Studies On Modification And Application Of Polyamide Composite Forward Osmosis Membranes

The rapid growth of industrialization and population has led to an ever-increasing demand for freshwater in human society,and the pollution of water resources has become increasingly severe.Membrane separation technology has developed rapidly in recent years and is playing an increasingly important role in the production of clean water.As a new type of membrane separation technology in the 21st century,forward osmosis(FO)has many unique advantages,such as low energy consumption,high pollutants rejection and low fouling tendency.Therefore,this technology has attracted a wide range of research interests and has shown great potential in the fields of desalination,wastewater reuse,food processing,and infiltration power generation.However,the application of forward osmosis technology is still in its infancy and it is difficult to realize large-scale industrial applications in a short period of time.One of the major problems is the development of a high-performance forward osmosis membrane.In this paper,we aim to improve the structure and hydrophilicity of the substrates of polyamide thin film composite(TFC)membranes.The relationship between the structure and properties of the substrates and the forward osmosis performance of TFC membranes was studied.Then two different membrane modification methods were used to enhance the water flux and salt rejection rate of FO membrane.Finally,the feasibility of forward osmosis technology for phenol wastewater treatment was evaluated.At first,we chose the three factors that have the greatest influence on the performance of the substrate,including the polysulfone concentration,casting solvent and porogen.The differences in the structure and properties of the substrates under different conditions were investigated,and the effects of these differences on forward osmosis performance were studied.The results of scanning electron microscopy(SEM),porosity and permeability tests showed that as the concentration of polysulfone in the casting solution increases,the exchange rate of the solvent and non-solvent during phase conversion process became slower.The precipitation process of polymer gradually changed from instantaneous liquid-liquid phase separation to delayed liquid-liquid phase separation,resulting in the reduction of the finger-like macroporous structure in the substrate and the decrease in porosity.Therefore,the FO water flux decreased.As the solvent of the casting solution changed from N-Methylpyrrolidone(NMP)to N,N-dimethylformamide(DMF),the membrane structure transited from the finger-like long pore to the sponge-like pore,and the porosity decreased significantly.Thus,the mass transfer resistance of the membrane was greatly increased.With the increase of porogen(polyvinylpyrrolidone,PVP)content,the finger-like long pore inside the substrate is more obvious and the pore connectivity is improved,resulting in increased water flux.However,the FO performance decreased when the PVP content is too high.The experimental results showed that the structure and properties of the substrate directly affect the forward osmosis properties of the TFC membrane,and the structure and morphology of the substrate mainly depends on the exchange rate of solvents and non-solvents during the phase inversion process and the content of additives.In the scope of this study,the optimal substrate was achieved when the content of polysulfone is 14～18%,the solvent is NMP,and the content of PVP is 0.5wt%.Based the research above,hydrophilic nano-SiO2 was used as a modifier.Different contents of nano-SiO2 particles(1 wt%,3 wt%,and 5 wt%)were added to the casting solution to prepare a thin film nanocomposite(TFN)forward osmosis membranes with improved permeability.The modified membranes were characterized by ATR-FTIR and EDX.The results showed that the SiO2 nanoparticles were only used as additives in the form of physical blending and no new chemical bond was formed with the polysulfone molecule.The measurements of the contact angle,porosity and permeability of the modified membranes showed that the nano-SiO2 modification improved the wettability,increased the porosity and reduced the structural parameters of the substrates,and greatly improved the pure water permeate flux.However,the nano-particles tended to agglomerate when the content of SiO2 particles was relatively high,but it did not affect the integrity and compactness of the active layer of the composite membrane.There was a clear trade-off relationship between the pure water permeability coefficient and the salt permeability coefficient of the modified membrane,and the B/A value increased with the increase of the nano-SiO2 content in the substrates,indicating that nano-SiO2 can be a good modifier to increase the water flux of the polysulfone membrane,it was unable to improve the selectivity of the TFN membranes.Finally,the FO performance test results showed that the nano-SiO2 modification could significantly increase the water flux of the composite forward osmosis membrane,and the water flux increases from 18.2 LMH for the TFC membrane to 41.9 LMH for the TFN-5 membrane.However,the reverse salt diffusion flux also increased with the increase of the nano-SiO2 content in the substrate.And excessive nano-SiO2 would have a serious negative impact on the performance of the forward osmosis membrane.Polysulfone substrates were modified by poly-dopamine(PDA)in three different forms(top surface,bottom surface,and both surfaces)using surface modification method.The PDA modified layer was characterized by ATR-FTIR,X-ray energy spectrum(XPS),SEM,contact angle and permeability properties.The results showed that the PDA layer was successfully prepared on the surface of the PSf substrate.A large number of hydrophilic groups were introduced onto the membrane surface,which improved the hydrophilicity of the surface of the PSf substrate and reduced the ICP effects in the forward osmosis process.The salt rejection of the TFC PDA@top PSf membrane was significantly increased,while the TFC PDA@bottom PSf membrane showed significantly improved water permeability.This indicated that the PDA layer played different roles under different modification methods.PDA modification on the top surface of the PSf film helped to improve the the integrity and compactness of the polyamide active layer in the interfacial polymerization process.Meanwhile,the PDA modification on the bottom surface of the PSf could significantly improve the hydrophilicity of the substrate,because dopamine molecules could easily enter into the membrane through the large hole on the bottom surface of the substrate and attached to the pore wall.Finally,the FO water fluxes of the three PDA-modified TFC membranes were significantly modified compared to the original membranes.The water flux of the TPC PDA@bottom PSf membranes was significantly higher than that of the original membranes.The TFC PDA@top PSf membrane had the lowest reverse salt diffusion flux and showed a significant improvement over the original membrane.Therefore,the TFC PDA@dual PSf membrane prepared by combining the advantages of both methods did not cause a large amount of loss of exfoliation while achieving high water flux,and overcomes the trade-off problem,so the performance of TFC PDA@dual PSf membrane was much better than that of the SiO2 modified one.Phenol removal from water in FO process using polyamide and AgCl mineralized thin film composite membranes has been systematically investigated.The influence of operating parameters on phenol rejection and phenol adsorption was investigated to elucidate the phenol transport behaviors in FO process.The factors included membrane orientation,phenol concentration,draw solution concentration,pH and ionic strength.Results show that phenol rejection could be improved by increasing the draw solution concentration or pH of feed solution.The highest phenol rejection could reach 97.0%for TFC membranes and 98.8%for MTFC membranes when the pH of feed solution was 1 1.The behavior of phenol adsorption on membrane surface may be related to the solute hydrophobic character,electrostatic interaction and reverse salt diffusion.The long-term fouling experiments show that the phenol fouling in FO process is reversible and can be easily cleaned by physical flush.Besides,the MTFC membranes have a better FO performance than the TFC membranes.Finally,the studies in this section have demonstrated that the forward osmosis process using polyamide thin-film composite(TFC)membranes and AgCl mineralized polyamide composite(MTFC)membranes is a viable and potential alternative method to treating phenol wastewater.