Mechanism And Regulation Of Ion Exchange Membrane Electrolysis Coupling Dual-membrane Process To Reclaim Nanofiltration Concentrate

The unconventional water treatment is an effective way to alleviate the crisis between freshwater supply and its demand.Nanofiltration（NF）has gradually become one of the mainstream methods for high-quality water purification due to the effective removal of various pollutants based on the environmentally friendly separation mechanism.Unfortunately,applying the NF process would produce a large volume of a concentrated stream（NF concentrate,accounting for 10%to 50%of the NF influent）characterized by not only low biodegradability but high inorganic salinity and organic concentration.Conventional disposal methods（such as surface water discharge,discharge to sewers,and subsurface injection）have historically incurred the local environment and the residents’health,causing the loss of ionic and water resources from the NF concentrate.However,further increasing the recovery rate would aggravate the combined fouling of the NF membrane.Thus,ion exchange membrane electrolysis was proposed to couple dual-membrane process,to recover valuable resources and alleviate combined membrane fouling.In this work,a two-step ion exchange membrane electrolysis was developed to investigate the recovery of divalent ions（calcium cations,magnesium cations,and sulfate anions）from NF concentrate.Then,the membrane electrolysis process was simplified to a one-step cation exchange membrane（CEM）electrolysis.The CEM electrolysis coupling with the UF process was investigated,including its recovery efficiency and the mechanism of membrane fouling mitigation.Finally,the CEM electrolysis/UF was performed prior to NF membranes to evaluate the foulants removal and membrane interface characteristics.The two-step membrane electrolysis system of anion exchange membrane（AEM）electrolysis plus CEM electrolysis was used for processing the real NF concentrate.It was found that the AEM electrolysis recovered calcium cations by forming calcium carbonate crystals and obtained magnesium cations by forming amorphous magnesium hydroxide in the cathode chamber.Meanwhile,the AEM electrolysis concentrated the sulfate anions into the anode chamber.The CEM electrolysis assisted by calcium hydroxide and sodium metaaluminate was followed,forming ettringite and amorphous magnesium hydroxide in the cathode chamber.The two-step membrane electrolysis totally reduced the calcium cations by 80.0%,the magnesium cations by 93.3%,and the sulfate anions by 43.1%.Besides,the crystallization process adsorbed and removed organic matter,achieving a total organic carbon（TOC）permeability with 69%.However,in the membrane electrolysis process,the AEM fouling was more severe than that of the CEM.The aluminum ion concentration in the effluent was also significantly increased.In order to overcome the negative effects of electrolysis,the CEM electrolysis process without chemicals addition was used as the pretreatment process to combine the UF process,forming the separated anode/ultrafiltration（SAUF）process and the separated cathode/ultrafiltration（SCUF）process,respectively.When treating the real NF concentrate,it was found that changing the electrolysis time can control the purity of the recovered precipitate.After using 30 min of the CEM electrolysis,the purity of calcium carbonate reached 90.6%.Prolonging electrolysis time to 90 min,the purity was reduced to 82.3%,while the recovery yields increased to 42.3 g Ca/（m² membrane）and 11.8g Mg/（m² membrane）.Interestingly,the fouling of the UF membrane has been significantly alleviated.In order to further study the mechanism of UF membrane fouling mitigation,the effects of calcium cations,magnesium cations and CEM electrolysis on the model foulants were investigated.The results showed that calcium and magnesium cations can form combined membrane fouling with model pollutants.Controlling the membrane electrolysis time can alleviate it.The flux improvement was different:humic acid>bovine serum albumin>sodium alginate.Hydrogen ions in the anode chamber eliminated the bridging effect of divalent ions and humic acid;moreover,the humic acid molecules were transformed into supramolecular structures,significantly alleviating the membrane fouling.The amorphous magnesium hydroxide adsorbed organic matter in the cathode chamber and forms a dynamic barrier on the UF membrane surface,reducing the fouling caused by organic matter.Therefore,the CEM electrolysis is expected to treat the NF concentrate water with natural organic matter（NOM）as the main organic pollutants to control the UF membrane fouling.Based on the two parallel processes of SAUF and SCUF,their effluents were mixed and applied to improve the NF performance.The fate of pollutants in the process and the interface characteristics of NF membranes were mainly investigated.The results showed that CEM electrolysis could maintain the NF membrane permeation at 6.01～8.41LMH/bar,reaching an increase of 123%～211%compared to the control system（dual-membrane process）.The CEM electrolysis reduced the salt salinity of NF influent（from1344μs/cm to 970μs/cm）,played the role of softening Ca²⁺/Mg²⁺（removal rates68.8%/30.9%）,and controlled the bridge between NOM and divalent cations.In addition,it was also found that CEM electrolysis can promote pollutants self-aggregation on the membrane surface and reduce the free energy of adhesion between the contaminants and the NF membranes,further enhancing the permeability of the organic fouling layer.Therefore,the effluent quality（salt content and organic matter）was improved,and the combined fouling on the surface of the NF membrane was reduced.According to the energy consumption estimation,it is demonstrated that the CEM electrolysis can effectively recover the divalent ions from the NF concentrate and optimize the total energy consumption with 1.436 k Wh/m³,lower than the conventional dual-membrane process.Overall,the CEM electrolysis/dual-membrane process can manage the NF concentrate,that is,recovering divalent ion resources,while controlling the combined UF fouling and the combined NF fouling,as well as reclaiming freshwater resources.The research findings could provide new thoughts for a combined NF process with characteristics of low chemicals addition and environmental friendliness,which is expected to accelerate the development of NF in unconventional water treatment.