Fabrication And Congo Red Adsorption Performance Of Hierarchically Porous Materials

Water is vital for life,and water quality is closely related to human health and development.With the rapid development of the global economy and modern industry,the demand for water resources continues to rise.Meanwhile,the increasing industrial wastewater has caused serious pollution to water resources and affected the sustainable development of human society.The excessive dyes and heavy metal ions in polluted water not only seriously affects the survival of aquatic organisms,but also poses a severe threat to human health.Among the various methods for water treatment,adsorption has attracted much attention due to its simplicity,efficiency,and low cost.As an important class of functional materials,hierarchically porous materials possess large specific surface area and interconnected porous networks,which can not only increase the contact area between the adsorbent and adsorbate,but also provide abundant adsorption sites and fast diffusion channels,thus leading to high adsorption efficiency.In this work,Congo red is used as the main model pollutant and several hierarchical porous adsorbents with different compositions and structures have been constructed,aiming at increasing the adsorption rate while improving the adsorption capacity.Moreover,the adsorption mechanism has been studied.The main research contents are as follows:Firstly,nearly monodispersed hierarchical flower-like nickel?II?oxide?NiO?microspheres were fabricated by a facile solvothermal reaction with the assistance of ethanolamine and a subsequent calcination process.Flower-like nickel?II?hydroxide microspheres assembled by thin nanosheets with uniform diameters of approximate 6.3?m were obtained after the solvothermal reaction.After heat treatment at 350°C,the crystal phase transformed to NiO,but the hierarchical porous structure was maintained.Due to the hierarchical porous structure with relatively large specific surface area and high positive zeta potential for the surface of NiO particles,the as-prepared microspheres exhibited outstanding performance for the adsorption of Congo red?CR?,an anionic organic dye,from aqueous solution at circumneutral pH.Based on the fitting results of adsorption isotherms,the theoretical maximum CR adsorption capacities of NiO and Ni?OH?₂ approach to 534.8 and 384.6 mg g^–1,respectively,much higher than the commercial NiO particles.The adsorption mechanism of CR onto the as-synthesized samples can be mainly attributed to electrostatic interaction between the positively charged sample surface and the anionic CR molecules.Secondly,hierarchical bristle-grass-like NH₄Al?OH?₂CO₃@Ni?OH?₂ core-shell structure is fabricated through a simple hydrothermal approach combined with subsequent chemical bath deposition.Electron microscopy characterization reveals that the ammonium aluminum carbonate hydroxide?NH₄Al?OH?₂CO₃?microfibers are uniformly covered with interconnected and vertically aligned nickel?II?hydroxide?Ni?OH?₂?nanosheets,forming well-ordered hierarchical pore structure.The NH₄Al?OH?₂CO₃ microfibers not only serve as substrate for the growth of the Ni?OH?₂nanosheets but also prevent the nanosheets from aggregating.The maximal adsorption quantity of the hierarchical NH₄Al?OH?₂CO₃@Ni?OH?₂ composite is 426 mg g^–1,which is higher than the bare NH₄Al?OH?₂CO₃ microfibers with larger specific surface area and the aggregated Ni?OH?₂ microspheres.Further analysis revealed that the CR adsorption performance not completely depends on the specific surface area,but is closely related to the surface charge and pore structure.The major adsorption mechanism is electrostatic interaction between the CR molecules with negative charge and the positively charged sample surface of the Ni?OH?₂ nanosheets at neutral pH.Thirdly,three-dimensional?3D?hierarchical porous Al₂O₃@ZnO core-shell composite was fabricated by the deposition of Zn5?OH?8?Ac?2·2H2O nanosheets on NH₄Al?OH?₂CO₃ microfibers and a subsequent calcination process.Al₂O₃@ZnO composite was uniform in size and possessed a structure constituted by an Al₂O₃ core and a shell of crumpled ZnO ultrathin nanosheets.NH₄Al?OH?₂CO₃ microfibers are crucial to the formation of a highly porous Al₂O₃@ZnO core-shell structure because they serve as the substrate of ZnO nanosheets and prevent the aggregation of nanosheets.Numerous pores remained on the ZnO nanosheets after calcination treatment,forming a highly open porous network conducive to accelerating adsorption rate.With the structural reconstruction of the Al₂O₃@ZnO sample in CR solution,CR molecules could be intercalated into the internal structure of the sample,thereby contributing to larger adsorption capacity.Compared with individual Al₂O₃ and ZnO,the as-synthesized Al2O3@ZnO core-shell composite exhibited higher adsorption capacity and faster kinetics for CR.Benefiting from the unique structure and highly porous texture,the Al2O3@ZnO core-shell microfibers have a large maximum adsorption capacity of the is 714 mg g^–1.Lastly,3D hierarchical NiFe layered double hydroxide?NiFe LDH?microspheres and graphene oxide-NiFe layered double hydroxide?GO-NiFe LDH?composite are fabricated using a facile one-pot hydrothermal reaction.Electron microscopy images demonstrate that the GO-NiFe LDH composite possesses a highly porous and well-ordered structure.Both sides of GO are fully covered by the ultrathin LDH nanosheets,resulting in the sandwich-like architecture.The presence of GO nanosheets in the composite prevented the LDH nanosheets from aggregating into microspheres,which in reverse led to the simultaneous exfoliation of GO nanosheets.Compared with NiFe LDH microspheres,GO-NiFe LDH composite has higher specific surface area and larger pore size and pore volume,contributing to the better adsorption performance.For the adsorption towards three anionic pollutants,namely,Congo red?CR?,methyl orange?MO?and hexavalent chromium ions[Cr?VI?],the GO-NiFe LDH composite exhibits markedly faster adsorption kinetics and higher adsorption capacity than NiFe LDH.The enhanced adsorption performance results from the more porous structure and the interaction between GO nanosheets and adsorbates.