Fabrication Of Composite Nanofiltration Membranes Based On Metal-Polyphenol Complex And Its Application In Water Treatment

As an emerging membrane separation technology,nanofiltration membrane separation technology can effectively separate 200-1000 Da molecules and multivalent ions,and has broad application prospects in dye wastewater treatment and seawater purification.At present,the nanofiltration membranes on the market are mainly composed of thin file composite membrane structures and the separation performance of it is mainly determined by the separation layer.Therefore,designing and optimizing the internal molecular network structure of the separation layer is the key to the preparation of high-throughput and high-selectivity nanofiltration membranes.Polyphenols are widely present in the roots and leaves of various plants.Due to the fact that the rich catechol or pyrogallol groups in tannic acid（TA）structure can readily coordinate with transition metal ions,and the formation of metal polyphenol complexes is guided by the adsorption of polyphenols and pH-dependent multivalent coordination bonds.The morphology and porosity of the separation layer can be controlled by adjusting the molecular structural units,so it is widely used in nanofiltration membrane separation.In this paper,TA and metal ions are used to construct selective layers of different membrane materials and surface modification to construct ZIF-8 membranes.The separation performance and modification mechanism of thin film composite membranes under different modification strategies were systematically investigated.A novel method based on layer-by-layer（LBL）self-assembly for preparation of TA-Fe³⁺complex based NF membrane was proposed.The membrane morphologies were characterized by SEM shows the top surface and cross-sectional SEM images of the membranes with different bilayer numbers and different pH values,explained the formation mechanism of the TA-Fe³⁺complex layer,and adjusted the pH value to make mono-complex state gradually transforms to tris-complex.The contact angle test showed that the hydrophilic groups and nanostructured protrusions on membrane surface endow the membrane a superhydrophilicity property.The optimal membrane showed a high-water permeability of 40.9 L m^-2 h^-1 bar^-1 and rejections higher than 93.9%towards various dyes with a molecular weight from 320 to 1017 Da.Moreover,the as-prepared NF membranes showed stable long-term separation performance and good ion/organic selective separation performance.This study not only proposed a facile fabrication process for TA-Fe³⁺complex based NF membrane,but also provided great potentials for sustainable molecular separation by the prepared membranes.A ZIF-8 membrane was fabricated by using a plant polyphenol tannic acid layer as an intermediate layer to overcome this issue.It was found that the abundant pyrogallol groups in TA layer could promote heterogeneous nucleation and contribute to a continuous and intergrown ZIF-8 thin film.Fourier transform infrared spectroscopy results prove that TA and ZIF-8 has successful immobilized on PES substrate surface,and X-ray photoelectron spectroscopy and thermogravimetric analysis showed that TA as an intermediate layer affects the growth of ZIF-8 film on the membrane surface.Scanning electron microscopy and atomic force microscopy found that the abundant pyrogallol groups in TA layer could coordinate with Zn²⁺ions,promote heterogeneous nucleation and contribute to a continuous and intergrown ZIF-8 thin film.The as-prepared ZIF-8 membrane showed a pure water permeance of 3.6 L m^-2 h^-1 bar^-1 with NaCl and Na₂SO₄ rejections of 64.7%and 92.2%respectively.This membrane is one of few MOF membranes that have a divalent salt rejection higher than 90%,demonstrating its great potential application in efficient desalination.A continuous and ultra-thin metal-phenolic network top layer on a polyethersulfone substrate surface was used as metal precursor,immersing into a 2-methylimidazole aqueous solution,and then a continuous and ultra-thin ZIF-8 top layer was facilely formed.X-ray photoelectron spectroscopy and X-ray diffraction demonstrated the formation of ZIF-8 on membrane surface.Fourier transform infrared spectroscopy,thermogravimetric analysis and ultraviolet-visible absorption spectroscopy illustrated(TA-Zn²⁺)₂/PES membrane surface to form ZIF-8.Scanning electron microscope,atomic force microscope and contact angle test system investigated the LBL process provides sufficient controllability of preparing a continuous and ultra-thin ZIF-8 top layer,which are crucial factors for nanofiltration membrane separation performance.The optimized ZIF-8membrane shows a pure water permeability of 5.1 L m^-2 h^-1 bar^-1,a 55.2%NaCl rejection and a 93.6%Na₂SO₄rejection.The novel zinc source of ZIF-8 synthesis disclosed in this study can provide a new incentive to MOF membrane preparation.