Study On Preparation Of Separation Membrane Using Columnar MOF Material And Its Adsorption Of Iodine From Aqueous Solution

There will be a large amount of radioactive iodide ions in the medical wastewater produced by radionuclide treatment.These radioactive iodide ions will cause serious damage to the ecological environment with the flow of water.The radionuclide of iodine is volatile.Although the concentration of radionuclides is relatively low compared with other components in wastewater,the half-life period(t_1/2)is relatively long.For example,¹²⁵I t_1/2is 60.14 days,¹²⁹I t_1/2is 1.57×10⁷years.The radionuclide of iodine needs to be captured and stored in time in the process of decay.There is an urgent need to develop an efficient method that can capture it in a short time and storage it safely with a minimum volume.However,it will be challenging.As a new type of porous material,Metal Organic Frameworks（MOFs）have been widely used for the capture of radioactive iodine in gaseous and solution by taking advantage of their large specific surface area and adjustable pore size.This article attempts to use a new column-layered double-wall MOF（{[Zn₃（DL-lac）₂（pybz）₂]·2.5DMF}_n,abbreviated as lac-Zn）as the membrane material,and make it into an adsorption membrane and use it to enrich the iodide ions in the aqueous solution.Lac-Zn is a rigid metal organic column formed by the left and right crossed of metallic zinc and flexible lactic acid.It is an emerging double-wall MOF material formed by connecting the organic group 4-（4-pyridyl）benzoic acid（Hpybz）to the double wall.Due to the interaction between iodine andπ-electron aromatic pores and the existence of special regular one-dimensional pores,lac-Zn shows excellent affinity for iodine.In addition,the first case that MOF adsorption membrane adsorbs iodine anions has already been recorded.In fact,during the permeation process in which the aqueous solution of iodine permeated the lac-Zn membrane,the iodine anions are adsorbed into the internal pores of the lac-Zn crystal by physical adsorption,and thus remains in the lac-Zn polycrystalline layer.The water permeates through the lac-Zn membrane.Here,the lac-Zn membrane is equivalent to a short and thick adsorption bed,which is conducive to obtaining a high flow rate under the same pressure drop,thereby it can increase the separation speed and output.Compared with conventional separation membranes,the membrane pore size of lac-Zn membrane is larger and the specific adsorption can also promote the high adsorption of iodine.In addition,the shorter penetration and diffusion paths,fast mass transfer and shorter separation time in the lac-Zn membrane also help to Promote the adsorption of iodine.In this paper,the secondary growth method is used to prepare lac-Zn polycrystalline membrane on the surface of the polytetrafluoroethylene carrier.The effects of the p H value of the solution,the coexisting anions in the solution and the flow rate of the solution in dynamic adsorption on the adsorption performance of lac-Zn membranes for iodine ions were studied.In order to improve the hydrolysis resistance of the lac-Zn membrane,we modified the lac-Zn membrane with pyrrole through a strategy of oxidation reaction,and studied the performance of the modified lac-Zn membrane about the adsorption of iodine ions in water.The specific research content mainly includes the following four aspects:（1）The lac-Zn polycrystalline membrane is prepared by the secondary growth method.The influence of the size of the seed crystal and the method of seed crystal deposition on the continuous density of the seed membrane is studied;the influence of the suction filtration method,the spin coating method and the synergistic action of the two methods on the continuous density of the seed crystal membrane is also studied.The focus is on the influence of concentration,volume and solution deposition sequence of seed crystal for the continuous of the seed crystal membrane.A continuous and dense micron-sized lac-Zn polycrystalline membrane is obtained by the secondary growth method of seed crystals.（2）The dynamic adsorption performance of lac-Zn membrane for iodide ions in aqueous solution is studied.The influence of experimental factors such as the p H value of the solution,the coexisting anions in the solution,and the flow rate of the solution in dynamic adsorption on the performance of the lac-Zn membrane in adsorbing iodide ions in the aqueous solution is mainly studied.When the p H value of the iodine aqueous solution is 7,the iodine ion removal rate of the lac-Zn membrane is 92.89%in the 6th min.The physical and chemical properties of the lac-Zn membrane before and after adsorption were also analyzed by PXRD,ATR-FTIR and XPS,and the chemical valence of iodide ions on the adsorbed lac-Zn membrane was studied by XPS.Explain the adsorption mechanism of lac-Zn membrane.（3）The modification of lac-Zn membrane with pyrrole in situ.Polypyrrole is a hydrophobic polymer material.There is a nitrogen element containing a lone pair of electrons on the aromatic ring of the pyrrole which can be doped with iodine to form a charge transfer complex,and helps to trap iodine.The lac-Zn membrane is used to adsorb the pyrrole to load the pyrrole on the surface of the polycrystalline membrane,and then the lac-Zn membrane loaded with the pyrrole is placed in a ferric chloride ethanol solution for in-situ oxidation reaction.And the lac-Zn membrane before and after the reaction is characterized by PXRD,Raman and FE-SEM spectra.Polypyrrole is formed on the surface of the lac-Zn membrane after chemical oxidation polymerization.After the lac-Zn membrane is immersed in water for 12 hours,the lac-Zn crystals on the surface began to hydrolyze,while the lac-Zn crystal structure on the surface of the modified lac-Zn membrane remained intact after being immersed in water for a week.In the static adsorption experiment,the saturated adsorption capacity of iodide ions on the modified lac-Zn membrane is about 397 mg/g,which is higher than the saturated adsorption capacity of the lac-Zn membrane before the modification of 183.49 mg/g,but it may still result in the disintegration of lac-Zn framework on the modified membrane when it immerses in the water system for a long time.（4）This paper also explored the dynamic adsorption performance of the modified lac-Zn membrane for iodine ions,and the effect of the p H value of the solution on the performance is studied.When the p H value of the iodine aqueous solution is 7,the iodine ion removal rate of the modified lac-Zn membrane in the 6th min can reach 93.66%,which is higher than the iodine ion removal rate of the lac-Zn membrane before modification;The long-term stability study of the iodine water adsorption of the Zn membrane.According to the research,under the same conditions,the saturation time of the modified lac-Zn membrane is 26h in iodine water.The saturation time of the modified lac-Zn membrane is longer than that without modification,which is 14.5h.It proves that its resistance to hydrolysis has been improved.The modified lac-Zn membrane after adsorption is characterized by PXRD,Raman and XPS,then the physical and chemical nature of the lac-Zn membrane is analyzed before and after adsorption.By the method of XPS,the chemical valence state of iodide ions adsorbed on the modified lac-Zn membrane is studied to prove the adsorption mechanism of the modified Lac-Zn membrane.