Development And Application Of PVDF Layer Self-assembled Adsorption Membrane

As one of the most dangerous heavy metals,mercury significantly threatens human health and the environment.Therefore,urgent research and development of adsorptive materials capable of removing mercury ions from aquatic environments is required.Polyvinylidene fluoride（PVDF）is one of the most used film-forming materials due to its excellent film-forming properties,thermal stability,and chemical corrosion resistance.However,its application in the adsorption field is limited because of its inherent hydrophobicity and lack of an active functional group on the surface.On the other hand,dopamine（DA）is widely used in material surface modification due to its high hydrophilicity and excellent reactivity.Using PVDF as a substrate,the strong adhesion-autopolymerization effect and excellent secondary reactivity of DA,as well as the advantages of LBL’s efficient composite membrane preparation technology,supplemented by freeze-drying,a series of DA-induced layer upon layer self-assembly PVDF-based membrane adsorbents（PVDF@PDA-LBL）were prepared.The following are specific research findings and their respective contents:（1）L-cysteine is a suitable ligand for complex heavy metals because it contains–SH,–NH₂and–COOH.Polydopamine（PDA）was coated on the PVDF base membrane by oxidative self-polymerization of DA using the non-solvent phase conversion method described in this paper.Self-assembly grafting of L-cysteine（PVDF@PDA-LCys）was then used to generate a modified membrane with an affinity for the Hg（II）ion.The membrane possesses the following qualities:Upon adsorption of Hg（II）at an initial concentration of 10mg·L^-1,the removal rate reaches 95.14%.It has good hydrophilicity,and the water contact angle drops to 33.67°after 1 s,significantly better than the PVDF base film（89.22°）.With excellent acid resistance,the Hg（II）ion removal rate remains at 70%even when the solution p H is as low as 1.5.After repeated use,the performance of the composite film remains unchanged.（2）Polyethylenimide（PEI）contains many primary and secondary amine groups in its molecular structure,making it an ideal polymer ligand for the complexation-based removal of various heavy metals.Based on PVDF@PDA-LCys,a PVDF@PDA-LCys-PEI membrane was prepared by covalently crosslinking glutaraldehyde（GA）with a PEI layer.The membrane has excellent selective adsorption and removal ability for Hg（II）ions,and the maximum adsorption capacity is 157.6 mg·g^-1.With good hydrophilicity,the water contact angle is 35.73°after 1 s.There is good acid resistance;the removal rate of mercury ions can still reach 83.92%when the solution p H is 1.5.The selective adsorption coefficient of PVDF@PDA-LCys-PEI for Hg（II）ions is 200～3000 times higher of magnitude greater than that of other heavy metals when multiple heavy metals coexist.After five repetitions,the Hg（II）ion removal rate can still exceed 85%.In addition,expensive Hg Cl₂is converted to low-toxicity Hg₂Cl₂via membrane treatment and crystallization,which facilitates removing Hg（II）ions from water and recovering metallic mercury.This paper describes the preparation of a PVDF-based composite membrane that not only contains many active functional groups for chelating heavy metals from functional monomers but also benefits from the substrate polymer’s chemical,thermal,and mechanical properties,resulting in excellent physical and chemical stability and heavy metal adsorption performance.Furthermore,by adjusting the types of organic functional groups on composite materials（e.g.,–SH,–NH₂,–OH–COOH）,achieving the efficient and selective removal of mercury ions from wastewater is possible,laying the theoretical groundwork for its practical application.