| Membrane-based filtration,due to its small grounding,easy operation and environmental-friendly,has become one of the leading technologies in the field of wastewater reuse and desalination.Among this,membrane distillation（MD）,a thermal separation process with many advantages such as its complete salt rejection and the recoverability of low-grade latent heat,would become an emerging desalination technology,attracting the attention of many researchers.Although membrane distillation has showed these potentials,the concerns of membrane scaling,fouling,and wetting are desired during actual use,and bring lots of difficulties to practical industrial applications.Nowadays,researchers mostly use the combination of nanoparticles（NPs）and fluorine-containing reagents to prepare superhydrophobic-omniphobic membranes in order to achieve stable MD operation.In fact,dip-coating of NPs cannot guarantee uniformity,and would also result in decreasing flux;meanwhile the influence of the physical properties of SiNPs（such as dimension and structure）on the performance of MD is lack of in-depth research.The NPs and fluorine in this modification method will be harmful to the environment the human body,so how to realize the superhydrophobic membranes in a more economical way under the premise of environmental friendliness is also a big problem.Details are presented as follows:（1）Synthesize 200 nm hollow mesoporous SiNPs,and choose two sizes of solid silica nanoparticles（S-SiNPs）at the same time,we apply them to the membrane surface by electrospray or conventional solution deposition,and finally fluoride the surface with FDTS.The six membranes obtained can achieve superhydrophobic-omiphobic properties.The water contact angle of the H200E-PVDF membrane is as high as 167.4±3.1°,and for mineral oil with a surface tension as low as 30.1 m N m–1,the contact angle is still as high as 136.5±3.3°.The hollow mesoporous structure can effectively restore the flux drop caused by the presence of the large-particle nanoparticle coating.The hollow mesoporous SiNPs coating with the same size（200 nm）can increase the flux by 20-50%than S-SiNPs,where the intrinsic cavities provided extra channels for vapor transport.More importantly,using a high-voltage electric field in the electrospray procedure resulted in a uniform distribution of SiNPs,thereby endowing an ideal membrane surface layer for a robust MD operation.When the dimension of NPs decreases,the coating tends to form micro-nanometer hierarchical structures and re-entrant textures,resulting in larger contact angles and more stable MD performance.（2）Through the stoichiometric reaction of OTS containing long-chain alkyl groups with water under ambient conditions,a superhydrophobic coating is formed on the electrospun polyvinylidene fluoride（PVDF）membrane.Since the reaction generates a compact and highly oriented self-assembled monolayer on the surface of the membrane,the modified membrane（O-PVDF-E）exhibits an ultra-high water contact angle（165.3±3.5°）and an ultra-low sliding angle（8±3°）compared to the pristine electrospun PVDF membrane（PVDF-E）;for the140m M SDS with a surface tension of 32.5 m N m–1,the contact angle of O-PVDF-E is also above 90°,indicating that the modified structure has good resistance to LST liquid.In addition,it showed better performance and more stable flux for the MD treatment of 3.5 wt%Na Cl solution and 14 m M gypsum solution than PVDF-E membrane.For the 3.5 wt%Na Cl solution,the flux of O-PVDF-E remains stable when the water recovery reached 50%,and there was no observed scaling on the surface,while the flux of PVDF-E decreased by 20%;for the 14 m M gypsum solution,the flux of PVDF-E decreases by 69%when the water recovery rate reached40%,while the flux of O-PVDF-E remains stable when the water recovery rate got 50%,without obvious scaling phenomenon.Overall,we developed a promising superhydrophobic surface modification for MD application,and highlight its great potential for treating salty water samples. |
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