Research On Ion Exchange Membrane Interface Fouling And Concentration Polarization Based On In-situ EIS Technology

Membrane fouling and concentration polarization?CP?easily occurred in ion exchange membrane?IEM?during electrodialysis.The electrochemical impedance spectroscopy?EIS?used in previous studies to monitor the electrochemical properties of the IEM was usually an off-line/ectopic monitoring,lacking information on the CP and fouling formation of the membrane interface during desalination.In this paper,an in-situ method for monitoring the ion mass transfer by EIS was established which could distinguish the contribution of membrane fouling and CP.Meanwhile,the change of ion flux in the desalination chamber was monitored to synchronously reflect the desalination performance of the membrane.On this basis,the method was applied to the IEM monitoring process of polymer flooding wastewater treatment.The main contents and findings are as follows:?1?An in-situ method for monitoring the ion mass transfer by EIS was established which can distinguish the contribution of membrane fouling and CP,and the equivalent circuit model of the monitoring system in different stages of desalination process was determined.In the initial stage of the desalination process,the Nernst diagram consisted of an impedance arc and a diffusion boundary layer,thus the equivalent circuit fitting element was selected as R_m+s(R_edlC_edl)R_dbl,however,in the later stage of the desalination process,fouling layer was formed in the membrane interface,the Nernst diagram transferred to two impedance arcs and a diffusion boundary layer,therefore,the equivalent circuit fitting element evolved into R_m+s(R_edlC_edl)?R_pC_p?R_dbl.The mass transfer impedance of the IEM interface during desalination process was monitored by in situ EIS and ectopic EIS,respectively.In situ EIS can obtain more comprehensive information on membrane interface fouling and CP.?2?The mass transfer properties of the membrane interfaces were significantly affected by the composition of solution.The presence of foulants in the solution significantly increased the ionic mass transfer impedance,especially at the later stage of the desalination process,the fouling layer would even form at the membrane interface due to the accumulation of foulants,thereby,the CP at the membrane/solution interface was promoted.The mass transfer properties of the membrane interfaces were compared in the condition of several different electrical foulants.The fouling degree of the membrane mainly depends on the interaction energy between the foulants and the membrane.When the interaction energy was negative,the force between the membrane and the foulants appeared to be attractive.The ion mass transfer impedance of the membrane interface increased with the increase of foulants concentration,especially when the foulants concentration reached the critical micelle concentration,the foulants was easily condensed at the membrane interface and the fouling layer was easily formed,which also promoted the phenomenon of CP at the membrane/solution interface.Unlike the foulants concentration,the interaction between the salt ions and the membrane increased with the salt ions concentration increase,thus the fouling and CP at each interface of the membrane was slowed down.Mass transfer impedance at each interface of the membrane after inverted electrode and ultra-pure water cleaning were compared.The cleaning effect of ultra-pure water cleaning was better than that of the inverted electrode.When irreversible fouling was formed in the membrane,even after cleaning the membrane performance cannot be restored,and the membrane fouling in the second process showed an accelerated state.?3?In the process of treating the polymer flooding wastewater with IEM,the interfacial impedance of the membrane increased gradually with the desalination process because the solution contains organic foulants such as polyacrylamide,and the formation of the fouling layer aggravated CP at the membrane/solution interface.The desalting performance of the membrane can be restored to the initial level by timely cleaning.As the increase of current density,the desalination rate increased,simultaneously,the accumulation of foulants aggravated at the membrane interface.In this study,the current density of 2m A/cm²can maintain high desalination rate and light membrane fouling.