| The ubiquity of fluoroquinolone antibiotics(FQs)in water environment has caused potential harm to the ecosystem cycling and people’s water safety.Nanofiltration has become an important water purification technology to solve water pollution and water shortage due to its high efficiency in rejecting small molecular organic matters.It has also been gradually applied to the deep purification process of secondary effluent from urban sewage treatment plants.However,the current study on the efficient removal of FQs from wastewater by nanofiltration technology is unsystematic,especially since the relationship among nanofiltration membrane materials,structures,and antibiotic properties is unclear.It restricts the application and development of nanofiltration in wastewater treatment.Hence,in this study,according to the oriented design principle,a series of functional composite nanofiltration membranes were constructed by adopted different fabrication strategies and based on the zirconium-based metal-organic frameworks(Zr-MOFs).The effects of structures and characteristics of Zr-MOFs on the permeability,rejection,antifouling and long-term stability of composite nanofiltration membranes were investigated.The rejection mechanism of the Zr-MOFs composite nanofiltration membranes for FQs removal was deeply discussed.Furthermore,the rejection performance of Zr-MOFs composite nanofiltration membranes for FQs removal in practical wastewater and the simulated secondary effluent system was preliminarily investigated.The specific research work and results are as follows:(1)Based on the UiO-66 MOFs,a highly water-stable Zr-MOFs(UiO66/PGP)composite nanofiltration membrane was constructed using the combination of phase immersion conversion and vacuum-assisted filtration method.As the doping amount of graphene oxide was 0.40 wt%,the polydopamine deposition time was 40 min and the loading amount of UiO-66 was 0.08 μg·m-2,the UiO-66/PGP composite nanofiltration membrane obtained an optimal structure,in which the polydopamine promoted the stable combination between the UiO-66 separation layer and porous substrate,thereby exhibiting high water flux(31.33±0.75 L·m-2·h-1·bar-1)and high stability when treating the dye and antibiotic wastewater.The results showed that the rejection mechanism of the UiO-66/PGP composite nanofiltration membrane for dyes and antibiotics was dominated by size exclusion,but the electronegative congo red,methyl orange,and sulfamethoxazole rejection was supplemented by electrostatic repulsion,while the electropositive methylene blue and ciprofloxacin rejection was supplemented by electrostatic attraction.After continuous operation under the cross-flow mode for 1000 min,the water flux of the composite nanofiltration membrane remained above 80.3%of the initial value,and the rejection rate of methyl orange and tetracycline hydrochloride were above 96.0%and 94.6%,showing a highly water-stable property.Besides,the UiO-66/PGP composite nanofiltration membrane also owned good antifouling performance and low economic cost,indicating that it has the potential application for the treatment of practical antibiotic wastewater.(2)Based on the amino-functionalization UiO-66-NH2 MOFs,a Zr-MOFs composite nanofiltration membrane with a hierarchical porous structure was constructed by an electrostatic layer-by-layer self-assembly method combined with molybdenum disulfide(MoS2).The orderly stacked lamellar MoS2 formed nanochannels to promote the rapid permeation of water and achieved high water flux(78.98±1.45 L·m-2·h-1·bar-1).The chitosan modified UiO-66-NH2(CMOF)also formed hierarchical nanopores and active sites to improve the rejection of ciprofloxacin and ofloxacin(R>94%).The results showed the average pore size of the MoS2/CMOF composite nanofiltration membrane was between the molecular diameter of water and antibiotic,and the membrane surface was positively charged.Therefore,the rejection mechanism for ciprofloxacin and ofloxacin was dominated by size exclusion and electrostatic repulsion.Meanwhile,the strong hydrogen bonding between the MoS2 and CMOF also promoted a hydration layer formed on the membrane surface,weakened the adhesive tension of ciprofloxacin and ofloxacin,and reduced hydrophobic adsorption to achieve efficient rejection.Further studies have shown that the MoS2/CMOF composite nanofiltration membrane could effectively resist membrane fouling and maintained stability in acid and alkali environments.For the treatment of FQs in domestic sewage,surface water,and livestock and poultry wastewater,the MoS2/CMOF composite nanofiltration membrane could also reduce the content of chemical oxygen demand,total phosphorus,total nitrogen,total chlorine,and free chlorine in the water.(3)To improve the interlaminar stability of composite nanofiltration membrane and expand its application,a hydrophilic interlayer modified ZrMOFs composite nanofiltration membrane was constructed by improved interfacial polymerization combined with interlayer modification strategy,applying to remove FQs in aqueous phase and organic phase.Based on the principle of Michael addition and Schiff base reaction,the effects of different concentrations of tannic acid and different deposition time on the morphology and structure of hydrophilic interlayer were investigated.The nanofiltration performance of the composite nanofiltration membrane was also investigated by regulating the content of UiO-66-NH2 in an aqueous solution.The UiO-66NH2 hybrid polyamide layer adhered firmly on a porous substrate and formed a relatively loose separation layer,under the bridging effect and gully effect of the hydrophilic interlayer.The mass transfer resistance in membranes or pores weakened,so the water flux of the membrane increased to 87.23±1.43 L·m-2·h1·bar-1.The nano-scale pores and cracks on the membrane surface were repaired to reduce membrane pore size and enhance surface electronegativity,so the rejection rate of norfloxacin,ciprofloxacin,and levofloxacin reached 92.94±1.60%,94.62±1.29%,and 96.92±1.05%.The effect of hydrophilic interlayer modification on the antifouling performance of the composite nanofiltration membrane was further studied by fouling cycle experiments.The presence of rich hydrophilic groups enhanced membrane hydrophilicity,and the flux recovery rate was stable at more than 93.40±2.57%.When the composite nanofiltration membrane was applied to remove levofloxacin in solvents such as ethanol,acetone,and isopropanol,it also maintained a good permeance and rejection rate(R>91%).(4)To further improve the antifouling ability and practicability of the composite nanofiltration membrane,a chitosan-modified Zr-MOFs composite nanofiltration membrane was constructed using the combination of improved interfacial polymerization and chitosan modified UiO-66-NH2 strategy.Results showed the morphological structure and nanofiltration performance of the composite nanofiltration membrane was best when the doping amounts of chitosan modified UiO-66-NH2(C-UiO-66-NH2)nanoparticles were 0.04 mg·mL-1.The water flux of the membrane reached 17.66±1.19 L·m-2·h-1·bar-1,and the rejection rate of norfloxacin and ofloxacin were 97.92±2.28%and 95.36±1.03%,respectively.Further studies indicated that the high rejection of norfloxacin and ofloxacin by the composite nanofiltration membrane was based on the synergistic effects of size exclusion,steric hindrance,electrostatic repulsion,and hydrophobic adsorption.Besides,the weakened hydrophobic adsorption on the membrane surface also declined the concentration polarization and cake layer formation,which effectively strengthened the antifouling ability of the composite nanofiltration membrane.The flux recovery rate of membrane fouling caused by bovine serum albumin was maintained at about 95.86±1.28%.Using the XDLVO model explores the antifouling mechanism of composite nanofiltration membrane,it was found the presence of hydrophilic groups such as carboxyl,hydroxyl,and carbonyl groups in the C-UiO-66-NH2 was the main reason for the enhancement of membrane antifouling ability.And when the influent was weakly alkaline,the composite nanofiltration membrane’s fouling tendency continued to slow down.Finally,the composite nanofiltration membrane was used to deeply purify the simulated secondary effluent.The rejection rate of norfloxacin and ofloxacin was maintained at over 90%after 15 days of continuous operation,and the effluent quality reached the government standard of the reuse of urban recycling water-Water quality standard for urban miscellaneous use(GB/T 18920-2020).This study gives a practical guiding significance for the application of Zr-MOFs composite nanofiltration membranes in urban sewage treatment plants and the prediction of membrane fouling caused by water quality changes. |
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