Flower-like ZIF-L/Membrane Composite For Adsorption And Separation Of Heavy Metal Ions

Humans and the environment are threatened by heavy metal ions such as Cd²⁺,Cr³⁺,and Pb²⁺in sewage.Membrane separation technology has been widely studied for the treatment of heavy metal ions due to its advantages of high separation efficiency,low energy consumption,and easy recovery and reuse.However,MOFs are highly susceptible to polymer encapsulation,which limits adsorption performance;compounding MOFs/polymer thin layers on a porous substrate can effectively alleviate encapsulation,but the small pore size of the substrate restricts the water flux of the membrane.Therefore,it is crucial to investigate separation membranes with both high flux and high barrier properties.In this work,a flower-type Zn-MOF material with high efficiency in adsorption of heavy metal ions was synthesized,and combined with high porosity nanofibers and polymer substrates to prepare nanocomposite membranes with high separation efficiency.Firstly,a flower-shaped Zn-MOF material with elongated branches（ZIF-L）was prepared using H₂O₂ as a modifier.The high aspect ratio“petal”structure of flower-type ZIF-L increases the effective adsorption area and exposes more active sites,and its adsorption performance far exceeds that of traditional polyhedral structure ZIF-L,and especially the adsorption of Cd²⁺was increased by 70.8%.However,flower-type ZIF-L is difficult to be recovered in water treatment,and the preparation of ZIF-L composite membrane can effectively solve this problem.The PI@PDA@PEI/ECH/ZIF-L-4 composite membrane was prepared by coating polydopamine（PDA）on polyimide（PI）nanofibers with high porosity and strong chemical stability,and bonding ZIF-L-4 to PI fibers with polyethyleneimine（PEI）/epichlorohydrin（ECH）as the binder.The composite membrane combined the high permeation capacity of PI fiber and the high adsorption capacity of ZIF-L-4.It can also ensure a high removal rate during rapid separation,where the permeation flux reached 1140.02 L/（m²·h·bar）,which can remove 80.53%of Cd²⁺,96.80%of Cr³⁺,and99.63%of Pb²⁺from the mixed heavy metal solution.In addition,ZIF-L-4 is firmly bonded to the fiber,demonstrating excellent cycling stability.However,due to the large pore size of the membrane,it can only treat low concentration heavy metal solutions based on adsorption.The dense ZIF-L-4 composite membrane can further promote the separation capacity by combining the size exclusion effect of the membrane and the high adsorption property of flower-shaped ZIF-L-4.The three-layer composite structure of PI@ZIF-L-4@PA membrane was constructed by depositing ZIF-L-4 on PI fibers by vacuum-assisted method,while an ultrathin polyamide（PA）layer was prepared on top of it by interfacial polymerization,which has a higher removal rate in the environment of heavy metal solution with high concentration.The membrane showed a higher removal rate in the environment of heavy metal solution with high concentration,especially can remove 99.99%of Cr³⁺from the solution.Therefore,through the above modulation of MOF morphology,selection of substrate materials and structural design of membranes,highly efficient heavy metal separation membranes were prepared,which provide new design ideas for the preparation of separation membranes with high barrier rate and high flux.