investigated respectively.Firstly, hierarchically porous of lithium/aluminum layered double hydroxides were synthesized via a facile hydrothermal route. 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 two types of hierarchical porous organization such as small mesopores (ca.4.5-10nm) and large mesopores (ca.40-50nm). The as-prepared Li/Al-CLDHs samples exhibit excellent adsorption capacity of158.7mg·g-1towards fluoride species in water. Thermodynamic and kinetic studies revealed that the adsorption process was spontaneous and endothermic in nature.Then, well-defined hydrophobic Zn/Al layered double hydroxides (LDH) hollow microspheres were prepared by a facile one-step hydrothermal method using sodium dodecylbenzenesulfonate (SDBS) and trisodium citrate dihydrate (TSC) as organic modifier and structure-directing agents, respectively. The basal spacing of the alkyl chains intercalated LDH (3.045nm) are slightly larger than unmodified (0.754nm). It can be inferred that the alkyl chains were stacked by perpendicular monolayer model. A possible formation mechanism for the controlled organization of primary LDH nanosheet building units into special three-dimensional nanostructures is proposed on the basis of the interaction between trisodium citrate molecules and LDH crystals. The equilibrium adsorption data were best represented by Langmuir isothermal model, showing the maximum adsorption capacity of285.7mg·g-1towards dimethyl phthalate in water Regenerative adsorption experiments confirmed that the organic intercalated zinc/aluminum layered double hydroxides can be repeatedly applied to the removal of organic pollutants in water, and keep a good adsorption stability.Finally, the graphene/zinc/aluminum composite layered double hydroxide were prepared via a facile one-pot hydrothermal method. As can be observed from TEM, the graphene/zinc/aluminum composite display a fold nanosheet morphology containing certain layered structure. The graphene/zinc/aluminum contains two types of hierarchical porous organization such as small mesopores (ca.4.0nm) and large mesopores (ca.10-50nm). Raman data confirmed that the graphene was composited well with zinc/aluminum layered double hydroxide compound. The results indicate that the adsorption capacity of the grapheme/zinc/aluminum composite is larger than zinc/aluminum layered double hydroxide. Pseudo-second-order kinetic equation can better describe the adsorption kinetics, Langmuir model provides the better correlation of the experimental data. The maximum loading capacities for removal of parachlorophenol were found to be48.5mg·g-1.