Preparation Of Low-dimensional Carbon-based Eletrochemically Switched Ion Permselectivity Membrane And Separation Of Heavy Metal Ion

Electrochemically switched ion permselectivity（ESIP）is a novel effective and environmentally benign ion separation technique based on electrochemically switched ion exchange（ESIX）method.ESIX,via reversible electrochemical regulating redox states of electroactive materials to control ion uptake and release processes,has realized selective removal of target ions.However,to realize continuous operation of the ESIX process,the film electrodes need to be exchanged frequently between waste and regeneration liquids,and the operation process was cumbersome.Thus,our group developed a unique ESIP system using low-dimensional carbon material as the ESIP membrane firstly,investigated the mechanism of action（electric double layer–ESIP membrane）of carbon material as electroactive ion exchange membrane,designed a pump type potential-electric field coupling ion transport system with high flux.To realize the continuous,efficient and selective separation of heavy metal ions,this paper explored a simple and effective,economical and applicable and energy-efficient and environmental electrochemically switched membrane separation system.In the ESIP system,the development of ESIP membrane with excellent permselectivity is the key issue.These membrane materials include inorganic metal hexacyanoferrates,conductive polymer and hybrid material.However,there are still some problems using the traditional ESIX films（metal hexacyanoferrates and conductive polymer）for the ESIP system.Such as,inorganic metal hexacyanoferrates have poor conductivity and weak adhesion to conductive substrates.Conductive polymer can induce volume swelling effect in the frequent oxidation/reduction pulse process,which is not conducive to the precise control of ion transfer channel of polymer.Therefore,a novel low-dimensional carbon materials represented by carbon nanotubes,graphene and carbon quantum dots,due to their high conductivity,corrosion resistance,high specific surface area,excellent electrochemical stability,non-toxicity,wide raw material source,and the electric double layer charge/discharge process produced by the regulation and control of oxidation/reduction potential with rapid and reversible ion adsorption/desorption characteristics,make the low dimensional carbon materials to be promising ESIP membranes.This investigation improves active adsorption sites of carbon material by grafting functional groups or constructing nanocomposite,and provides fast ion transfer channels by combining three-dimensional（3D）porous structure,which has shown efficient removal properties for heavy metal ions.The combination of these low dimensional carbon materials and ESIX function material significantly improves the separation performance of electroactive membranes for heavy metal ions and has the practical value.The main results are as follows:（1）The（3D）porousMWCNT/PTFEmembranewasfabricatedon polytetrafluoroethylene（PTFE）by pressure filtering technique,and the membrane was used in the ESIP system for the selective separation of Pb²⁺.According to the carbon-based membrane,a novel electric double layer-ESIP membrane（EDL-ESIPM）mechanism was proposed,and the ESIP behavior of Pb²⁺in the MWCNT/PTFE membrane by modulating the positive and negative charge densities on the EDL of the membrane together with the application of an external electric field was investigated.The results showed that the MWCNT/PTFE membrane had higher permselectivity for Pb²⁺.The highest Pb²⁺flux across the membrane was 0.718 g m^-22 h^-11 occurred under the optimization operation condition.The system reduced the Pb²⁺concentration in the solution from 30 to 0.36 ppm with a current efficiency of 46.1%.In addition,the MWCNT/PTFE membrane has good reproducibility,stability and practical application potential,but its ion flux is still to be improved.（2）In order to realize the high flux separation of Pb²⁺,a 3D polypyrrole（PPy）@graphene oxide（GO）composite membrane was fabricated via one-step electrochemical co-deposition method.The composite membrane made full use of the double layer performance of GO,and the pseudo capacitance effect of the PPy provided high driving force of the ion transport in the membrane.In this paper,a novel ion pump system was designed according to the coupling effect of cell voltage and double pulse potential,which enhanced the high flux separation of Pb²⁺.Numerical results revealed that the highest Pb²⁺flux was 4.70 g m^-22 h^-1.In this ion separation process,the removal percentage of Pb²⁺and current efficiency were 96%and51.9%,respectively.（3）The 3D branched poly（ethylenimine）（BPEI）-functionalized carbon quantum dots（BPEI-CQD）/polypyrrole（PPy）/polystyrenesulfonate（PSS）composite membrane was fabricated on polytetrafluoroethylene（PTFE）membrane by pressure filtering technique,and the membrane was used in the ESIP system for the selective separation of Cu²⁺.In addition,the selectivity of BPEI-CQD/PPy/PSS membrane was studied based on the ESIP system with adsorption-diffusion model.In the BPEI-CQD/PPy/PSS membrane,PPy/PSS showed a cation exchange property and provided extra mechanical support for BPEI-CQDs,and the functional groups on the surface of BPEI-CQDs have higher selectivity to Cu²⁺.The experimental results showed that the BPEI-CQD/PPy/PSS composite membrane has excellent selectivity and ion separation properties for Cu²⁺.In different BPEI-CQD content and membrane thickness,the permeation,adsorption and diffusion selectivity of BPEI-CQD/PPy/PSS membrane were investigated.In the ESIP test,under optimization operation condition,the highest flux and removal percentage of Pb²⁺were 1.08 g m^-22 h^-11 and 97.2%,respectively.（4）A novel 1D/2D core-shell type g-C₃N₄@MWCNT composite carbon material was synthesized by adopting MWCNTs adhesion with graphite carbon nitride（g-C₃N₄）,and the3D porous g-C₃N₄@MWCNT composite membrane was prepared through the vacuum pressure filtration technology under the action of crosslinking agent poly（vinyl alcohol）（PVA）.Based on these advantages of MWCNTs and g-C₃N₄,such as,MWCNTs have high specific surface area,excellent electrical conductivity,good stability and fast ionic charge and discharge performance,g-C₃N₄ has rich of carbon and nitrogen,easy to enrichment of the target ion,low cost and easy to synthesis,the artful combination of the two conformed to the condition of ESIP membrane.When g-C₃N₄@MWCNT membrane was used in ESIP system,the g-C₃N₄@MWCNT membrane showed high permselectivity for Zn²⁺and separation performance.In the optimal operation conditions,the maximum flux of Zn²⁺was 10 g m^-22 h^-1,and the current efficiency was 48%.