Preparation Of Polyamide Composite Reverse Osmosis Membrane And Its High-flux Desalination Performance

Reverse osmosis?RO?technology can efficiently get rid of small molecular contaminants and salts from water and is believed as one of the promising ways to address the water shortage problem.However,the current mainstream RO membranes still own poor permeability,the energy consumption and operating cost of RO process are still high,which limits the application and promotion of RO technology.The reason lies in the trade-off between membrane permeability and selectivity,which makes it difficult to ensure the desalination performance of polyamide RO membrane under high-flux conditions.Therefore,it was attempted in this paper to achieve an improvement in the permeability of polyamide thin film composite?PA-TFC?membranes,without compromising its desalination performance.In order to achieve this goal,the support layer and rejection layer of PA-TFC membranes were modified to reduce the resistance of both layers.First,hydrophobic SiO₂ nanoparticles were used to blending modify polyvinylidene fluoride?PVDF?materials to prepare a SiO₂/PVDF membrane.Then the SiO₂/PVDF-PA-TFC membrane was prepared by the interfacial polymerization process with m-phenylenediamine and trimesoyl chloride as polymerization monomers.The interaction between SiO₂ nanoparticles and PVDF polymer molecules was studied and the effect of the SiO₂ content on the structure and properties of PVDF membrane was investigated.A 2000 mg/L Na Cl solution was used to simulate high-salt wastewater to evaluate the permeability and desalination performance of SiO₂/PVDF-PA-TFC membrane.The results showed that at the 0.5 wt.% silica content,the SiO₂/PVDF substrate had the best compression resistance property and the permeability of the SiO₂/PVDF-PA-TFC membrane was improved to 3.16 L m^-2 h-1 Bar-1.The hydrophobic SiO₂ nanoparticles addition induced the PVDF crystal structure variation from ? phase to ? phase.This variation enhanced the hydrogen bonds between the PVDF polymer molecules,narrowed the finger-like pore size and thickened the pore wall of the SiO₂/PVDF membrane.The enhanced intermolecular hydrogen bond and the variation of the finger-like pore structure could improve the compression resistance property of the PVDF substrate,reduce the support layer resistance and enhance the permeability of the PA-TFC membrane without damaging its desalination performance.In consideration of that the dense structure of polyamide rejection layer was the main source of membrane resistance,the rejection layer of TFC membrane was modified with multiwalled carbon nanotubes?MWCNT?to improve the TFC membrane permeability.First,the hydrophilic MWCNT framework was deposited on PVDF substrate surface by negative pressure filtration and then the polyamide material was filled into the framework by the interfacial polymerization process under a negative pressure condition.Utilizing this method,the novel MWCNT/PA composite rejection layer was fabricated successfully.The effects of hydrophilic MWCNT framework on the surface morphology,hydrophilicity,permeability and antifouling performance of polyamide rejection layer were analyzed.The results showed that the MWCNT/PA composite rejection layer significantly enhanced the TFC membrane permeability,while maintaining its desalination performance.The permeability and salt rejection of TFC membrane were raised to 5.65 L m^-2 h-1 Bar-1 and 98.6% respectively,under the optimized MWCNT density of 1.92 g/m2.This phenomenon was attributed to the larger amount of hydrophilic MWCNTs which were embedded into the rejection layer working as water channels.In above experiment,it was found that most of the MWCNTs in the polyamide rejection layer were inclined or horizontal,which did not fully utilize the one-dimensional water channel function of MWCNTs.Therefore,mesoporous silica nanoparticles?MSN?with abundant three-dimensional nanochannels were applied to modify the rejection layer of the TFC membrane to further improve the TFC membrane permeability.Vinyltrimethoxysilane?VTMS?was grafted on the PVDF membrane surface by a free radical polymerization method firstly.The effects of grafting temperature,grafting time,azodiisobutyronitrile?AIBN?concentration and VTMS concentration on the VTMS graft density of PVDF membrane surface were investigated.The results shew that under optimized conditions of a grafting time of 120 min,a grafting temperature of 72 ?,an AIBN concentration of 8 wt.%,as well as a VTMS concentration of 9 wt.%,the VTMS graft density on the PVDF membrane surface was improved to 3.04 mg/dm2.Then MSNs were uniformly grafted on the VTMS/PVDF membrane surface through a dehydration reaction between the hydroxyl groups on the surface of hydrophilic MSNs and the silicon hydroxyl groups?Si-OH?generated by hydrolysis of a methoxide group?Si-OCH3?in VTMS.The effects of grafting temperature,grafting time,p H value and MSN concentration on the MSN graft density of VTMS/PVDF membrane surface were further investigated.The results shew that under optimized grafting conditions of 2 h grafting time,40 ? grafting temperature,p H=10,5 mg/m L MSN concentration,the graft density of MSNs on the VTMS/PVDF membrane surface was improved to 8.73 mg/dm2.Finally,MSNs were embedded into the polyamide rejection layer through an interfacial polymerization process between MPD and TMC.The morphology and permeability of the MSN/PA-TFC membrane were further investigated.The results shew that under the optimized MSN graft density of 8.73 mg/dm2,the permeability of MSN/PA-TFC membrane was improved to 6.14 L m^-2 h-1 Bar-1 and the salt rejection was maintained above 95%.This phenomenon can be attributed to the large amounts of MSNs which had been grafted into the polyamide rejection layer.The three-dimensional nanochannels within MSNs were able to work as particular paths for water molecules to pass through and thus improve the permeability of polyamide rejection layer without compromising its desalination performance.In order to further improve the permeability of the TFC membrane,the support layer and the rejection layer of the TFC membrane were modified simultaneously.A SiO₂/PVDF-MWCNT/PA-TFC membrane and a SiO₂/PVDF-MSN/PA-TFC membrane was prepared,respectively.The effects of the SiO₂/PVDF substrate on the MWCNT/PA rejection layer and the MSN/PA rejection layer were investigated.The experimental results shew that the permeability of SiO₂/PVDF-MWCNT/PA-TFC membrane and SiO₂/PVDF-MSN/PA-TFC membrane was further improved to 6.73 L m^-2 h-1 Bar-1 and 6.56 L m^-2 h-1 Bar-1,respectively.Due to the decrease of the MSN graft density on the SiO₂/PVDF substrate surface,the permeability improvement of the MWCNT/PA-TFC membrane was greater than that of the MSN/PA-TFC membrane.