Preparation Of Positively Charged Crosslinking PVC Ultrafiltration Membranes And Their Performance For Salt/Dye Mixtures Separation

To explore a simple and effective route for preparing modified polyvinyl chloride（PVC）ultrafiltration（UF）membranes with novel separation property,a series of positively charged crosslinking PVC UF membranes were prepared by alkylation of amino units such as-NH₂ or-NH on multi amines with the active C-Cl units on PVC in this paper.In order to prompt multi amines to react with PVC,immersion method,surface deposition and blending method were investigated respectively.The rejection of these prepared membranes to positively charged particles were studied.Firstly,the positively charged PVC based UF membrane was prepared by immersing the PVC hollow fiber membrane into aqueous solution of tetraethylenepentamine（TEPA）,and then heating for the crosslinking between PVC and TEPA.The isoelectric point of the prepared PVC-TEPA membrane was at about p H=9.3,which showed typical characteristics of positively charged membrane.In the filtration process of this membrane,two separation mechanisms including steric hindrance and Donnan exclusion contributed to the rejections of the anionic dye Congo red with larger size and the cationic dye Vitoria blue B with smaller size.The rejections of two dyes were both as high as 99%.The dense layer structure throughout the whole membrane formed by crosslinking had a significant enhancement effect on the mechanical properties of the membrane.Secondly,polyethyleneimine（PEI）was deposited on the outer surface of PVC UF membrane by filtering PEI solution,according to the ultrafiltration characteristics of PVC membrane.And then the positively charged PVC UF membrane was prepared by heat treatment to promote the crosslink between PVC and PEI.This membrane also had the capacity of rejecting organic dyes,which result from the positively charged characteristics.After secondary crosslinked by glutaraldehyde to fix PEI and to reduce the pore size,the rejection of the membrane to 1000 ppm Ca Cl₂ at 0.3MPa rose to 37%,while the flux was stabilized at about 30L/m²·h.Finally,a novel in-situ polymerization polymerization routed was proposed.The casting solution of PVC and PEI in dimethylacetamide（DMAC）was prepared at low temperature to inhibit crosslinking,then liquid film was casted and heated to promote crosslinking.The positively charged PVC/PEI UF membrane was obtained via a typical NIPS process by immersing the crosslinked liquid film into water bath.Following this operation,the crosslinking degree of PVC/PEI membrane was more than 60%,and PEI was retained stably in the PVC/PEI membrane and made the membrane positively charged.Basing on two separation mechanisms of steric hindrance and Donnan exclusion,the rejection of the membrane to the cationic dye Vitoria blue B reached over 96%.Together with the low rejection to inorganic salts,and high flux of 100L/m²·h at 0.1 MPa,the prepared PVC/PEI UF membrane exhibited both the higher flux and the higher rejection in the separation of dye/salt mixtures in aqueous solutions.The above results revealed that the PVC based membrane could be stablly positively charged via the chemical crosslinking between C-Cl in PVC and amino contained reageants,which would be great potential for preparation of positively charged PVC membranes.Basing on the positive characteristics,the prepared membrane should be the new candidates for the separation of dye/salt mixtures with the higher flux and rejection.