Fabrication And Desalination Performance Of Non-Metallic-Doped Silica Membranes

The shortage of freshwater resources caused by population growth,climate deterioration and industrial pollution has become one of the biggest challenges for the development of human society.Seawater desalination is currently the most effective way to alleviate the shortage of fresh water resources.Membrane desalination represented by reverse osmosis is the most common seawater desalination technology.Compared with reverse osmosis,pervaporation has unique advantages and has potential application prospects in seawater desalination,especially the treatment of high-salt wastewater,and high-performance membrane materials are the key to realizing industrial application of pervaporation desalination.The silica membranes have attracted widespread attention due to its good thermal stability,chemical stability and anti-fouling properties.However,pure silica membranes are easily cause hydrolysis of Si–O–Sibonds when exposed to hydrothermal environment,leading to a degradation of network structure and selectivity decreases.In this thesis,bis（triethoxysilyl）ethane（BTESE）used as the silicon precursor,and the non-metallic doping strategy aims to improve the hydrothermal stability and desalination performance of silica membranes.The research content of this thesis mainly includes the following two parts:（1）B-SiO₂ hybrid silica membranes were fabricated with BTESE as silicon precursor and triethyl borate（TEB）as boron source by the sol-gel method.The effects of fabrication conditions such as the ratio of boron to silicon（B/BTESE）,the ratio of water to silicon（H₂O/BTESE）,the ratio of acid to silicon（HNO₃/BTESE）and calcination temperature on the micropore structure and desalination performance of the membrane were systematically investigated.The FT-IR,XRD,XPS and other related characterizations confirmed that boron elements were successfully doped into the network structure of silica membranes,forming hydrothermally stable Si–O–B bonds,which can significantly affect the microporous structure and desalination performance of the membrane.The optimized B-SiO₂hybrid membrane exhibited good desalination performance and hydrothermal stability.The water flux achieved 23.2 kg m^-2 h^-1 and salt rejection nearly 100%when desalinating 3.5 wt%NaCl solutions at 60 ^oC.In addition,the salt rejection of the membrane remained above 99%in the whole 240 h cycle test,which shows excellent stability.（2）In order to further improve the desalination performance and stability of the silica membrane,F-SiO₂ hybrid membrane with ammonium fluoride（NH₄F）as the fluorine source and BTESE as the silicon precursor was prepared by the sol-gel method.The effects of fabrication conditions such as the ratio of fluorine to silicon（F/BTESE）,the ratio of water to silicon（H₂O/BTESE）,the ratio of acid to silicon（HNO₃/BTESE）and the concentration of BTESE in the synthetic sol on the membrane pore structure and desalination performance were systematically investigated.The optimal preparation condition of F-SiO₂ membranes is BTESE:NH₄F:H₂O:HNO₃=1:0.05:200:0.10,the concentration of BTESE is 1 wt%and the calcination temperature is 300 ^oC.The optimized F-SiO₂ hybrid membrane shows excellent desalination performance and acid resistance.The water flux and salt rejection towards 3.5 wt%NaCl solution at 30 ^oC can achieved 13.3 kg m^-2 h^-1and 100%,respectively.In addition,the membrane remains over 99%salt rejection after immersing in 90 wt%acetic acid solution for 1 month,which shows excellent acid resistance and stability.In this thesis,B-SiO₂ and F-SiO₂ hybrid membranes were prepared by the non-metallic doping strategy,and the effects of preparation conditions on the microporous structure and desalination performance of silica membranes were discussed.The research results have certain guiding significance for the desalination application of silica membranes.