| Hypersaline brine concentration not only contributes to zero liquid discharge（ZLD）,but also facilitates the deep development of water and salt resources.In the process of ecological civilization construction under the goal of"double carbon",the relationship between water,energy and environment has been highly concerned.Membrane distillation（MD）,which holds the advantage of modular unit and using low-grade or waste heat,has the potential to concentrate hypersaline brine.However,the problems of low stability caused by high-concentration inorganic salt erosion and organic pollution,as well as the trade-off between stability and water flux have not been effectively resolved,which limits the industrialization of MD technology.Aiming at solving the difficult problems in the hypersaline brine desalination by MD,a series of strategies to improve water flux and stability were proposed from the aspects of process improvement,membrane preparation and mechanism analysis.Herein,concentrated seawater is selected as the hypersaline brine case.The strategy of super-slippery/superhydrophobic interface coupled feed-side non-penetrating aeration was propposed against salt erosion.This strategy can not only effectively alleviate the typical inorganic salt erosion problem,but also increase the water flux by 47 wt%compared with the traditional MD system,overcoming the trade-off relationship between water flux and stability.Based on the relationship between aeration intensity and water flux,a critical condition was found for feed-side aeration that allows the bubbles to pass through the membrane pores,unveiling that low-intensity large bubbles are more suitable.Super-slippery/superhydrophobic interface coupled with feed-side non-penetrating aeration can achieve higher water recovery in a shorter time compared to conventional MD forms without aeration.The synergistic effect can not only concentrate the individual sodium chloride and calcium sulfate to supersaturation,but also concentrate the concentrated seawater until a large amount of inorganic salt crystallizes out with a water recovery rate of 90 wt%.Based on the results,we elucidated the mechanisms governing the enhanced water flux and salt resistance driven by the synergistic effect from thermodynamics and dynamics respectively.Thermodynamically,we analyzed the contribution of the super slippery interface in increasing the energy barrier of heterogeneous nucleation on the feed-membrane interface.Dynamically,we analyzed the effects of“slip boundary effect”,vortex,and the movement and bursting of bubbles on reducing temperature/concentration polarization and mitigating membrane scaling.In addition to process improvement,water flux and desalination stability are also improved through research on novel membrane materials.Electrospinning can prepare interconnected porous membranes with high permeability,but membrane wetting is prone to occur due to poor structural stability.To solve this problem,we propose an in situ three-dimensional membrane pore stabilization method,which uses the difference in polymer solubility and solvent volatility during emulsion electrospinning to in-situ reinforce the pore structure.This method is simple and can scale up production.The results show that the water flux of the interconnected porous membrane with 3D stabilized pores（3D interconnected porous membrane）was 24 L·m-2·h-1,which exceeded the water flux of aeration system in the previous chapter.In addition,3D interconnected porous membrane was continuously operated for one week in hypersalin brine without wetting and could achieve a water recovery rate of 86 wt%for concentrated seawater.In order to illustrate the effect of the pore structure on the MD performance,two comparison membranes with different pore structures were added,that is,common interconnected porous membrane and the interconnected porous membrane with 2D stabilized pores（2D interconnected porous membrane）.Result showed that the membrane wetting occurred rapidly with the decrease of salt rejection for the common interconnected porous membrane.And the water flux dropped sharply within the first hour of operation and a water pocket（internal wetting）was observed in the 2D interconnected porous membrane.However,the distillate conductivity of 2D interconnected porous membrane remained stable without any increase.Based on the experimental results and theoretical analysis,the wetting mechanism of internal wetting caused by condensation and external wetting caused by feed solution was revealed.In addition,this chapter revealed that the excellent wetting resistance of 3D interconnected porous membranes is endowed by the uniform and stable pore structure in three-dimensional space.For the compound pollution containing high salt,surfactant,and oil at the same time,a composite interconnected porous membrane composed of PVA armor and 3D interconnected porous membrane was developed.Utilizing the time difference of PVA from solution to complete gelation,an interlocking structure was formed between PVA armor and the substrate,leading to high mechanical stability of the composite membrane.The gradient pore structure of the PVA armor induced by lyophilization is dense toward the hydrophobic substrate and opens toward the feed solution,which not only retains the interception function,but also reduces the mass transfer resistance.The water flux of the composite membrane without the gradient pore structure was only 17 L·m-2·h-1,while the composite membrane with the gradient pore structure structure was 20 L·m-2·h-1.Results show that the composite membrane with PVA armor achieves synchronous resistance to surfactant wetting,oil contamination and salt erosion.Based on the experimental results and theoretical analysis,the synchronous anti-wetting,anti-scaling and anti-fouling mechanisms of the composite membrane with PVA armor was analyzed.The hydrophilicity of PVA armor prevents oil from contacting hydrophobic substrate.The compactness of PVA armor can reduce the deposition of inorganic salts on hydrophobic substrate.The PVA armor can slow down the delivery of surfactants to the evaporation front through size effects,hydrogen bonding interactions,steric hindrance,and frictional resistance.In addition,the evaporation front control system formed by the pull effect of the PVA molecular network on the feed liquid and the hydrophobic substrate controls the movement of the evaporation front to the permeation side.As a result,the composite membrane with PVA armor exhibited excellent stability during desalination of concentrated seawater containing both surfactant（0.1 mM）and oil（0.5 wt%）.This paper developed MD technology for hypersaline brine concentration from the aspects of process improvement,material innovation and mechanism analysis.The results not only overcome the trade-off relationship between stability and water flux,but also provide a series of strategies for resisting high-concentration inorganic salt erosion and organic pollution,which is of great practical significance for treating hypersaline water streams. |
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