Preparation And Desalination Performance Of Cordierite Ceramic Membranes Modified By Organosilicon Compounds

The production of freshwater by desalination has great significance in human life and economics.Membrane distillation（MD）is considered a promising technique for desalination.As a popular candidate for MD desalination,FAS（fluoroalkylsilane）-modified ceramic membrane suffers from insufficient stability,such as membrane wetting and decline of salt rejection during long-term operation,because of the degradation of hydrophobicity.The present work aims to enhance the hydrophobic and desalination stability of ceramic membranes via organosilicon compounds modification.The modification is realized by two methods,namely,polydimethylsiloxane（PDMS）soaking and constructing a porous SiO₂@PDMS hydrophobic protective layer on the top of the membrane.The phase composition,pore structure,surface morphology,N₂permeability,surface wettability,surface roughness,and chemical structure of porous cordierite ceramic membranes were characterized by an X-ray diffractometer,bubble point method,scanning electron microscope,N₂ permeation device,optical contact angle measuring instrument,optical profilometer,and solid-state nuclear magnetic resonance spectrometer,respectively.The desalination performance and long-term stability of the ceramic membranes were evaluated in detail.Porous cordierite ceramic membranes were prepared by dry-pressing and solid-phase sintering process.The concentration of polymethylmethacrylate（PMMA）and ball-milling time greatly influence the pore structure of the porous ceramic membranes.The cordierite membranes with a PMMA（10μm）concentration of 40 wt%and the ball-milling time of 16 h possess a desirable pore structure with a mean diameter of 0.364μm.The ceramic membranes were soaked in a PDMS solution for modification.The modified membranes have a water contact angle（WCA）of only about 130°and exhibit a liquid entry pressure（LEP）of less than 100 k Pa.The soaking time has little effect on the hydrophobic property and N₂ permeance of the membranes.The concentration of PDMS solution has a significant impact on the N₂ permeance of the membranes.The incorporation of SiO₂ fails to improve the hydrophobicity of the ceramic membranes,and results in a considerable decrease in N₂ permeance.The PDMS soaking modification can not ennhance the desalination stability of the ceramic membranes.A novel strategy is proposed to enhance the long-term desalination stability of FAS-modified cordierite ceramic membranes by constructing a porous SiO₂@PDMS（polydimethylsiloxane）protective layer on their tops to obtain a stable hydrophobic property.The results show that such a layer can be fabricated using a two-step spraying method with hydrosoluble NaCl particles as templates.The optimal spraying parameters are as follows:spraying NaCl solution for 15 s at a distance of 20 cm;spraying SiO₂@PDMS solution for 15 s at a distance of 10 cm.The incorporation of SiO₂ nanoparticles into PDMS generates a loose and porous structure in the layer and renders an enhanced hydrophobicity.At a SiO₂/PDMS mass ratio of 0.30（N15s-P15s-0.30S）,the dissolution of NaCl templates leaves behind cavities within the layer,which helps to reduce the vapor transport resistance.The WCA reaches 155.8°,surface free energy（SFE）decreases to 0.22 m N m^-1,the surface roughness rises from 4.156 to 7.582μm,and the LEP increases remarkably to 165 k Pa.In addition,the SiO₂@PDMS layer exhibits exceptional resistance to both mechanical and chemical corrosion.The N15s-P15s-0.30S membrane has a water flux of 8.15 kg/m²h and a salt rejection of 99.99%under the following conditions:the feed temperature of 80℃,the feed flow rate of 300 m L/min,and the NaCl concentration of 3.5 wt%.The FAS-modified membrane is stable on desalination within only 9 h,and the salt rejection decreases to 48%due to the water leakage on the permeate side.The N15s-P15s-0.30S membrane is highly stable upon long-term desalination（>300 h）and preserves a steady water flux of 8.10 kg/m²h and a salt rejection close to 100%.The SiO₂@PDMS layer is responsible for the outstanding desalination stability since it prevents membrane wetting by reducing the direct contact of the reflux feed solution to the underlying cordierite ceramic membranes.After desalination for 300h,membrane fouling occurs on the surface of the N15s-P15s-0.30S membrane,resulting in a decline of the WCA to128°and a slight decrease of the N₂ permeance.The N15s-P15s-0.30S membrane was also used for natural seawater desalination.For the seawater after filtration,the salt rejection is larger than 99.99%.The Ca²⁺and Mg²⁺ions in seawater are easy to form insoluble scalings to block the pores on the surface of the membrane,resulting in the rapid decline of water flux and WCA.The scalings on the surface can be effectively removed by dilute hydrochloric acid,and the water flux can be restored.For the seawater after hardness removal and pH adjustment,the salt rejection is larger than 99.99%.NaCl crystals precipitate on the membrane surface and block a fraction of pores,leading to a slower decline of water flux and WCA.The fouling membrane after water cleaning can effectively restore water flux,N₂permeance and WCA.It shows that removing hardness and adjusting the pH of seawater before desalination can significantly inhibit the generation of fouling.