Synthesis And Nanofiltration Performance Study Of Composite Membranes Modified By Metal Organic Frameworks

The development and utilization of membrane technology has become the focus of current and future research.According to the difference in membrane pore size and driving mechanism,membrane technology can be classified into microfiltration,ultrafiltration?UF?,nanofiltration?NF?,reverse osmosis?RO?and forward osmosis?FO?.Nanofiltration?NF?is defined in the range between ultrafiltration?UF?and reverse osmosis?RO?,which can effectively separate high-valent ions,small organic molecules and large viruses substances.The application of NF technology is very extensive,especially in the fields of environment,desalination and pharmaceutical biotechnology.However,the single-component nanofiltration membrane is difficult to meet the application requirements of NF in terms of pore size,retention efficiency,water filtration efficiency,etc.,which limits its development to certain extent.To solve this problem,modification methods commonly include interface polymerization?IP?,graft modification and blend modification has been introduced.Presently,metal organic framework materials?MOFs?that constructed by hybrid organic and inorganic are widely used in the modification of membrane materials due to their extremely high surface area and high porosity.Based on the above observations,the classic MOFs material ZIF-8 was selected to construct a continuous crystal functional layer on the surface of the membrane by in-situ modification,and the DNA was introduction to improve the water stability of the ZIF-8 composite membrane.Furthermore,UiO-66-NH₂ with good water stability was used to prepare a positively charged NF composite membrane,and the structure and separation performance of that were studied.This study provides a theoretical and practical basis for the further development and application of NF.Firstly,the polysulfone?PS?ultrafiltration membrane was modified by in-situ growth to prepare a nanofiltration composite membrane with excellent dye retention performance.The a nanofiltration composite membrane ZIF-8/PDA/PS was characterized by IR,XPS,XRD,SEM and AFM,which confirmed the formation of a continuous and compact ZIF-8crystal layer.The nanofiltration performance test of the composite membrane shows that the rejection was almost than 99%for dyes(98.83%for methyl orange and 99%for isatin with the water flux about 20 L·m^-2·h^-1·bar^-1.This work demonstrates a facile method for preparing ZIF-8/PDA/PS composite membranes with outstanding rejection and high-water flux in a simplified enviropmental friendly way.Secondly,using porous polysulfone ultrafiltration membrane as the support layer and the self-polymerization L-dopa to prepare the intermediate layer,two kinds of composition membrane ZIF-8/L-PDA/PS and DNA@ZIF-8/L-PDA/PS were successfully prepared by in-situ growth methhod.The characterization of SEM,AFM and XRD demonstrated that the two types of ZIF-8 membranes both have a continuously dense crystalline layer and higher roughness.What drives attention is,the introduction of DNA molecules can outstanding enhance the hydrophilicity of composite membranes?29.2°?compared with pure ZIF-8 membrane?66.9°?through water contact angle test.Meanwhile,the nanofiltration properties of two prepared ZIF-8 membranes are performed using six different dyes and four kinds of salt,respectively.The results demonstrate that ZIF-8membranes show both similar excellent rejection for dyes and salt?i.e.,dye:>300 Da,almost 99%;salt:Al₂?SO₄?₃,>90%?,however,the DNA-introduced ZIF-8 membrane exhibits higher pure water flux.Furthermore,but the DNA-introduced ZIF-8 membrane also show an excellent water stable and anti-fouling property during long-term filtration performance,which would be of great significance for the reuse of dye wastewater.Finally,the UiO-66-NH₃⁺@PS hybrid matrix composite membrane with selective separation of negatively charged dyes was designed and prepared.Using the prepared UiO-66-NH₃⁺and UiO-66-NH₂ MOF materials as fillers,PS as the cast film material,the screen sieve as the support material layer,DMAC as the solvent,and polyglycol as the pore-forming agent,two NF modified PS composite membranes?UiO-66-NH₂@PS and UiO-66-NH₃⁺@PS?prepared.The IR,XRD,SEM and AFM characterization results confirmed the presence of UiO-66-NH₃⁺and UiO-66-NH₂ particles both inside and on the surface of the membrane,and the roughness of the membrane surface after modification was significantly improved,confirming the success of hybrid modification.Two composite membranes UiO-66-NH₃⁺@PS and UiO-66-NH₂@PS were characterized by EDS,confirming the existence of-NH₃⁺ion in the UiO-66-NH₃⁺@PS composite membrane,which proves that the composite membrane has a positive charge.Through the study of the NF performance of the composite membrane,it was proved that the special rejection properties of UiO-66-NH₃⁺@PS membrane for negatively charged dyes due to the Daonan effect.Among them,the UiO-66-NH₃⁺@PS composite membrane with a doping amount of 5%has the highest rejection rate?>97%?for the charged dye.According to the research on the organic solvent resistance of the composite membrane,it is confirmed that the composite membrane has good organic solvent resistance.