Surface Hydrophilic Modification Of PVDF Micro-filtration Membranes Based On Atomic Layer Deposition And Mechanism Of Fouling Control

Membrane fouling becomes the main obstacle of development restriction of membrane-based water treatment technology.Membrane modification can realize effective control of membrane fouling primarily.Atomic layer deposition?ALD?appears as an approach of chemical vapor deposition for thin films preparation based on order and surface self-saturated reactions.The ALD technique integrates outstanding features of uniform deposition,controllable target and process.Besides,the ALD can achieve alternate deposition of different precursors,fabricating ideal modified substances with specific properties.Membrane surface modification by ALD can prevent blockage of membrane pores effectively,which becomes the frontier technology in the membrane modification filed.However,some problems still need to be resolved.The membrane flux might decrease due to nucleation growth of modified materials on membrane surface.The modified material with superior anti-fouling performance and suitable conditions for membrane fouling control also need to be determined.Moreover,the anti-fouling mechanism of modified membranes needs to be interpreted.In view of the above problems,firstly this project developed a surface activation method for polyvinylidene fluoride?PVDF?membranes using nitrogen dioxide and metal organic precursors.The modified materials could be uniformly deposited on inert membrane surface at lower deposition cycles.This thesis took zinc oxide?ZnO?deposition as an example and analyzed the changes of some essential characteristics such as growth property of the deposition layer and surface morphology of the membrane.The growth rate of deposition layer on activated membrane surface was1.6-2.4 times higher than that without activation.The surface of modified membrane with activation presented relatively smooth.Besides,the deposition layer distributed uniformly on activated membrane surface.Furthermore,this study mainly focused on the hydrophilicity,permeability and separation performance of modified membranes.The pure water flux increased by 31.5%and the retention rate reached97.8%towards bovine serum albumin?BSA?after ZnO modification with 100deposition cycles.In addition,the processes of membrane activation and hydrophilic modification were analyzed.An active layer of ZnO was formed both on PVDF membrane surface and pore walls after 10 cycles of nitrogen dioxide and diethylzinc pre-deposition.The hydrophilic oxygen-containing functional groups could accelerate uniform growth of deposition layer on membrane surface.This study verified the activation method could be also applied for titanium dioxide?TiO2?and aluminum oxide?Al2O3?modification of the PVDF membrane.In order to determine the anti-adsorptive fouling potential of ALD hydrophilic modified membranes,this study selected PVDF-TiO2,PVDF-Al2O3 and PVDF-ZnO modified membranes with activation as research targets,and was first focused on their hydrophilicity and permeability by comparison.Results showed that the hydrophilicity of modified membranes presented the following order PVDF-ZnO>PVDF-Al2O3>PVDF-TiO2 under superior deposition cycles?100?.The enhancement of hydrophilicity of PVDF-ZnO modified membrane was the most significant.The water contact angle decreased by 56.2%.Accordingly,the pure water flux increased by 31.5%.Static adsorption fouling test found that the PVDF-ZnO modified membrane exhibited superior anti-fouling performance compared to PVDF-TiO2 and PVDF-Al2O3 modified membranes,presenting minimum values of flux decline percentage and increased trans-membrane resistance.Besides,the equilibrium adsorption amount of BSA and sodium alginate?SA?in mixed foulants?mass ratio of 1:1?on PVDF-ZnO modified membrane were 0.8-6.7 mg/g and 16.2-18.1 mg/g lower than those on PVDF-TiO2 and PVDF-Al2O3 modified membranes,respectively.Filtration fouling experiment found that the increase of solution ion intensity or function of coexisting bivalent cations could not only result in increase of flux decline extent by 3.7-11.1%,but also lead to rise of trans-membrane resistance by20.8-74.2%.The negative effects induced by change of foulant system on PVDF-ZnO modified membrane were lower than those on PVDF pristine membrane.The fouling process of PVDF-ZnO modified membrane was reversible.In order to interpret the anti-fouling mechanism of hydrophilic modified membranes,this study analyzed the adsorption style and process of foulant on modified membrane surface by isotherm models and thermodynamics.The process for foulants adsorbed on modified membranes were identified as monolayer adsorption.Also,the adsorption process was spontaneous,endothermic and entropy increased.The number of adsorption sites on modified membrane surface was limited,especially the PVDF-ZnO modified membrane showed the lowest saturated adsorption capacity towards BSA and SA foulants,only 14.28 and 10.43 mg/g respectively.The adsorption capacity of PVDF-ZnO modified membrane decreased by 44.9%compared to that of PVDF pristine membrane.The adsorption force between foulant and modified membrane surface declined.The fouling type and trend of superior modified membranes were determined by fitting of filtration models.The major fouling mechanism of PVDF-ZnO modified membrane was standard blocking,accompanied with intermediate blocking and complete blocking simultaneously.The shielding effect of coexisting cations and negative effect of calcium bridging could lead to long time of intermediate blocking.The complete blocking began to appear after collecting less filtrate.Moreover,the change of interaction energy between foulant and modified membrane was interpreted by XDLVO theory.The attractive energy of membrane decreased by 0.75 kT while the repulsive energy increased by3.78 kT towards BSA foulant after ZnO hydrophilic modification,which became the significant cause of the enhanced anti-fouling property.The repulsive energy between BSA foulant and PVDF-ZnO modified membrane decreased obviously with the increase of solution ion intensity or addition of divalent cations.BSA foulant needs to overcome higher obstacles to approach membrane surface in the presence of Mg²⁺compared to Ca²⁺in solution.In conclusion,this study obtained the PVDF-ZnO hydrophilic modified micro-filtration membrane,realizing improvement of the anti-fouling performance,permeation flux and retention property simultaneously.It was of great significance for membrane fouling alleviation,promotion of theory and practice of the membrane-based water treatment technology as well as sustainable development of the ecological environment.