Controllable Fabrication And Desalination Performance Of BTESM-derived Microporous Silica Membranes

With the growth of the world economy,the problem of water pollution is becoming increasingly serious,and the shortage of fresh water resources has become one of the key problems restricting global development.The pervaporation desalination technology using membrane method can efficiently provide fresh water resources,and high-performance membrane separation materials play a key role for application.Silica membranes have attracted wide attention due to their unique subnanoscale porous structure and tunable physicochemical properties.However,there are few related reports on the application of seawater desalination and high salinity wastewater treatment and its pervaporation mechanism.In this dissertation,a new organosilane with Si-C-Si bond,bis?triethoxysilyl?methane?BTESM?,was used as the silane precursor.By adjusting the synthesis conditions,high-performance BTESM-derived microporous silica membranes were fabricated.The membrane preparation conditions,characterization and desalination performance were systematically investigated.The details results are as follows:?1?Silica membranes with high performance were prepared by sol-gel method using BTESM as silane precursor and hydrochloric acid as catalyst.The optimum preparation conditions were as follows:the sol molar ratios of BTESM:HCl:H₂O=1:0.33:100 and the calcination temperature of 200 ^oC.By changing the amounts of hydrochloric acid and water,and the calcination temperature,the pore size of silica membranes was precisely regulated to be below 0.60 nm.For pure water,3.5 wt%and15.0 wt%NaCl as feed solutions,the permeated water flux of the optimized BTESM-SiO₂ membranes were 30.6,15.9 and 12.2 kg m^-2 h^-1,respectively at 60 ^oC.When the feed temperature rised to 90 ^oC,the water flux of this membrane were 92.2,46.6 and 32.8 kg m^-2 h^-1,respectively,and the NaCl rejection was almost unchanged and close to 100.0%.The presence of Si-C-Si bonds in the precursor could improve the membrane stability meanwhile increase the permeated water flux,enabling accurate sieveing of water molecule and different salt ions.Under the operating conditions of feed temperature ranges of 30-75 ^oC and salt concentration of 0.3-5.0wt%,the optimized BTESM-derived silica membrane also showed excellent water flux and salt rejection in the Na₂SO₄,MgCl₂,MgSO₄ and CaCl₂ salt solutions.In these processes of pervaporation desalination,the apparent activation energy E_j range was about?15.8±0.4²8.7±0.7?kJ mol^-11 and the permeability activation energy E_p range was about?-14.5±0.6^-27.5±0.3?kJ mol^-1.Thus,the difference between the two type activation energies was similar to the evaporation enthalpy of water(42.0⁴9.0kJ mol^-1).Such the optimized BTESM-derived silica membrane was stable tested in10.0 wt%NaCl solution at 60 ^oC at different pH ranges of 5-8 for 120 h.The water flux and NaCl rejection were maintained at 10.5±0.5 kg m^-2 h^-1 and 99.9±0.1%,respectively.Such the BTESM-SiO₂ membrane has the potential industrial application prospects in seawater desalination and the high salinity wastewater treatment.?2?Sr-doped BTESM-SiO₂ membranes and Sr-doped TEOS-SiO₂ membranes were successfully prepared by the sol-gel method with BTESM,tetraethyl orthosilicate?TEOS?as silane precursors and SrCl₂ as strontium source,respectively.It was confirmed by FTIR,XRD,TEM and other characterization results that Sr was doped into the SiO₂ framework during the hydrolysis-condensation reactions,forming the hydrothermally stable Si-O-Sr bonds.It could significantly affect the membrane surface morphology,microstructure properties and hydrophilicity,thus improving the desalination performance of these membranes.Compared with the membranes prepared under other conditions,the Sr-7.5 silica membrane?the sol molar ratio as TEOS:EtOH:HCl:H₂O:SrCl₂·6H₂O=1:3.8:0.085:6.2:0.075,calcination temperature of 600 ^oC?showed excellent desalination perforamnce.For the 3.5 wt%NaCl solution,the optimized Sr-7.5 membrane exhibited a high water flux of 20.7 kg m^-2 h^-1 and NaCl rejection of nearly 100.0%at 60°C.In addition,the membrane demonstrated excellent hydrothermal stability for ten times in the continuous cycle tests at temperature ranges of 35-60 ^oC.