Study On Modification Of Polyamide Reverse Osmosis Membrane To Improve The Renewable Chlorine-resistance

Polyamide(PA)reverse osmosis(RO)membranes have high efficiency,low energy consumption and excellent separation performance,and have been extensively used in(waste)water treatment.In order to control membrane bio-fouling and inhibit reproduction of bacteria,continuous/intermittent chlorination treatment processes using active chlorine chemicals are in vogue.However,the residual chlorine will attack the top PA active layer of RO membranes,which results in the degradation of PA membrane integrity and the deterioration of performance and operation life eventually.Therefore,the development of PA RO membranes with chlorine resistance is meaningful for the long-term,stable and efficient operation of reverse osmosis.Novel material exploitation with intrinsic chlorine-resistance suffers from a long product design period and difficulty in marketing,therefore failing to keep pace with the needs of industry and academia.Modification of membrane has become a hotspot due to its strong designable ability.However,there still exist two main drawbacks in existing research.Firstly,modified membrane always suffers from deterioration in membrane flux due to increased osmotic resistance.Secondly,most of the modified materials are lack of reversible chlorine-resistance ability.Based on the above reasons,this paper takes steps to modify polyamide membrane through three methods including nano-PEDOT incorporation,UBOB-grafting and PANI electrostatic self-deposition.It aims to improve the membrane's chlorine-resistance on the premise of maintaining excellent membrane performance and endow the membrane with a renewable anti-chlorine ability,allowing the chlorinated membranes to be recovered to the original state by reductive treatment.The contents and conclusions of this paper are as follows:(1)Nano-PEDOT incorporated into active layer to improve chlorine-resistance.PEDOT nanoparticles synthesized via reverse micelles method appear to be particle-shaped,which have good dispersion both in water and in organic solution.The diameter of nanoparticles is between 50-100 nm,which is smaller than that of the polyamide layer.Nano-Mixed matrix membranes are prepared by dispersing PEDOT nanoparticles into reaction solution during interfacial polymerization.The results show that modified membrane maintains good separation performance due to the enhanced hydrophilicity.PEDOT has relatively low oxidation-reduction potentials because of its 3 and 4 site are connected with oxygen.Therefore,active chlorine prefers to undergo redox reaction with nanoparticles doped in the membrane instead of damaging the polyamide separation layer.After being exposed in 5000 ppm·h hypochlorite,the rejection of modified membrane still maintains 99.2%of the original flux,compared with that of bare membrane dropping to 96.2%.The results of chlorine exposure experiments reveal that the nano-PEDOT incorporation improves the chlorine resistance of the polyamide membranes.(2)UBOB grafted polyamide reverse osmosis membrane to generate renewable chlorine-resistance:UBOB dissolved in ethyl alcohol is successfully grafted by using residual acyl chloride groups on the surface of pristine membrane as reaction sites.Taking water contact angle,Zeta potential and membrane performance as assessment index,the optimal UBOB concentration condition for grafting is 1.5 wt%.At this point,water contact angle of the membrane surface increases from 63.9° to 79.1° and Zeta potential goes up from-49.8 mV to+11.0 mV.Although the flux of membrane slightly decreases,the rejection starts to rise.In view of the reversible redox reaction between UBOB and chlorine,this work designs a "chlorination-reduction" cycle experiment.The normalized rejection of modified membrane retains at 99.3%versus 96.0%of bare membrane after three cycles.The results prove that the chlorine-resistance of modified membrane can be recovered.(3)Polyaniline self-deposition modified polyamide membrane with renewable chlorine-resistance:The oxidation of aniline hydrochloride with ammonium peroxydisulfate proceeding on the membrane generates aniline cationic.The subsequent oxidative polymerization of aniline forms PANI coating and spontaneously deposits itself on the surface of commercially negative-charged XLE membrane via electrostatically interaction.The molecular structure of modified membrane is conveniently assessed by Fourier transform IR(FTIR)spectroscopy.The bands at 1378 cm-1 and 1590 cm-1 attribute to the presence of PANI.PANI coating reduces flux but improves chlorine-resistance obviously.After double reduction treatments and tertiary chlorination,the normalized rejection of modified membrane remains 101%unchanged,while the unmodified XLE membrane drops to 96.7%.By comparing the variation of separation performance of membrane combined with the characteristic peaks of PANI,it is confirmed that the deteriorated performance caused by chlorination can be recovered by subsequent Na2S2O3 reduction.In summary,this paper adopts three different modification strategies to improve the membrane's chlorine-resistance without compromising separation performance.The polymers with low oxidation-reduction potential are selected in order to preferentially react with active chlorine,and realize the regeneration of chlorine-resistance membrane in virtue of their reversible redox capacity.