The High-performance Polyamide Composite Nanofiltration Membrane With ZIF-8 Intermediate Layer Was Prepared By The Interface Synthesis Method

Nanofiltration?NF?has the characteristics of low operating pressure,no phase change,low energy consumption,no intermediate products,high separation accuracy,high ion selectivity,and high environmental stability.It has been widely used in drinking water treatment,desalination,wastewater treatment as well as the separation,purification and concentration in industry.And it has become one of the main technologies for solving global problems such as water shortages and environmental pollution.The preparation of nanofiltration membranes with high permeability and high selectivity has always been attractive.Introducing nanoparticles into the polyamide thin film composite?TFC?NF membranes is an effective modification strategy.Zeolitic imidazolate framework-8?ZIF-8?is a classic metal-organic framework?MOF?with hydrothermal stability and chemical stability,and its synthesis protocol is simple.The use of ZIF-8 as the additive is extremely attractive for TFC membrane preparation,but its effective introduction is still a challenge.In this study,ZIF-8 fabricated by interfacial synthesis method was served as an interlayer between the polyamide?PA?top layer and the substrate of commercial poly?ether sulfone??PES?microfiltration membrane.The effects of poly?sodium 4-styrenesulfonate??PSS?on the properties of ZIF-8 loaded substrate and the PA layer were comprehensively studied in terms of morphology,structure,surface hydrophilicity/hydrophobicity,surface potential,etc.And the potential mechanism of PSS was discussed.Moreover,the separation performance of the prepared TFC NF mambranes was also evaluated.In the protocol of ZIF-8 interfacial synthesis,PSS was added to the aqueous solution containing Zn²⁺.PSS delayed the ZIF-8 nucleation rate,and promoted the heterogeneous ZIF-8 nucleation.The PSS ionomers were also able to stablize the ZIF-8 form at the water/organic interface,benefiting their homogeneous coverage on the PES mambrane.As a result,the subsequent loading of ZIF-8 onto the PES membrane was not presented in the form of the settlement of individual ZIF-8 nanoparticles,but in the form of layer formed by the interconnected nanoparticles,which greatly improved the ZIF-8 coverage and uniformity on the substrate.When PSS added into the Zn²⁺solution was 1.0 wt%to prepare the PES-ZIF-8-PSS-1.0 membrane,the surface morphology,structure parameters,surface hydrophilicity/hydrophobicity,surface potential and pure water flux of the as-obtained membrane were significantly changed compared with the PES membrane.For the PES membrane,the average roughness?R_a?,rood-mean-square roughness(R_rms),pore diameter,porosity,and the intrinsic contact angle were 51.3 nm,73.5 nm,232.7±12.6 nm,18.9±1.8%,and 37.8±2.4°,respectively.However,for the PES-ZIF-8-PSS-1.0 membrane,the R_a,R_rms,pore diameter,porosity,and the intrinsic contact angle were 33.6 nm,45.1 nm,87.2±10.5nm,34.7±2.1%,and 23.9±3.6°,respectively.In addition,the PES-ZIF-8-PSS-1.0membrane surface showed a more negative potential.The uniform ZIF-8 layer obtained through interface synthesis improved the characteristics of the substrate.Substrates with smaller pore size,higher porosity,more negative surface charge,and lower roughness were obtained.These changes affected the diffusion and distribution of piperazine?PIP?,which in turn affected the interfacial polymerization?IP?process,resulting in the formation of a thinner,smoother,looser,and more negatively charged ployamide?PA?layer.The pure water flux of the TFC NF membrane containing the modified ZIF-8 interlayer was 9.6±0.3 L·m^-2·h^-1·bar^-1,almost twice as high as that of the pristine TFC NF membrane.Meanwhile,the rejections against sodium sulfate?Na₂SO₄?,rhodamine B?RB?,and acid fuchsin?AF?were 91.1±1.4%,94.5±1.4%,and 97.2±1.2%,respectively,which were comparable to the pristine TFC NF membrane.In this study,a new method of preparing TFC NF membranes by introducing ZIF-8 interlayer based on interfacial synthesis was proposed,providing a new idea for preparing high-performance NF membranes by using MOFs.