| With the progress of society and the increase of human activities,the discharge of various industrial wastewater and domestic sewage has been increasing year by year In this case,there are many kinds of hazardous pollutants with different structural scales in water environment.Among them,heavy metals,phenols and oil pollutants have been an important topic for researchers due to their poor biodegradability,high toxicity,fast migration rate,difficult to handle,and high treatment cost.Numerous engineering practices and studies have shown that adsorption is one of the most effective method to remove such refractory pollutants from wastewater.Therefore,there is great significance in the field of water treatment to develop the adsorbent with excellent performance,and then carry out its adsorption properties in actual application.Layered double hydroxides(LDHs)are a family of brucite-like layered hydroxides that consist of di-and trivalent metal ions.Due to their unique two-dimensional layered nanostructures,high positive structural charge density and strong ion exchange capacity,LDHs have great potential in developing new kind of adsorption materials.However,LDHs tend to aggregates irregularly and hardly re-disperses in aqueous solution.At the same time,owing to the strong hydrophilicity,the affinity of LDHs is poor to the hydrophobic organic pollutants and they preferentially attract polar water molecules to their surface.In addition,the difficulty in solid-liquid separation and recycling also limit their practical application.In the exploration of adsorption mechanism,the relevant literature is based on the qualitative interpretation of the results of macroscopic experimental,and the systematic theoretical analysis of ion exchange reaction and hydrogen bonding interaction at the molecular level are lack.In this research,based on the comprehensive analysis of numerous literatures,different preparation methods were employed to construct LDHs hybrid materials with unique nanostructures in order to improve the water dispersibility and increase the specific surface.As a result,the adsorption performance of LDHs for heavy metals and phenols was enhance.Moreover,the LDHs were fabricated with carbon nanomaterials and magnetic nanoparticles to enhance the oil-water separation performance through the synergy and compatibility between the components.In this paper,the morphological structure and surface properties of the LDHs-based hybrid composites were investigated by various analytical characterization techniques.The adsorption performance of such hybrid composites for removal of hazardous pollutants,such as hexavalent chromium(Cr(Ⅵ)).2,4-dichlorophenol(2.4-DCP).and different types of emulsified oils were studied.The adsorption mechanism was explored by analyzing the results of macroscopic experiments and microscopic theoretical calculations.The research results of this work can provide theoretical guidance in developing and applicating of LDHs-based adsorption materials.The main research works are listed as follows(1)Direct delamination of lactate-intercalated LDHs into single layer LDHs nanosheet in aqueous solutions,which termed LDHNSs.The obtained LDHNSs had better dispersibility,more adsorption sites,larger contact areas,lower diffusion resistance than conventional LDHs.Therefore,the LDHNSs exhibited excellent adsorption properties for Cr(VI).The experimental results shown that the time required to reach adsorption equilibrium within 8 min when the Cr(VI)initial concentration was 20-200 mg/L.LDHN S s was particularly effective for the Cr(VI)uptake under alkaline conditions.The optimal dosage was 2.0 g/L.Adsorption data were found to be better fitted by the Langmuir isotherm,and the maximum adsorption capacity of LDHNSs for Cr(VI)were 108.91 mg/g.116.47 ms/g.and 125.97 mg/g,at 288 K.298 K.and 308 K,respectively.Thermodynamic parameter indicated that the adsorption process was spontaneous and endothermic.The results of macroscopic experimental and density functional theory proved that the adsorption mechanism were electrostatic attraction,ion exchange,and hydrogen bonding(2)The hollow nanoshell of layered double oxides(HNLDOs)was synthesized using sacrificial template method.The HNLDOs exhibited superior adsorption capacity for 2.4-DCP due to its strong dispersibility,rich porosity,high surface area,and good permeability.The experimental results shown that the maximum adsorption capacity of 2.4-DCP onto the HNLDOs was 566.08 mg/g,490.44 mg/g,and 451.44 mg/g at 293 K,303 K,and 313 K.respectively.The adsorption of 2.4-DCP onto HNLDOs reached equilibrium within 480 min.when the initial 2.4-DCP concentration range from 5-300 mg/L.The optimal pH for 2,4-DCP removal was achieved at 7.0.The adsorption process could be better described by Langmuir isotherm model.Thermodynamic parameter shown that the adsorption process was spontaneous and exothermic.Molecular dynamics simulation and density functional theory analysis implied that the adsorption of 2,4-DCP onto the HNLDOs was mainly dominated by chemisorption,which was associated with the "memory effect" of LDOs.In addition,some hydrogen bonds formed between 2,4-DCP and the LDHs host layer.The adsorption-desorption experiments shown that the HNLDOs had good recyclability,which made it a promising adsorbent that possesses great potential for treating chlorophenol wastewater.(3)Using GO as the substrate,the MGO/LDHs composites with different LDHs:MGO mass ratio were prepared by a two-pot reaction.The composites exhibited excellent oil-water separation performance due to the synergism of the individual components.The oil removal experiment indicated that the maximum oil removal efficiency for the crude O/W emulsion was achieved at mass ratio of LDHs/MGO=1:1.and the maximum oil removal efficiencies for the white oil and decane O/W emulsions were achieved at mass ratio of LDHs/MGO=1:3.Under the suitable dosage and mass ratio of LDHs to MGO,99%of emulsified oil could be removed by MGO/LDHs composites.Moreover,experimental data were best fitted to pseudo-second-order kinetic equation.The emulsified oil removal efficiency was decreased with the increasing temperature and increased with the increasing ionic strength.The results of dynamic interfacial tension and optical microscope images confirmed that the adsorption of emulsified oil droplets by MGO/LDHs composites involved two processes:(1)the composites adsorbed on the oil/water interface and then interacted with the SDBS;(2)the composites wrapped with oil droplets were attracted and collected by an external magnetic field.In addition,MGO/MgAl-LDHs composites could maintain good oil removal efficiency after reusing over four cycles.(4)The MCNTs/LDHs composites with different LDHs:MCNTs mass ratios were fabricated by a two-pot reaction,which using carboxylated carbon nanotubes(CNTs)as substrate.Due to the presence of carboxylated CNTs,the composites had a certain interfacial activity.The presence of LDHs makes the composites had a high positive charge density,which was favorable for the contact of composites with emulsified oil droplets.Therefore,the composites had superior oil-water separation performance.The oil removal experiment indicated that the maximum oil removal efficiency for the crude oil-in-water emulsion was achieved at a mass ratio of LDHs/MCNTs =1:1.and the maximum oil removal efficiencies for the paraffin oil and hexadecane oil-in-water emulsions were achieved at a mass ratio of LDHs/MCNTs =1:5.Under the suitable dosage and mass ratio of LDHs to MCNTs.99%of emulsified oil could be removed by MCNTs/LDHs composites.The adsorption of emulsified oil droplets onto composites was endothermic reaction.The initial pH of emulsion did not have much influence on the oil-water separation process.The oil removal efficiency was increased with increasing ionic strength.According to the characterization of dynamic interfacial tension and the optical microscope images,the real oil-water separation process involves interfacial covering.interfacial adsorption,and magnetic separation.The composites had good cyclic oil removal performance after reusing over five cycles.which indicating strong application potential of such carbon-based nanomaterials/LDHs composites in real oily wastewater treatment. |
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