Preparation And Modification Of Forward Osmosis Membrane In Seawater Desalination

Aromatic polyamide forward osmosis membrane is the main material in the field of seawater desalination and brackish water desalination.However,the non-tolerance of polyamide forward osmosis membrane to chlorine seriously limits the application o f forward osmosis technology,which increases the operating cost of membrane techno logy in the fields of wastewater purification and advanced treatment.At present,the m ost effective way to improve the chlorine resistance of polyamide forward osmosis me mbrane is to modify the supporting membrane and active layer by physical or chemic al modification.In this study,cyclopropylamine and cyclohexylamine were used as functional ma terials to graft modification on the surface of polyamide forward osmosis membrane,so as to improve the chlorine resistance of forward osmosis membrane.In this study,p olyacrylonitrile was used as the supporting material,and the supporting membrane wa s prepared by immersion precipitation phase transformation method,and then the acti ve layer of polyamide was prepared by interfacial polymerization method.The grafting modification of cyclopropylamine and cyclohexylamine was compl eted in the process of interface polymerization.The modified membranes were charac terized by various means.The physical morphology and roughness of the membrane s urface were observed by SEM and AFM.The changes of chemical elements and funct ional groups on the membrane surface were studied by using FTIR and XPS spectra.I n the evaluation and test of membrane separation performance and retention performa nce,cross-flow experiment was carried out by self-made membrane pool.The membr ane was chlorinated by sodium hypochlorite solution in a static test.The results are as follows:(1)The influence of polymer concentration,solvent type,shock temperature,sho ck time and amount of lithium chloride on the performance of forward osmosis memb rane was investigated.The results showed that the optimum conditions for the prepara tion of polyamide forward osmosis membrane were as follows:the optimal solvent for organic matter was N-methylpyrrolidone;The optimal concentration of PAN was 16.5wt%.The optimum oscillation temperature is 65℃.The optimal oscillation time was 150min.The optimal supplemental amount of lithium chloride was 1.5wt%.(2)The surface morphology of cyclopropylamine and cyclohexylamine modified forward osmosis membranes is affected by the grafting concentration.The surface ro ughness of the modified membranes grafted with 0.5wt%cyclopropylamine and cyclo hexylamine is increased by 89.5%and 44.5%,respectively,compared with the origina l membranes.The surface roughness of the modified membranes grafted with 1.5wt%cyclopropylamine and cyclohexylamine decreased by 28.5%and 48.0%,respectively,compared with the original membranes.(3)The grafting of cyclopropylamine and cyclohexylamine can significantly imp rove the chlorine resistance of polyamide forward osmosis membrane.After the chlori nation treatment of 20000pppm·h,the salt reverse flux of the grafted 1.0wt%and 1.5 wt%cyclopropylamine modified films was 19.6gm-2h-1 and 14.3gm-2h-1,respectively,and the rejection rate of sodium chloride was increased by 19.2%and 23.9%,respecti vely.The water fluxes of grafted 1.0wt%and 1.5wt%cyclohexylamine modified mem branes were 26.22 Lm-2h-1 and 17.88 Lm-2h-1,respectively,and the rejection rate of so dium chloride was increased by 15.5%and 19.7%,respectively.(4)Chlorine resistance of cyclopropylamine and cyclohexylamine modified mem branes is related to the pH value of the solution during chlorination.It was found that the form of free chlorine was highly dependent on the pH value,so the degradation m echanism of Cl on PA(polyamide)layer was different at different pH values.Under al kaline conditions,the membrane rejection rate of salt is better than that of acid conditi ons,because OH-can remove the chloride of amide bond reversibly.After 20000ppp m·h chlorination at pH 11,the salt rejection rates of grafted 1.5wt%cyclopropylamine and cyclohexylamine were 0.9%and 13.4%higher than those at pH 9,respectively.