Studies On Preparation And Performance Of Modified Forward Osmosis Membranes

With the growing demand for fresh water resources of the development of human society and the increasingly serious problem of water pollution,membrane separation technologies have been increasingly applied in water treatment process.Forward osmosis(FO),as an emerging branch of membrane separation technology,has attracted wide attention among oversea and domestic researchers due to its many advantages such as low energy input,low fouling and environmentally friendly in recent years.Currently FO has shown great potential in wastewater treatment,seawater and brackish-water desalination,food and pharmaceuticals processing,controlling drug release and electrical power generation.However,FO still faces some critical challenges,one of which is the need for membrane development.FO membranes with improved properties are the key to solve the challenges and play an important role in the future industrial application development.Based on synthesis of FO membranes with high performance,different membrane materials were selected and four modified FO membranes were synthesized through different methods of preparation and modification in this thesis.The effects of different modification methods on surface properties and FO performance of modified FO membranes were studied and the FO performances of modified FO membranes were compared with commercial FO membrane.Nano silica dioxide(nano-SiO2)modified cellulose acetate(CA)forward osmosis membranes were prepared using the addition of nano-SiO2 in the CA casting solution by phase inversion via immersion precipitation technique.SEM analysis indicated that the addition of nano-SiO2 did not change the asymmetric structure of CA membrane but made the spongy structure of the membrane appeared more compact.The data of contact angle,porosity and tensile strength showed that the hydrophilicity,porosity and tensile strength of the membranes were improved with the increase of the content of nano-SiO2 in the CA polymer system.ATR-FTIR and EDX analysis illustrated that the nano-SiO2 did not react with the C A but consisted in the CA organic system as an additive.The results of determination of FO performance showed that the water flux and salt rejection were simultaneously improved in the nano-SiO2 modified CA FO membranes.However,it is noteworthy that the degree of FO performance improvement resulting from nano-SiO2 modification is very limited.It is concluded that the modified FO membrane is not sufficient to meet the demand of ideal FO membranes.Modified cellulose acetate(CA)membranes using carboxylated functionalized MWCNTs(FMWCNTs)were synthesized by phase inversion via immersion precipitation technique.ATR-FTIR analysis confirmed the existence of FMWCNTs in the modified membrane system and also illustrated that FMWCNTs reacted with CA and formed crosslink structures on the surface of the modified membrane which could be seen from SEM images.SEM analysis showed that the incorporation of the functionalized MWCNTs did not change the asymmetric structure of CA membrane but made the finger-like structure more compact.The data of contact angle,porosity and tensile strength showed that the surface hydrophilicity,porosity and tensile strength increased with the increment of FMWCNTs content in the casting solution.The results of determination of FO performance showed that the synthesized CA/FMWCNTs FO membranes exhibited superior performance than CA and commercial CTA-W FO membranes.The water flux(in both AL-FS and AL-DS modes)and salt rejection were simultaneously improved in the CA/FMWCNTs membranes.However,further addition of FMWCNTs might affect the immers-ion precipitation process and decrease the selectivity of the membranes.BaSO4-based mineralized thin-film composite(TFC)forward osmosis(FO)membranes were fabricated through depositing barium sulfate on the surface of the prepared polysulfone/polyamide(PSf/PA)membranes by adopting an approach named surface mineralization.BaS04 particles were deposited by an alternate soaking process(ASP)with aqueous solutions of barium chloride(BaCl2)and sodium sulfate(Na2SO4),separately.EDX and XPS analysis confirmed the existence of BaSO4 in the modified membrane system.The calculation results of mineralization degree(MD)demonstrated that MD of modified membranes could be controlled through changing the ASP cycle number.SEM analysis showed that BaSO4 particles were evenly distributed on the membrane surface and the coating did not increase the thickness of the skin layer of the TFC membrane but made the membrane surface denser and smoother which were illustrated by AFM data.Contact angle and zeta potential data indicated that the surface hydrophilicity and negative charge of the mineralized TFC membranes were both enhanced with the increase of mineralization degree.The results of determination of FO performance showed that the synthesized BaSO4-based mineralized TFC membranes exhibited superior FO performance than original TFC and commercial CTA-W FO membranes.The surface mineralization simultaneously enhanced water flux and salt rejection of membranes in both AL-FS and AL-DS modes.However,excessive deposition of BaSO4 might decrease the permeability and selectivity of the membranes.AgCl-based mineralized thin-film composite(TFC)forward osmosis(FO)membranes were fabricated through depositing silver chloride on the surface of the prepared polysulfone/polyamide(PSf/PA)membranes by adopting an approach named surface mineralization.AgCl particles were deposited by an alternate soaking process(ASP)with aqueous solutions of silver nitrate(AgNO3)and sodium chloride(NaCl),separately.The calculation results of mineralization degree(MD)demonstrated that MD of modified membranes could be controlled through changing the ASP cycle number.SEM analysis showed that AgCl particles were evenly distributed on the membrane surface and the coating did not increase the thickness of the skin layer of the TFC membrane but made the membrane surface smoother which were illustrated by AFM data.Contact angle and zeta potential data indicated that the surface hydrophilicity and negative charge of the mineralized TFC membranes were both enhanced with the increase of mineralization degree.The results of determination of FO performance showed that the synthesized AgCl-based mineralized TFC membranes exhibited superior FO performance than original TFC membranes.Fouling tests using the model foulant BSA showed that the mineralized membranes displayed lower water flux decline ratios in BSA aqueous solution and higher water flux recovery ratios after simple hydraulic washing.However,we note that excessive deposition of AgCl could add an additional layer of resistance to permeation through the membrane and thus decline the water flux,selectivity and antifouling property of the membranes.