Preparation And Properties Of DOPA/PEI Co-deposited Composite Films In MBfR

Membrane Biofilm Reactor(MBfR)is a new and efficient wastewater treatment technology that combines bubble-free aeration with biofilm treatment.Currently,commercial MBfR membranes are not yet available,and the development of commercial membranes suitable for MBfR systems is the key to the widespread industrial application of MBfR systems.In this paper,on the basis of the group's previous separate dopamine(DOPA)modified composite membranes,the group's homemade polyvinylidene fluoride(PVDF)hollow fiber membranes were modified on the membrane surface using the dopamine/polyethyleneimine(DOPA/PEI)co-deposition system,expecting to obtain modified composite membranes with excellent oxygen transfer performance,high stability and good bioaffinity.Firstly,the paper determined the key influencing factors and optimized the range of experimental conditions through single factor experiment.Then,under the condition of four factors and three levels,the response surface method was further used to optimize the surface modification conditions of DOPA/PEI system with the total oxygen transfer coefficient as the main evaluation index,and the interaction between various factors was investigated.The results showed that PEI could accelerate the reaction rate of DOPA polymerization without forming large particle polymer precipitation,and the short time modification effect was better than the DOPA modified membrane alone.The best modification condition was DOPA:PEI=1:0.625,DOPA concentration 1.36 g/L,reaction temperature 25?total deposition time 4 h,heat treatment temperature 40?.The performance of PDA/PEI-coated PVDF modified film obtained under the optimal modification conditions was far better than the original film.The oxygen mass transfer performance of PDA/PEI-coated PVDF modified film was 2.104 times higher than that of the original film(0.77×10^-2 min^-1 to1.62×10^-2 min^-1).The mechanical properties of the modified film were similar to that of the original film.The bubble point pressure was increased from 20 KPa to 80 KPa,and the surface hydrophilicity of the film was greatly improved,decreasing from 85.8°to 20.5°.By FESEM,FTIR and XPS analysis,it showed that PDA-PEI polymer was successfully synthesized on the membrane surface,and the main reaction was Michael addition and a small part of Schiff base reaction;the adhesion layer on the membrane surface was relatively dense and uniform.In addition,it showed that DOPA/PEI solution had better reusability,and the properties of each batch of DOPA/PEI modified membrane were relatively stable,but the performance was not completely consistent by the reusability experiments of DOPA/PEI and DOPA solution.According to the existing research,it is speculated that the solution can be recycled for a higher number of times in a high proportion of PEI environment.By SEM,AFM and XPS test analysis,it showed the physical properties of the modified film composite layer of DOPA/PEI solution differed relatively little from batch to batch.The difference of each batch of modified film composite layer was related to the material loss in solution and the continuous solution reaction.The DOPA/PEI solution had good repeatability.Further experiments on the stability of DOPA/PEI modified and DOPA modified membranes showed that PDA/PEI polymer layer had better stability,and could still maintain good chemical stability under extreme conditions of strong acid and strong base,while polar acetone solution caused great damage to the polymer layer.PDA/PEI-coated PVDF modified composite film had a small attenuation rate of the total oxygen transfer coefficient under extreme and conventional conditions.The attenuation rate of K_La in ultrasonic,strong acid,neutral,strong base and acetone solution was 4.3%,1.2%,4.3%,7.4%and 28.4%,respectively.PDA/PEI-coated PVDF modified composite film had good physical/chemical stability.