Research On Intelligent Modification Of Membrane Surface Based On Membrane Cleaning Response

Membrane fouling is the bottleneck of membrane separation technology that restricts its high efficiency and wide application.So the interpretation of membrane fouling mechanism and the development of membrane pollution control strategy has become one of the key scientific themes of membrane technology.In order to realize the functional design of the polyvinylidene fluoride(PVDF)membrane,it needs to be modified by amines to introduce the active site of amino groups.In this paper,the amination mechanism of PVDF membrane was studied and the nucleophilic reaction activity of three kinds of amines with different molecular structures was compared.The results showed that the order of nucleophilic reaction activity of the three kinds of amines was piperazine>ethylenediamine>diethylamine.In addition,the effects of reaction temperature,amine concentration and pH value on the amination reaction degree were also discussed.This study provided a new method for surface modification of PVDF membrane.At present,a popular way to improve the antifouling ability of the membrane is to improve its hydrophilicity.However,during the long-term operation,the modified membrane would still be polluted.Suitable chemical cleaning is still a necessary way to ensure the stable operation of membrane separation system.Therefore,the cleaning efficiency is an important factor affecting the performance of membrane separation process.This paper proposed an online chemical pulse membrane cleaning method by designing a membrane surface with cleaning responsiveness in order to improve the membrane cleaning efficiency in continuous membrane filtration process.On the one hand,it was still devoted to improving the antifouling performance of the membrane;on the other hand,the on-line chemical pulse cleaning could be realized due to the existence of pH responsive molecules.Specifically,periodically changed the pH value of the feed liquid to make the molecular chains expand and shrink periodically,so as to destroy the integrity of the fouling layer and then the pollutants could be easily removed from the membrane surface under the effect of fluid shear stress,thus a higher flux recovery rate was obtained.The water contact angle measurements showed that the hydrophilicity of the modified membrane increased significantly.The swelling degree results showed that the modified membrane had pH responsiveness,which made the mutual transformation between chemical energy and mechanical energy possible.More importantly,the modified membrane showed outstanding anti-fouling capability and online chemical pulse cleaning effect when used to separate soybean oil-water emulsion and BSA solution.Meanwhile,the flux recovery rates after,five times circulation could still reach 90.3%and 95.2%,respectively.Finally,based on the fact that the pollutants encountered in the process of membrane technology for water treatment usually have the characteristics of hydrophobic and oleophilic.The perfluoroalkyl was introduced to the hydrophilic membrane surface,and thus a discontinuous and oleophobic "fluorine island"structure was formed on the membrane surface.The introduction of oleophobic structure could reduce the interaction between pollutants and membrane surface,thus further improving the antifouling performance of membrane surface.In addition,the chemical pulse effect of pH responsive molecules would cause the molecular chains swell and stretch,and thus destroyed the integrity of fouling layer,so that pollutants could be easily eluted from the membrane surface under fluid shear stress,thereby improving the cleaning efficiency of the membrane.The results showed that the soybean oil contact angle of perfluoroalkyl modified membrane under dry condition was up to 96°.In addition,the soybean oil contact angle of perfluoroalkyl modified membrane under wet condition could maintain above 36°,which was higher than that of the hydrophilic modified PVDF membrane of 25°(pH=6)and the original PVDF membrane of 0°.The results of continuous soybean oil-water emulsion filtration experiment showed that the antifouling performance of original membrane,hydrophilic modified membrane and perfluoroalkyl modified membrane increased gradually,and their flux attenuation rates were 81.5%,62%and 21.4%,respectively.In addition,the perfluoroalkyl modified membrane also showed excellent online chemical pulse cleaning effect,its flux recovery rate of soybean oil-water emulsion could reach 96.3%after five times circulation.