Study On The Mechanism Of Oxidation Resistance Of Reverse Osmosis Membrane Modified With Peroxide

The polyamide skin of aromatic polyamide composite reverse osmosis(RO)membrane is vulnerable to active chlorine attack,which leads to the structural degradation and the decreased membrane performance.In order to solve this problem,this paper explored a method of improving both the membrane separation performance and the oxidation resistance by treating with persulfated,and made a preliminary discussion on the treating mechanism.RO membranes were soaked in potassium persulfate(K2S2O8)aqueous solution,and the following factors were observed: the temperature of the immersion solution,the concentration of K2S2O8,the soaking time,the heat treatment temperature and the heat treatment time.The results showed that the optimum modification was obtained when the soaking condition was 5 wt% K2S2O8,for 100 min at 60?,and the subsequent heat treatment condition was 5 min at 120?,in this condition the flux increased by 43.2%,the rejection increased by 0.2%.Separation experiments of small molecular alcohols(ethanol,isopropanol,n-butanol)showed that the highest rejection of the modified membrane was achieved at 5 wt%,100 min.Static oxidation experiments showed that the modified membranes exhibited better oxidation resistance,in which the best oxidation resistance performance was obtained at 5 wt%,100 min.The results of SEM and AFM showed that proper modification only reduceed the surface roughness and would not destroy the "peak-valley structure" of the membrane surface,while the high-strength treatment indeed destroyed the surface morphology.The IR spectra of the modified films were not changed.the XPS results showed the content of O elements increased,which indicated that the carboxyl groups on the surface of the modified films increased.Zeta potentials showed that the negative charge of the modified films increased,which was consistent with the increase in carboxyl content on the membrane surface.The contact angle after modification decreased,i.e.,the hydrophilicity increased.Based on the aforesaid results,the mechanism of "interfacial layer cross-cortex surface oxidation" is proposed,that is,when the commercial RO membrane is modified by potassium persulfate,polysulfone of the base film is crosslinked with the polyamide of the skin layer through activated groups to form a dense interface layer;meanwhile,the polyamide skin is oxidated by persulfate,its structure gets looser and thinner,which causes a loss of solute rejection,so the interface layer takes the role of rejection.As the interface layer structure becomes more compact,the solute retention capacity is stronger,so the membrane rejection increases;at the same time,the looser and thinned skin layer results in the increase of flux.The stability of the interface after cross-linking improves,which makes it withstand higher strength of oxidation,thereby enhancing the membrane's oxidation resistance.