High Flux And Superhydrophobic Composite Membrane With Gradient Structure For Membrane Distillation

As the major demand for China industrial development,high salinity wastewater treatment and resources recycle utilization are critical approaches to alleviate water scarcity.However,large-scale membrane separation process via reverse osmosis(RO),nanofiltration(NF)and microfiltration(MF)create highly saline wastewater as an undesirable byproduct,which when discharged directly on land,causes extensive damage to environment.As a friendly and efficient technology,membrane distillation(MD)can simultaneously achieve zero-liquid discharge and resource utilization of high salinity wastewater.However,the insufficient hydrophobicity and fouling resistance of the MD membranes can induce membrane wetting and fouling,which constrains the large-scale application of MD.Therefore,how to prepare superhydrophobic,antifouling and high-flux membranes for membrane distillation via a simple and efficient method is a key issue to be solved urgently.In this paper,the superhydrophobic and high-flux PP composite membranes were prepared through a novelty simple and efficient method and applied to the MD for highly saline wastewater treatment,and its flux stability and antifouling mechanism were thoroughly investigated.Firstly,plasma surface treatment and fluorination modification methods were proposed to prepare hydrophobic microporous polypropylene(PP)composite membranes.Plasma treatment was introduced to enlarge pore size and construct microscale rough structure,thus enhance mass transfer efficiency.Then,the surface chemical reaction of 1H,1H,2H,2H–perfluorodecyltriethoxy-silane reduced surface free energy and enhanced surface hydrophobicity.During the process,the surface morphology and chemical composition of the membranes could be accurately controlled by changing the plasma treatment time and power to achieve hydrophobicity improvement.The resultant F-K-PP/N membrane exhibited excellent superhydrophobicity with a WCA of 150°under the optimal conditions of 50 W treatment for 3 minutes.When applied to VMD for high salinity NaCl and MgCl2 mixed solutions treatment,the F-K-PP/N membrane showed highly initial flux of 18.53 kg/(m~2·h),which was 11%higher than that of the original PP membrane.The fouling rate was only1.27%,which was only 1/3 of the original PP membrane,showing more stable flux and desalination performance.Furthermore,on the basis of plasma surface treatment and hydrophobic modification,the micron and nanoscale SiO2 particles were coated on the surface via surface coating and in-situ sol-gel method to construct hybrid micro/nano gradient rough structure and synergistically improved the hydrophobicity and antifouling properties of the membrane surface.The effects of particle size and loading mass fraction of SiO2 particles on the membrane surface morphology,roughness and hydrophobicity were investigated.The experiment results showed that optimal microscale SiO2 particle size was 0.37μm.Compared with the original PP membranes,the resultant F-nmSiO2-PP/N membranes showed a larger vapor-liquid interface and enlarged effective evaporation area.After hydrophobic modification via fluorination,the average WCA of the obtained F-nmSiO2-PP/N composite membrane could reach 160°and the highest WCA up to 168°,which proved the robust superhydrophobicity and fouling resistance of the F-nmSiO2-PP/N composite membranes.The theory of high efficiency MD membrane preparation was established.The surface gradient structure were optimized and antifouling mechanism was revealed from the perspective of thermodynamics and kinetics through membrane surface dynamic fluid simulation,boundary layer concentration polarization model and membrane surface nucleation barrier model.The simulation results of the fluid velocity and volume distribution at the membrane interface showed that the gradient rough structure could effectively increase the effective evaporation area and intensify the microfluidic field disturbance,thus reducing the negative effect of concentration polarization and eventually prevent crystal deposition.By constructing the model concerning membrane surface roughness and nucleation energy barrier,the effects of membrane surface roughness and hydrophobicity on heterogeneous nucleation energy barrier and the phenomena of crystal nucleation and scaling on the rough surface were evaluated.The results showed that the growth of crystal nuclei on the membrane surface can be avoided via particle size and spatial distribution density optimization to further improve antifouling ability.The fabricated F-nmSiO2-PP/N composite membranes were applied to VMD test for high salinity wastewater treatment.In the application of VMD,the flux stability and antifouling performance were investigated under different feed concentrations and operating condition.The results showed that during the long-term continuous and batch processes,the F-nmSiO2-PP/N composite membranes exhibited extremely high initial flux over 22 kg/(m~2?h),and the fouling rate is only 1/12 that of the original PP membrane,showing excellent antiwetting,antifouling properties and high flux stability.When exposed to a high concentration of 15 wt%NaCl solution,the prepared antifouling and superhydrophobic F-nmSiO2-PP/N composite membranes showed a high flux of about 20 kg/(m~2?h)during the entire operation,and the fouling rate was only 3.4%of the original membrane,maintaining excellent fouling resistance and flux stability under high salinity systems.The initial flux of F-nmSiO2-PP/N composite membranes under different operating conditions is up to about 29%higher than that of the original membrane,and the fouling induction time is up to about 8times.The results proved that the fabricated F-nmSiO2-PP/N composite membranes exhibited excellent comprehensive performance during V MD among various membrane materials.