Application And Mechanism Of Anion Effect Enhancing Separation Efficiency Of Polymer Inclusion Membrane Electrodialysis

At present,metallurgy,special steel smelting and yellow phosphorus production process will produce a large number of vanadium-chromium waste slag.The accumulation of vanadium-chromium waste slag over a long period of time will cause serious heavy metal pollution to the nearby soil,rivers and groundwater,seriously endangering the ecological environment.In turn,vanadium and chromium are strategic metals in China and have a very important role in a number of fields.Therefore,in view of the high hazard and high value of vanadium-chromium slag,the problem of resource recycling of vanadium-chromium slag needs to be solved.In addition,since nitrate is readily absorbed and excreted by living organisms,and in an oxygen-poor environment such as in the digestive tract,nitrate is reduced to the toxic nitrite,which oxidizes cells so that they lose the ability to transport oxygen.Nitrite can also react with secondary amine organic compounds to produce carcinogenic nitrosamines.Therefore,efficient and highly selective separation and removal of nitrates from water is necessary.In this paper,the polymer inclusion membrane electrodialysis（PIMED）system is applied to the separation of vanadium and chromium metals and other anions for the first time,and a series of studies are conducted to improve the efficiency of the PIMED system for the separation of vanadium and chromium anions by using the anion effect,and to explain the various competitive adsorption behaviors occurring between ions and the liquid/membrane interface from the perspective of the interface between the feed liquid and the polymer inclusion membrane（PIM）.The mechanism of ion-membrane interaction was analyzed from the microscopic perspectives of hydrophobicity,molecular structure,molecular polarity,and charge center atomic electronegativity,and then the separation efficiency of PIMED was further enhanced by relying on the precise modulation of the anion effect to achieve the deep separation of vanadium and chromium and other anions.This study not only provides a new technological process for the separation of various ions,but also provides a reference for explaining the mechanism of membrane separation process in the future.The main conclusions of this paper are as follows:Firstly,PIMs containing six different ionic liquids were prepared,and a total of three membranes,PVDF-HFP/Cyphos IL 101,PVDF-HFP/Aliquat 336,and PVDF-HFP/Aliquat 336[NO₃],were selected for experimental use and characterized.SEM-EDS results showed that the three PIMs were dense and uniform,with flat surfaces,no obvious pits and protrusions,no cracks,and no penetration holes.AFM images show that the surface roughness of PVDF-HFP/Cyphos IL101 membrane（127nm）is slightly lower than the other two membranes（178 nm,173 nm）.By measuring the contact angle of PIMs,it was found that all of them have good hydrophilicity.FT-IR images showed that the carrier and substrate were mixed well to form a membrane.Testing the membrane resistance of PIMs revealed that the membranes with carriers Aliquat 336,Aliquat 336[NO₃],had lower membrane resistance than the Cyphos IL 101 carrier membrane（33.6±1.2Ω·cm²）.5.9±0.9Ω·cm²and 6.1±0.8Ω·cm²,respectively.The mechanism of interfacial competition behavior of the anion effect on the separation of Cl^-,NO₃^-process was investigated by the constructed PIMED system.The performance of PIMs and four commercial membranes,ACS,AMX,ASVN and AMVN,in the separation process of Cl^-,NO₃^-were compared.The EIS and LSV characterization results showed that the impedance and linear resistance of all membranes were greater for the transport of NO₃^-than for the transport of Cl^-.In addition,the PVDF-HFP/Cyphos IL101 membrane was the best for the separation of Cl^-and NO₃^-,with a separation coefficientβ（NO₃-/Cl-）of 650 for both anions,and the Cyphos IL101 membrane was also very effective for the separation of Cl^-and SCN^-,with a separation coefficientβ（SCN-/Cl-）of 12644.5 for both ions.was even 12644.5.The comprehensive analysis found that the affinity of different anions for the liquid/membrane interface differed greatly,and the affinity magnitudes of ions and membranes obeyed the order of SCN^->NO₃^->Cl^-,which was very consistent with the Hofmeister sequence.The Hofmeister sequence in the competitive behavior of ions at the liquid/membrane interface was explained from the perspectives of electrochemical characterization,hydrophobicity,molecular structure,molecular polarity,and charge center atomic electronegativity of ions or membranes.This not only provides a deep separation of NO₃^-from Cl^-and SCN^-from Cl^-in the PIMED system using the anion effect,but also provides a theoretical guide for the subsequent understanding of the mechanism of the competition behavior of anions and vanadium-chromium metal ions at the liquid/membrane interface.In the previous study,it was found that the competitive adsorption interaction between the anion and the liquid/membrane interface is consistent with the Hofmeister sequence,and the main work of this chapter is to use the previous research results to precisely modulate the magnitude of the anion effect to enhance the selective separation of chromium from chromium-vanadium alkaline solutions by PIMED,and to optimize the relevant experimental parameters to achieve the best separation effect.It was found that the separation of the two metal ions was not favored by adding either high or low concentrations of Na Cl to the feed solution,and the best Cr（VI）extraction（96.4%）and reverse extraction（94.7%）were exhibited when 0.4 mol L^-1Na Cl was added to the feed solution and the experiments were performed using PVDF-HFP/Aliquat 336 membranes,while the V（V）（95.5%）and Cr（VI）（81.6%）were also higher than the other two PIMs,respectively,with a separation coefficientβ（Cr/V）of 93.6.Its separation coefficientβ（Cr/V）reached 93.6.Optimizing the parameters,the optimal separation efficiencyβ_（Cr/V）=314.6 was obtained.in addition,the increase of temperature also improves the metal ion transport efficiency.