Polydopamine Electropolymerization Modified Carbon Nanotube Nanofiltration Membrane For Dye Wastewater Treatment

Dye wastewater has the characteristics of high turbidity,large chroma and high toxicity.Meanwhile,the discharge of dye wastewater in China is large and the composition is complicated,which make it an emergency to develop a high efficiency and low energy consumption dye wastewater treatment technology.Nanofiltration is one of the most important technology of dye wastewater treatment owing to its strong universality and low energy consumption.Recently,bio-inspired adhesion technology has gradually become one of the important preparation methods of nanofiltration membrane by virtue of its substrate universality,implicity and versatility.Nevertheless,there are some significant limitations for traditional bio-inspired adhesion method,e.g.time consuming,low dopamine conversion ratio,hard in PDA thickness control and low PDA layer stability and homogeneity.This research developed a electropolymerization route of PDA on membrane to overcome the bottleneck of existing chemical deposition method.This metod significantly shortens the generation time of PDA,improves the utilization rate of DA molecules,and prepares a uniform and controllablePDA layer.Firstly,we developed an electropolymerization polydopamine-carbon nanotube membrane(ePDA-CNT).A CNT membrane was prepared by vacuum filtration,acting as the conductive basis for electropolymerization.The electropolymerization of dopamine was performed on CNT membrane by cyclic voltammetry(CV)with different potential cycles in a range of-0.8 to 1.2 V.The cyclic voltammogram shows two complete shapes of phenolic hydroxyl-quinone structure conversion.Meanwhile,we study the structureactivity relationship between electropolymerization parameters and ePDA-CNT membrane.It was found that the surface morphology,chemical structure and hydrophilicity of ePDA-CNT membrane can be adjusted by controlling the number of scan cycles of CV.The ePDA-CNT membrane has good hydrophilicity with positive charges in the p H range of 4.00～9.78.The electropolymerization of dopamine on membrane can significantly shortened thePDA layer preparation time by increasing the redox reaction rate of the DA polymerization.The self-limiting nature of electropolymerization helps control the DA polymerization on the surface of the conductive material,increase the DA conversion ratio,and ensure the uniform growth of ePDA layer.Secondly,based on the metod of electropolymerization of polydopamine on CNT membrane above,we adjusted CV potential range to-0.5～+0.5V to control the the redox process,and prepared a potential-regulation polydopamine-carbon nanotube membrane(p PDA-CNT).In this specific CV potential range,only partial of the redox processes can happen.The change of scanning potential range affects the redox process and the overall reaction rate of DA polymerization in each cycle,giving p PDA a different physical structure and chemical composition from ePDA.It can tell that p PDA mainly grows along the outer wall of CNT,while ePDA spread in the CNT layer,indicating that p PDA is much denser than ePDA.Also,the p PDA layer has a large number of phenolic hydroxyls,which makes the zeta potential of the p PDA-CNT membrane much lower than that of the ePDACNT membrane at the same p H.Finally,the permeation and rejection performance of ePDA-CNT membranes and p PDA-CNT membranes were measured by using methylene blue(MB)and congo red(CR),which are two dyes with different molecular weight and charge.Also,the long-term operation stability and hydraulic flushing resistence were evaluated.The different preparation parameters cause differences in the surface morphology,chemical composition and charging properties,which in turn affect the membranes’ permeation,dye rejection rate,long-term operation stability and hydraulic flushing resistence.Under the influence of the Donan effect,the ePDA-CNT membrane shows better rejection ration of cationic dyes and better resistance to cationic dye fouling,while the p PDA-CNT membrane is more conducive to the rejection of anionic dyes and resistance to anionic dyes fouling.Also,the differencea in chemical composition make the ePDA-CNT membranes easier to exfoliate under rapid hydraulic flushing.