Removal Of Colloids And Salts From Water By Integrated Electrodialysis-ultrafiltration Technology

With the development of technology,colloids are playing an increasingly important role in people’s production and life.Colloids are ubiquitous from food to industrial development,so the production and complexity of saline colloidal wastewater has increased.The most commonly used methods for the treatment of high-salinity colloidal wastewater are coagulation,adsorption and membrane separation.However,these methods are still limited and troubled by the problems of high cost,low removal rate and secondary pollution.Therefore,it is still necessary to develop a new clean treatment process to achieve good desalination effect while removing the colloid.Electrodialysis（ED）has some necessary conditions for colloid destabilization,such as electrolyte,electric field force and p H,so ED can be used for colloid destabilization and aggregation based on DLVO theory.Ultrafiltration is a separation method based on the physical retention of porous membranes,driven by pressure differences.In this study,two treatment methods,electrodialysis and ultrafiltration,were coupled to form an integrated electrodialysis-ultrafiltration（ED-ULF）technology,where colloids were destabilized and aggregated by ED and then retained by ultrafiltration.This study attempts to aggregate small size colloids to alleviate the flux reduction caused by adsorption within the membrane pores and improve the removal rate of colloids.Furthermore,this study is also devoted to exploring the mechanism of colloidal destabilization in the ED process,and complementing the gap in the mechanism while deepening the understanding.Molecular colloid BSA was used for ED treatment.The changes of colloid morphology and dynamics under laminar flow,salt concentration,current density and p H were compared.The mechanism of colloid instability under these factors was analyzed by DLVO theory.The results showed that laminar flow induced aggregation mainly through the collision of colloidal particles during flow,which could only magnify the colloidal particle size by 2.28 times at most and had the worst effect on on BSA destabilization and aggregation.The variation of p H can lead to the change of spatial structure and surface charge of proteins,which leads to colloidal aggregation.The effect of p H on colloidal aggregation was also limited,and the maximum particle size can be magnified to 17 times.The electric field force can destabilize the colloidal particles by breaking the force balance of particles,and can make the colloidal particles aggregate to70 times.The salt concentration had the best effect on the aggregation of BSA,resulting in a 278-fold enlargement of the colloidal particle size and a significant flocculation.This was because the electrolyte compressed the double electric layer of BSA and changed its electrostatic repulsion.Meanwhile,floc production under high salt conditions may be the culprit of ED membrane stack blockage.In addition,by simulating the actual data,an empirical formula of the relationship between the thickness of the double electric layer（i.e.Debye length）and the particle size of the colloid was proposed,which can provide reference for the particle size amplification of the colloid in the ED process.The particle colloid Na₂SiO₃·9H₂O was selected for ED experiment to verify the general aggregation effect of ED on colloid and eliminate the interference of special factors.The results showed that ED also had a good aggregation effect on particle colloids.Changing the current density can cause Na₂SiO₃·9H₂O to aggregate up to 67.31times,while changing the salt concentration can enlarge the particle size up to 65.12times and produce fine-grained aggregates.The decrease of p H made SiO₃^2-continuously condensed and crosslinked to form gel,which can aggregate the particle size to more than 60 times（61.88）.The particle size variations of BSA and Na₂SiO₃·9H₂O under different parameters were compared,and it was found that the aggregation of BSA by ED was graded while the aggregation effect of Na₂SiO₃·9H₂O was relatively average,which was determined by the structure of the two colloids themselves.In addition,the desalination rate of ED could reach 90.00～99.66%,so it could play a good desalination effect.The Na₂SiO₃·9H₂O colloidal solution after ED was used in the ultrafiltration process to explore the effect of ED on the decline rate of specific flux and the removal rate of SiO₃^2-in the ultrafiltration process.The results showed that when the current density raised from 100 A/m² to 200 A/m²,the particle size was enlarged by 2.94 times,the flux decline rate decreased from 20%to 7.35%,and the removal rate of SiO₃^2-reduced from 93.06%to 85.99%.After a 20-fold increase in salt concentration（from0.05 mol/L to 1 mol/L）,the particle size was magnified by 10.74 times,the specific flux reduction rate was only 3%,and the removal rate was less than 1%（0.65%）.When the solution changed from alkaline to neutral,the particle size increased by 2.94 times,and the reduction rate dropped from 18.46%to 14.81%.The removal rate became negative due to the transformation of the Siform.In addition,molecular weight cut-off（MWCO）of ultrafiltration membrane and the type of colloid were also included in the research scope to better evaluate the feasibility of integrated electrodialysis-ultrafiltration technology in mitigating membrane pore blockage and colloid removal.