Fractal Interfacial Interaction Of Membrane-fouling Layer And Establishment Of Permeation Model Of Membrane Fractal Structure

Membrane fouling is always the bottleneck problem in membrane application,therefore,a number of studies have been investigated on fouling in detail.Based on previous research,new methods have been provided to solve the membrane fouling problem in terms of membrane structure and fractal interface interaction.By introducing the fractal dimension theory to describe the microstructure and morphology of membrane,the relationship between the membrane microstructure fractal dimensions and the conventional structure parameters as well as the membrane separation performance have been verified.Furthermore,the membrane microstructure fractal dimension permeability model was derivated by modified the Hagen-Poiseuille equation to clarify the dependence of membrane permeation performance on the character of membrane microstructure fractal dimensions.Whatsmore,the xDLVO theory was used to calculate the interfacial free energy between membrane-foulant and foulant-foulant.Combined the membrane fouling trend and irreversible membrane fouling resistance,characterization of membrane fouling by the foulant with different structures and properties has been discussed to reveal the effect of fractal interface between membrane and the fouling layer.The main results were as follows:?1?The field emission scanning electron microscopy FESEM and professional image processing software ImageJ were used to characterize the six different molecular weight cut-off molecular weight PES membranes and the 100 kDa PVDF membrane.Image processing and data analysis were carried out to obtain the membrane surface opening ratio?,pore density,membrane pore roundness,maximum pore diameter?_max,minimum pore diameter?_min;and parameters such as membrane porosity,average pore diameter,membrane separation layer thickness,and pure water contact angle were also measured.?2?The fractal dimension D_f of 50 kDa PES membrane at different amplification multiples?20000X,50000X,100000X?was determined by small island method,box dimension method and correlation function method.The first and second methods calculated the D_f at different magnifications,the difference is larger,and the D_f calculated by the correlation function method has good stability,the difference is less than 0.2%.Therefore,the correlation function method is preferred as the method for measuring the membrane area fractal dimension.The fractal dimension D_S and D_T were calculated by P-d relation and fractal scaling relation respectively.?3?The relationship between membrane pore fractal dimension D_s and membrane pore shape,membrane area fractal dimension D_f and membrane pore structure,average tortuosity fractal dimension D_T and streamline bending degree was qualitatively analyzed;The relationship between D_f,D_T and membrane surface porosity?was described by linear fitting and polynomial fitting.D_f and?show a good positive correlation,and D_T has a good negative correlation with?.The larger the D_f,the more fully the pores develop in the membrane,the more relaxed the corresponding streamline,and the smaller the D_T,the better linear negative correlation between D_f and D_T.?4?Based on the Hagen-Poiseuille formula,the membrane pore fractal dimension D_S and the tortuosity fractal dimension D_T were introduced.Considering the hydrophobicity and membrane pore fractal characteristics of membrane materials,combined with the fractal theory and method,membrane fractal structure percolation model was established.There are no empirical constants in the model expression,and each parameter has a clear physical meaning.?5?The fluorescence characteristics,functional group information and particle size distribution of three characteristic pollutants BSA,HA and SA were characterized by three-dimensional fluorescence spectroscopy,FTIR spectroscopy and laser nanometer particle size analyzer.The functional groups and roughness of the films were characterized by FTIR and atomic force microscopy.?6?Based on the contact angle data of pollutants and membrane surface and the extended Young's equation,the parameters of pollutants and membrane surface tension,such as?⁺??^-and?^LW were calculated;Membrane pollutants between pollutants and pollutant-interface free energy were calculated further by using xDLVO theory;In the stage of adhesion and cohesion,polar force energy dominates,and has strong correlation with pollutants and surface?^-/?⁺.According to the total interfacial interaction,the degree of pollution of different characteristic pollutants on the membrane surface can be identified as BSA>HA>SA,and the anti-pollution ability of different kinds of membranes can be evaluated as 1 kDa PES>30kDa PES>100 kDa PES>100 kDa PVDF by using the total interfacial interaction energy.?7?The relative flux of different ultrafiltration membranes for different characteristic pollutants decreased,and the attenuation of specific flux was not consistent with the predicted results of interfacial free energy.The linear fitting coefficients R² between the initial membrane fouling trend and the free energy of the membrane-pollutant interface at the adhesion stage of 100 kDa PES and 100 kDa PVDF caused by BSA were 0.22889 and0.01472,respectively.The correlation was weak,and non-interfacial factors were dominant.In the linear fitting between the sum of the free energy of the membrane-pollutant/pollutant-pollutant interface and the irreversible membrane fouling specific resistance,the linear correlation coefficients are greater than 0.81,and the linear negative correlation is good,which proved that hydrophobic organic compounds mainly formed the resistance of irreversible membrane fouling,while hydrophilic organic pollutants had less physical irreversible membrane fouling.