Studies On The Treatment Of Water-soluble Dyes Using Metal-organic Frameworks PCN-222

Metal-organic frameworks（MOFs）is a porous organic-inorganic hybrid material assembled from metal ions and organic ligands.Metal-organic framework materials are porous,possess larger surface area,various structure and function,high thermal stability and mechanical stability,and good photoelectric properties.PCN-222（MOF-545）is a famous three-dimensional porphyrin MOFs material with one-dimensional channel structure.The theoretical diameter of the channel can reach 3.7 nm.It is formed by the ligand bonding of the Zr6 group and the tetraxyl porphyrin molecule.PCN-222 is very stable in boiling water and even 2M hydrochloric acid,which is very rare in MOFs materials.The porphyrin center of PCN-222 can combine different kinds of transition metals,it can also do not contain any metal.PCN-222（Fe）possesses peroxidase activity,in the presence of H₂O₂,the reductive substrate molecular oxidation decomposition can be made.In recent years,water pollution has become increasingly serious.Dyestuff wastewater has great influence on environment and human health due to its high chromaticity,high toxicity and difficult degradation.Therefore,the treatment of dye wastewater has become a hot research topic.There are many methods for the treatment of dye wastewater,and the physical adsorption method is the most commonly used method for the treatment of dyes due to its simple operation and high efficiency.Activated carbon is the most commonly used adsorbent,but its biggest disadvantage is that it is difficult to regenerate.The chemical oxidation method is also a common treatment method,which uses the oxidant such as H₂O₂ to destroy the hair color group of the dye molecules,thereby achieving the purpose of removing the dye color.The chemical oxidation method can be used in combination with the physical adsorption method to effectively dispose the dye wastewater.MOFs has the advantages of ultramicro density,discrete structure,large surface area,simple synthesis and good thermal stability,which are suitable for the application of physics and chemistry.As more and more MOFs materials have been synthesized,the research of MOFs treatment of dye wastewater has also been reported.In this study,we have synthesized PCN-222 and PCN-222（Fe）,using an optical microscope,x-ray diffraction（XRD）,nitrogen adsorption/desorption measurement,Fourier transform infrared spectroscopy（FT-IR）and ultraviolet-visible absorption spectra to characterize them.The BET surface area is 2236m2/g and 2249m2/g,with the aperture size of 32? and 30? of PCN-222 and PCN-222（Fe）respectively.The Zeta potential determination of PCN-222 has been detected,the p HPZC（PZC=point of zero charge）of PCN-222 is p H8.0.When the p H of the solution is 8.0,the whole MOFs is electrically neutral.When the p H of the solution is 3.0 and 8.0,the Zeta potential of PCN-222 is 23.5 m V and-13.6 m V respectively.The general MOFs is either positively charged or negatively charged,which limits the type of absorption of the dye.From the Zeta potential of MOFs itself,PCN-222 is suitable for dye adsorption materials.In the experiment of the influence of solution p H on PCN-222 adsorption,the adsorption of methylene blue（MB）increased with the increase of p H,and when the p H was 9.0,the adsorption capacity reached a maximum of 906mg/g.Methyl orange（MO）reached a maximum adsorption capacity of 589mg/g at p H= 5.0.When the mixture of MB-MO is absorbed,the adsorption capacity increases obviously under any of the p H conditions.The maximum adsorption of MB can reach 1239mg/g,and the maximum adsorption capacity of MO can reach 1022mg/g.Thus,the dye molecules have a strong catalytic effect on each other’s adsorption.We compared PCN-222 with other MOFs to the amount of adsorption of MB and MO.It is found that most MOFs are selective to dye adsorption,and there are only 4 MOFs that can simultaneously absorb MB and MO,and the phenomenon of mutual promoting adsorption is not reported.In addition,we also found that PCN-222 has a different degree of adsorption toward malachite green（MG）,rhodamine B（Rh B）,acidic indigo（IC）,acid orange（AO）,acid magenta（AF）and fluorescein disodium（FD）,the six different anion and cation dyes.The adsorption capacity of MG is 2880mg/g.In order to make the adsorption easy to apply,we use PCN-222 powder to make a small adsorption column,and decolorize the MB,MO,MG,IC,Rh B,AF,AO,FD and MB-MO mixture.When the dye flows through the adsorption column,it is obvious that the effluent is colorless and transparent.In addition,the dye residues in MOF can be quickly cleaned out with different elution.The removal efficiency of MB and MO can still reach 95.2% and 92.7% after 8 repetitions of PCN-222.Moreover,the results of the photoscope and XRD characterization indicate that no change has occurred in the MOF structure.The PCN-222 is an excellent adsorbent.We have also discussed the adsorption dynamics,isotherm and thermodynamic models.The adsorption process conforms to the pseudo-second-order kinetics model,and the adsorption process can reach saturation over time,and there is a shared and transfer between the MOF and the dye.In addition,by fitting the isotherm,the adsorption process conforms to the Langmuir isotherm model,indicating that the adsorption is the single molecular layer adsorption,and the maximum adsorption capacity of the theory of MB and MO is 909.1mg/g and 591.7mg/g respectively.Through the study of thermodynamics,the gibbs free energy of the adsorption process can be reduced,and the adsorption process is proved to be spontaneous.The change of enthalpy and entropy are both negative,indicating that the adsorption process is the exothermic process and the chaos is reduced.The adsorption process is dominated by enthalpy.The results of Raman spectroscopy analysis show that the “π-π conjugate” function is not the main force between the dye and the MOF.Charge is the main force between them.We propose a “push-pull” model to explain the occurrence of mutual promoting adsorption.Under low p H conditions,the imidazole ring of TCPP combined with the new H+,MOF has a positive charge overall.On the contrary,under the condition of high p H,the H+ is dissociative from the imidazole ring of TCPP,and the MOF is negatively charged.MOF surface charge can be positive and negative,providing the necessary conditions for the absorption of anionic and cationic dyes.We believe that MO molecules can “push” the formation of the molecules of MB dimer,and the MB molecules on the surface of PCN-222 can be used in turn to “pull” MO molecules.And vice versa,so there will be an increase in adsorption.Finally,we have used PCN-222（Fe）as a simulated peroxidase to remove 8 dyes,and proved that PCN-222（Fe）could indeed fade the dye in the condition of adding H₂O₂.This indicates that as a kind of porphyrin MOFs,PCN-222 can not only remove dyes by physical adsorption,but also can be used as a catalyst to oxidize dyes and thoroughly destroy the dye’s chromophoric group.By studying the treatment of dye wastewater from PCN-222,we found the potential of MOFs containing porphyrin ligand in the treatment of dye wastewater,and also provided new ideas for the treatment of industrial wastewater.