Calcined Products Of Mg-Al LDH/SWCNT Nanocomposites For Expeditious Removal Of Phenol And 4-chlorophenol From Aqueous Solutions

With the rapid development of the chemical and petrochemical industries,a lot of phenolic pollutants are produced every day.Phenolic wastewater has a bad impact on the environment.Phenols are classified as priority pollutants due to their high toxicity,carcinogenicity and refractory properties.As the production and use of phenolic compounds continue to increase,the discharge of organic wastewater containing phenolic pollutants rapidly increases,and the resulting environmental problems are also increasing.Therefore,the treatment of organic wastewater in recent years has attracted worldwide attention.Government regulations prohibit the direct discharge of organic wastewater containing phenolic pollutants as they cause serious and long-term environmental and ecological problems.There are many methods to remove phenolic pollutants from industrial wastewater,including physical,chemical and biological techniques,such as precipitation,flotation,membrane filtration,condensation,adsorption,oxidation,and aerobic or anaerobic treatment.Among these methods,adsorption is considered a suitable method for the removal of phenolic pollutants from wastewater due to its high efficiency,simple design,easy operation and low maintenance cost.Existing adsorbents,such as activated carbon which is the most widely used adsorbents for treating industrial wastewater due to their microporous structure and large surface area.However,activated carbon has low adsorption capacity for pollutants,and activated carbon has strict requirements on the pH range of wastewater.These issues have raised our interest in developing effective and adaptable strong adsorbents.In this work,the calcined products of layered double hydroxide/single-walled carbon nanotubes nanocomposites(CLDH/SWCNT)were prepared and their physicochemical properties were characterized.In addition,the adsorption properties of CLDH/SWCNT composites for phenol(PH)and 4-chlorophenol(4-CP)were also investigated.The main research contents are as follows:(1)CLDH/SWCNT was successfully prepared by two steps.Firstly,LDH/SWCNT composites were prepared by in-situ coprecipitation method to grow LDH in situ on the surface of SWCNT tube wall.The prepared LDH/SWCNT composites were then calcined to form calcined products of the LDH/SWCNT composites,which were defined as CLDH/SWCNT nanocomposites.(2)The micro-and macroscopic structures,physicochemical properties,and surface wettability before and after fabrication of LDH on SWCNTs were characterized by field-emission scanning electron microscopy(FE-SEM),X-ray diffractometer(XRD),Fourier transform infrared spectroscopy(FT-IR),X-ray photoelectron spectroscopy(XPS),zeta potential analyzer,nitrogen gas adsorption analyzer and contact angle analyzer.From the FE-SEM images,it can be clearly seen that LDH had been successfully compounded onto the SWCNT surface.And after calcination,the dispersibility was greatly improved.It is very interesting to note that after calcination,many new pore-like structures appeared in CLDH/SWCNT nanocomposites,which we call"pore channels".The characterization of the specific surface area and pore volume of the nitrogen adsorption analyzer showed that it is the presence of a large number of pore channels that greatly increases the specific surface area and pore volume of CLDH/SWCNT.This phenomenon is very advantageous for the excellent performance of CLDH/SWCNT as an adsorbent.(3)The effects of the composition ratio of the adsorbent,the dosage,the reaction time and the initial pH on the adsorption of phenol and 4-chlorophenol by CLDH/SWCNT were investigated.The results showed that under suitable conditions,CLDH/SWCNT as an adsorbent showed a high adsorption capacity(maximum adsorption capacity of phenol and 4-chlorophenol is 219.0 mg/g and 255.6 mg/g,respectively)and excellent removal ability of phenol and 4-chlorophenol(the removal efficiencies of phenol and 4-chlorophenol were 91.7%and 99.5%,respectively).(4)Through characterization and experiments,the interaction between adsorbent and contaminant was studied in detail.During the adsorption process,the following interactions may occur between CLDH/SWCNT and phenols:(?)hydrophobic interaction.Due to the presence of carbon nanotubes,the hydrophobicity of CLDH/SWCNT was enhanced,compared with pure LDH.Therefore,there may be a hydrophobic interaction between adsorbent and adsorbate.(?)?-?electron-donor-acceptor(EDA)interaction.It was generally believed that the strong adsorption of aromatic compounds on SWCNT was mainly due to the ?-? EDA interaction between aromatic molecules and polarizable graphene sheets of SWCNT.The strong adsorptive interaction between phenols and CLDH/SWCNT was caused by the electron-donating effect of the hydroxyl group,resulting in a ?-? EDA interaction between the adsorbates and the ?-electron-depleted regions on the graphene surfaces of CLDH/SWCNT.(?)Electrostatic interaction.It can be seen,from the characterization of the zeta potential analyzer,the surface charge of CLDH/SWCNT carry positive charges,which may enable the binding of negatively-charged phenolic anions through electrostatic interaction.(5)The cycle regeneration ability of CLDH/SWCNT was evaluated by 10 cycles of calcination-adsorption cycles.Compared with the first calcination-adsorption cycle,the removal efficiencies of phenol and 4-chlorophenol in the 10th calcination-adsorption cycle decreased by 3.7%(from 91.7%to 88.0%)and 1.0%(from 99.7%to 98.7%),respectively.This result indicated that CLDH/SWCNT has excellent recyclability as an adsorbent.The very stable 3D structure of CLDH/SWCNT and the memory effect of LDH play important roles in the ability of the adsorbent to maintain high removal efficiency in multiple calcination-adsorption cycles.