Fabrication And Desalination Performance Of Rare-earth-doped Silica Hybrid Membranes

With the rapid growth of economy and industrial pollution,shortage of fresh water resources has gained considerable attention over the world.Membrane desalination technology represented by reverse osmosis?RO?technology has been rapidly developed to be the most commonly applied technology for seawater desalination.Compared with RO,pervaporation desalination technology has more advantages and becomes one of the most potential technologies for seawater desalination and high-salinity wastewater treatment.However,there are few reports on membrane materials with high water permeated flux and high rejection over 99.9%at room temperature.The pore size of silica membrane is generally about 0.30 to 0.50nm,which has an adjustable pore size and excellent stability,thus it is very suitable for the membrane desalination process.Based on the application field of seawater desalination,the La/Gd-codoped and La/Y-codoped silica hybrid membranes were developed in this work.The preparation,characterization and desalination performance of SiO₂ hybrid membranes were systematically investigated.The details results are as follows:?1?The La/Gd-codoped silica membranes were successfully fabricated by sol-gel method using 1,2-bis?triethoxysilyl?ethane?BTESE?as silane precursors and adding La and Gd nitrates,with nitric acid as catalyst.The optimum membrane preparation conditions were obtained as follows.The sol ratio was as BTESE:?La+Gd?:HNO₃:H₂O=1:0.075:0.1:60?molar ratio,La:Gd ratio of 25:75?,membrane calcination temperature was 300 ^oC,and the pore diameter of porous support was 100 nm.For pure H₂O,0.3 wt%and 3.5 wt%NaCl solutions,the water permeated flux of the optimized La₂₅Gd₇₅-SiO₂ membranes were 16.6,14.3 and 10.2 kg·m^-2·h^-1,respectively,and their NaCl rejection were nearly 100%at 60 ^oC.Rare earth doping was beneficial to improve the hydrothermal stability of silica membranes.Therefore,such the La₂₅Gd₇₅-SiO₂ membrane exhibited excellent hydrothermal stability with temperature cycles and precise separating of water molecules and salt ions in different salt solutions.?2?La/Y-codoped microporous silica membranes were successfully prepared by sol-gel procedure with BTESE,La?NO₃?₃·6H₂O,Y?NO₃?₃·6H₂O and HNO₃ as sol precursors.It was confirmed by various characterization results that La/Y was doped into the SiO₂ framework during the sol-gel process,forming hydrothermally stable Si-O-RE?RE=La or Y?bonds.It could significantly influence membrane surface morphology,microstructure properties and hydrophilcity thus to improve the desalination performance.The optimized La₂₅Y₇₅-SiO₂?La:Y=25:75,mole ratio?membrane exhibited the highest water permeated flux of 10.3 kg·m^-2·h^-1 and NaCl rejection of nearly 100%towards 3.5 wt%NaCl feed solution even at room temperature of 25°C.It was attributed to the lowest water contact angle?strongest hydrophilcity?,smallest membrane pore size?finest microstructure?and lowest value of apparent activation energy?lowest mass resistance?of water permeation transport for La₂₅Y₇₅-SiO₂ membranes,which was compared to other membranes prepared under other conditions.Moreover,the membrane demonstrated excellent stability of simulated continuous tests of seawater desalination for up to 200 h,and showed good hydrothermal stability in continuous test at 25-60 ^oC.Such these rare-earth-doped silica hybrid membranes will have potential application prospects in seawater desalination and high-salinity wastewater treatment.