Water shortage is a global problem faced by human society throughout development process. Water purification and reuse are the key methods important means to alleviate this problem. Membrane separation technology has been widely applied in water treatment industry in recent years due to its simple technology, high automation and high effluent quality. However, the application of membrane technology in water treatment is limited by membrane fouling. To control the irreversible pore blockage fouling is the core issue in membrane water treatment. Electrochemical methods are usually employed as pretreatments prior to membrane filtration to control membrane fouling due to the coagulation, oxidation and aeration effects, together with electric field. In addition, employing the membrane as electrode, and taking advantage of the electrorepulsion and reaction on membrane electrode would be promising to improve the ion selectivity and enhance water permeability, breaking the “trade-off” effect.
An electro-membrane reactor (EMR) was developed, in which ultrafiltration membrane modules are placed in the electric field zone. It’s revealed that the combined effect of coagulation and electric field mitigated membrane fouling and improved the effluent water quality. Higher current densities and weakly acidic pH in the EMCR favored faster generation of large flocs and effectively reduced membrane pore blocking. The water permeability of the formed cake layers on the membrane surface was increased due to an increase in cake layer porosity and hydrophilicity, which was induced by the applied electric field. An Electro-coagulation/oxidation membrane reactor (ECOMR) was developed by altering the electrode material and arrangement based on ECMR. The hydrophilicity and porosity were further enhanced due to the synergistic effect between electrocoagulation and electrooxidation with electric field, leading to the enhancement in anti-fouling performance. Electrooxidation modulated the morphology of the cake layer by breaking up humic acid molecules (i.e., carboxylic functional groups and aromatic structures), giving a more porous cake layer, achiving higher water permeability. Electrocoagulation prevented the contaminates from attaching to membrane and increased the hydrophilicity of the cake layers, resulting in 55.07° decrease in contact angle and 29.12% increase in water permeability. Moreover, the cake layers formed in the ECOMR exhibited lower affinity for the membrane surface, which meant they were easier to clean off. Compared to traditional electrocoagulation-ultrafiltration process, ECMR has a smaller footprint and could achieve significant water permeability and energy savings.
Utilizing the excellent conducitivity of graphene, we fabricated graphene conductive membrane, and investigated its selective separation under electric field. The structure and performance optimization of graphene-based membrane were delineated. The oxygen-containing group modification strategy improved the hydrophilicity of membrane surface, as well as enhanced the nanochannel size for water and the charge density around the nanochannel, leading to improved filtration performance and enhanced water permeability. The influence of thermal reduction and flake (MoS2) intercalation on the ion penetration process of graphene oxide membrane (GOM) has been investigated. The results indicated that thermal reduction and MoS2 intercalation could work in concert to control the size-sieving effect and ion-nanochannel interactions to achieve fine selective separation, leading to ~25.5 times improvement in Cu2+ ion selectivity over Na+. The response of its desalination performance under electric field exhibited apparant principle. When the direction of electric field was along the filtration direction, the rejection declined, and when the the direction of electric field was opposite to the filtration direction, the rejection increased. The response degree was positively correlated to the electric field strength. The results indicated that the response to electric field could be tuned by the interlayer spacing and surface charge density in concert. Narrower interlayer spacing and higher charge density would lead to obvious response to electric field. The response mechanism was controlled by cations.
Ultizing the conductivity of graphene, a conductive graphene hydrogen membrane (GHM) was fabricated. The conducive membrane module was employed in EMR as electrode and membrane (namely membrane electrode). The results showed that when the membrane worked as an anode, the membrane fouling increased due to the electro-adsorption. When the membrane worked as a cathode, the in-situ electro-cleaning would be achived by the in-situ oxidation and electrorepulsion effect on membrane surface, which changed the molecular weight distribution and content, leading to an increase in water flux by 9.83%. Electrostatic repulsion was the primary factor (83.01%) that endowed the membrane with anti-fouling performance, which was confirmed by controlling the reactions occurred on electrodes. The irreversible fouling rate was inversely proportional to background electrolyte as well as cleaning time, leading to 98% decrease at most. The membrane fouling control and in-situ electro-cleaning were achieved by applying conductive membrane into EMR, giving important scientific support to the evolution and application of EMR system.