Preparation And Separation Performance Of Reverse Osmosis Composite Membranes By Low Temperature Plasma Direct Polymerization

In the global situation of fresh water resources shortage,reverse osmosis technology,as the main technology of seawater desalination and brackish water regeneration,has been paid more attention by more countries and regions in the world.How to improve the efficiency of reverse osmosis desalination and reduce its energy consumption through new technical methods has become a hot topic of academic research.As the core of reverse osmosis technology,the development of reverse osmosis membrane has experienced many iterations.Composite membrane has become the most widely studied membrane because of its excellent anti fouling performance and stable physical and chemical properties.A new method of preparing reverse osmosis composite membrane with high separation performance by depositing functional layer on the surface of excellent base membrane by plasma direct polymerization has also been developed.Compared with other traditional modification and membrane preparation methods,low-temperature plasma direct polymerization has lower requirements for the selection of functional monomers,and the reaction process is simple and rapid.The obtained polymer film has strong crosslinking,stability and separation performance.The reaction system is always kept at a lower temperature,simple,efficient,green and energy-saving,It is a new method with practical application value that can effectively prepare high performance composite membrane.In this study,the PS(polysulfone)ultrafiltration membrane material with excellent hydrophilicity and stability and no desalination effect was selected as the base membrane.The DBD(Dielectric Barrier Discharge)method was used to discharge the gaseous monomer to excite the low temperature plasma at normal atmospheric pressure,and the polymer film functional layer with desalination performance was deposited on the surface of the base membrane,thus the reverse osmosis composite membrane with improved salt cut-off rate and flux was prepared.Through the separation performance test and micro-characterization of the prepared composite membrane,the influence of various experimental conditions and the interaction between factors on the final separation effect was explored,and the analysis was verified by combining the micro-characterization results.At present,there is no perfect theoretical basis for the selection of functional monomers in plasma direct polymerization.Firstly,the paper summarized the existing research results,and the characteristic functional groups are used as variables to cluster analysis of organic substances.Five functional monomers with different properties are selected,and the evaporation capacity of monomers at different temperatures is tested to obtain the evaporation performance curve and equation,In order to control its MFR(mass flow rate)in the subsequent experiments.Secondly,five kinds of functional monomers were used as experimental materials,and the influencing factors of preparation conditions that may affect the process of plasma direct polymerization were expanded.After the pre-test was carried out to determine the appropriate level of each factor,the orthogonal test of 6 factors and 5 levels was designed,and the polymer functional layer was deposited on the surface of PS based film by DBD method to prepare composite film,Under the pressure of 1.5MPa,the separation performance of the composite membrane was tested with 2000ppm Na Cl and Mg SO₄ solution.The effects of functional monomer type,monomer mass flow rate v,discharge power P,discharge duration T,electrode spacing D and swelling duration t₀ on the salt rejection rate and flux of the composite membrane were investigated by using range,variance and regression analysis methods,and the significant effects of various factors on the performance indexes were obtained.The results showed that the type of functional monomer was a highly significant influencing factor,and 4-vinylpyridine(4-VP)monomer showed the best desalination performance under various experimental conditions.Under the optimal experimental conditions,the rejection rate of monovalent salt ion was 40.113%,the flux was 33.070 L/m²·h,the rejection rate of divalent salt ion was 39.050%,the flux was 30.660 L/m²·h.The electrode spacing has little effect on the performance index;Each factor had the same effect on the salt interception rate of Na Cl and Mg SO₄ solution,but had different effect on the flux;According to the factor effect curve,the best combination of preparation conditions can be obtained.SEM-EDS microscopic characterization of the experimental group with better desalination performance showed that different polymer groups were formed on the surface of the membrane,and the proportion of C element increased significantly,which proved that the products of plasma state direct polymerization of various functional monomers had successfully adhered to the surface of the base membrane.Furthermore,single factor experiments were designed with 4-VP monomer with better desalination effect to investigate the influence of monomer mass flow rate v,discharge power P,discharge time t and swelling time t₀ on the separation performance of composite membrane.Under the pressure of 1.5MPa,the separation performance of the composite membrane was tested with 2000ppm and 6000ppm Na Cl and Mg SO₄ solutions.The results showed that under the same reaction conditions,the desalination effect of the high concentration solution was better than that of the low concentration solution,and the rejection ability of the monovalent ion was better than that of the divalent ion;The influence of swelling time t₀,discharge time t and discharge power P on the separation performance was significant.The influence of gas monomer mass flow rate v on the flux of divalent salt solution was fluctuant,which was less significant than other factors and consistent with the previous analysis;The optimal preparation conditions were as follows:swelling time t₀=16h,discharge time t=7min,discharge power P=60W,and gas monomer mass flow rate v=23mg/s.At the same time,SEM micro characteri-zation of typical value test group can obviously observe the difference of membrane surface morphology at different levels,which provides support for analysis and inference.Finally,the composite membrane was prepared with 4-VP monomer with better desali-nation effect by response surface method.The separation performance of the composite membrane was tested with Na Cl and Mg SO₄ solutions of 2000ppm and 6000ppm under 1.5MPa pressure.The interaction among three discharge conditions,i.e.discharge time t,discharge power P and monomer mass flow rate v,which are directly related to the plasma polymerization process was explored,The effects of first and second order factors on the separation performance of composite membranes were studied.The results show that the polynomial regression equation obtained by fitting each performance index fits well with the actual value of the test,and the multivariate correlation coefficients of the established continuous variable surface model are greater than or very close to 0.99,with high accuracy.AC(discharge duration×Monomer mass flow rate)was the weakest secondary factor,which was not significant;The salt cut-off rates of high and low concentration monovalent and divalent ion salt solutions generally reach the highest point at P=100W,t=7?10min,v=18?20mg/s,and reach the low-est point at P=20W,t=1min,v=3mg/s.The trend of flux changing with preparation conditions is negatively correlated with the trend of salt cut-off rate.The conclusion is consistent with the previous analysis,which further shows that the experimental conclusion is scientific.Under the conditions of P=100W,t=10.1min,v=16.5mg/s,the salt rejection rate of the membrane for low concentration monovalent salt ion is 37.877%,and the flux is 36.660 L/m²·h;For high concentration monovalent salt ions,the salt cut-off rate was 48.133%,and the flux was 23.660L/m²·h;For low concentration divalent salt ions,the salt cut-off rate was 35.021%,and the flux was 31.660 L/m²·h;For high concentration divalent salt ions,the salt cut-off rate was 43.133%,and the flux was 21.660 L/m²·h.