Co-deposition Of Polydopamine-Polyethyleneimine For Monovalent Selective Cation Exchange Membrane Fabrication

The preparation and application of high-performance monovalent selective cation exchange membrane can greatly promote the promotion of electrodialysis technology in the fields of seawater desalination,refined salt,and lithium extraction from concentrated seawater.The development of biomimetic adhesive technology represented by polydopamine（PDA）has expanded new directions for membrane surface modification.The use of polyethyleneimine（PEI）can accelerate the rapid deposition of PDA on the membrane surface,and the formed co-deposit modified layer is beneficial to improve the separation performance of the membrane to a certain extent.In this paper,in order to improve the practical application performance of monovalent selective cation exchange membranes,Fe³⁺was introduced to induce PDA and PEI to achieve rapid deposition on the surface of common cation exchange membranes.On the basis of this method,the modification process was further enhanced by DC electric field.A high-performance monovalent selective cation-exchange membrane was fabricated by Fe³⁺induced rapid co-deposition of PDA-PEI.The prepared membranes were characterized by means of UV spectroscopy,scanning electron microscopy,EDX elemental analysis and Zeta potential,and the regulation mechanism of Fe³⁺concentration on the surface morphology,chargeability and selectivity of PDA-PEI modified membranes was analyzed.The results show that the oxidation and complexation of Fe³⁺can accelerate the polymerization of dopamine（DA）,further affect the co-deposition of PDA-PEI,and shorten the modification time.The surface properties and separation performance of the modified membranes can be affected by changing both Fe³⁺concentration and PEI molecular weight.When the Fe³⁺concentration was 0.0001 mol/L and PEI molecular weight was 70000 Da,the modified membrane prepared by deposition for 30 min had the Na⁺/Mg²⁺selectivity of 8.3,and low electrical membrane resistance and good stability.Based on the chargeability of PDA and PEI,the Fe³⁺induced PDA-PEI co-deposition process was enhanced by electric field,and the modification time was further shortened.A fairly high-selectivity composite membrane was obtained.The prepared membranes were characterized by means of scanning electron microscopy,elemental analysis,Zeta potential and water contact angle.The effects of electric field direction,Fe³⁺concentration and mass ratio of DA and PEI in the modified solution on the surface and separation performance of the modified membranes were investigated.The results show that when the modified layer faces the electric field cathode,the prepared modified membrane has higher selectivity and only a slight increase in membrane resistance due to the combined effect of the electric field and Fe³⁺on the DA polymerization process.When the concentration of Fe³⁺was 0.001mol/L and the mass ratio of DA:PEI was 1:1,the modified membrane prepared by electric field strengthening just for 5 min had the Na⁺/Mg²⁺selectivity as high as 12.8,and it also showed an excellent stability.