Preparation Of Polypropylene Nonwoven Anion Exchange Membrane And Its Application To Electrodeionization

With the increasing attention paid to environmental protection and energy in today’s society,ion exchange membrane has become a research hotspot because of its unique selective permeability and excellent performance in material separation,purification and energy storage.Electrodeionization(EDI)is a green environmental protection technology for removing metal ions and inorganic anion ions from wastewaters,which will be widely used in the field of wastewater treatment.In this thesis,two kinds of modified polypropylene nonwoven anion exchange membranes were made,and the chloride anions in wastewater were concentrated and separated with EDI device filled with the membranes.This is of great significance to promote the preparation and application of modified polypropylene nonwoven anion exchange membranes.First of all,polypropylene nonwoven fabric was used as matrix,methacryloyl oxyethyl trimethyl ammonium chloride(DMC)as a main monomer,and the polystyrene nanoparticles modified by PDMC as an additive(Nano-CPS),grafting modification and dosing of the fabric were implemented under ultraviolet(UV)irradiation.Another anion exchange membrane was made by thermal grafting polymerization with methacrylamido propyl trimethyl ammonium chloride(MAPTAC),and then bismuth subcarbonate was synthesized in situ on the grafted polypropylene and sequentially coated as well.After grafting and loading treatments of polypropylene nonwovens,the waterproofing treatment was adopted.Finally,the nano-CPS loaded and grafted polypropylene nonwoven anion exchange membrane was applied to EDI device to purify anion wastewaters.The findings and conclusions are drawn as follows:(1)The grafting rate of polypropylene nonwoven anion exchange membrane increased with the increase of nano-CPS content at the constant concentration of DMC.The maximum grafting rate was 98.8%at the content of nano-CPS emulsion of 50%,while that was 74.6%in the absence of nano-CPS.(2)The results of SEM and infrared spectra showed that the grafting of DMC and loading of nano-CPS are successfully achieved on the polypropylene nonwoven fabric.DMC was essentially grafted onto polypropylene macromolecular chains as well as the inner and outer surfaces of nonwoven fabric.The nano-CPS made the grafted fabric structure more uniform and dense,which will help to inhibit the transmembrane water migration.(3)The water contact angle,the areal membrane resistance,and the tensile strength at breakage of the grafted nonwoven ion exchange membranes decreased while the ion exchange capacities of the grafted nonwoven increased at the increased dosage of nano-CPS.This is essentially determined by the hydrophilicity of nano-CPS nanoparticles.(4)The grafting rate of polypropylene anion exchange membrane first increased and then decreased with the increase of the content of crosslinker N,N-methylene bisacrylamide(MBAM)at the constant concentrations of both monomer and initiator,with maximum grafting rate of maptac of 94.4%at the MBAM concentration of ca.2.8%.The grafting rate increased with the increase of ammonium persulfate(APS)while the total content of two initiators was fixed.The grafting rate of MAPTAC was the largest 89.5%with the ratio of APS to dibenzoyl peroxide(BPO)at 4:1.(5)The results of SEM,infrared spectra and X-ray diffraction showed that MAPTAC has been grafted onto the polypropylene macromolecular chains,and bismuth subcarbonate has been successfully attached onto the polypropylene nonwoven.(6)The ion exchange results verified that bismuth subcarbonate possessed ion exchange capacity to carbonate,hydroxyl and chloride ions.With the increase of basic bismuth subcarbonate content,the water contact angle and the hydrophobicity of the grafted and loaded polypropylene ion exchange membrane increased(completely contrary to nano-CPS).The ion exchange capacity of the resultant membrane increased first and then decreased,while the areal surface resistance decreased first and then increased.(7)Also with the increase of basic bismuth subcarbonate content,the water uptake of the resultant membrane first decreased and then increased,while the mechanical properties of the resultant membrane first increased and then decreased.The dosage of bismuth subcarbonate should not exceed 0.8-1.0g.The reason for the turning point of the mentioned above properties and performances is that the excessive dosage leads to the deficiency of the adhesion and coating of quaternary ammonium resin,and the actual loadings of bismuth subcarbonate in the membranes are diminished.In addition,both bismuth subcarbonate and nano-CPS have anion exchange capacity,but they have the opposite effect on the hydrophilicity and mechanical properties of ion exchange membrane.Bismuth subcarbonate is better for the modification of ion exchange membrane.(8)EDI results showed that,after 29-hour operation,the chloride ion exchanged by the lab-made homogeneous membrane was 25.0 ppm and that of commercial heterogeneous membrane was 17.7 ppm.The result indicates better separation performance of lab-made membrane in comparison to that of commercial heterogeneous membrane.