Fabrication And Separation Performances Of Composite Membranes With Passive/Active Antifouling Mechanisms

Nowadays water shortages are critical global challenges faced by mankind.With low cost,easy operation process and no secondary pollution,membrane separation technology is widely used in the wastewater treatment field.However,membrane fouling is the biggest challenge in the membrane purification processes.Direct contact with various kinds of foulants in the raw water,the foulants will block the membrane,leading to caking formation,organic adsorption,biological fouling and so on.Affected by the membrane fouling,the water flux will decline in the filtration process and the consumption for backwashing will increase.Passive antifouling strategy and active antifouling strategy are the main methods to solve the membrane fouling problem.In this manuscript,a strong hydrophilic hydration layer was used by weakening the interaction between pollutants and the rough membrane surface.In addition,by combining separation,adsorption and the Fenton process,the membrane could resist the fouling actively by directly eliminating the pollutants.The main work is as follows:Affects by the delayed non-solvent induced phase separation（NIPS）process,roughness and micro/nanostructure poly（vinylidene fluoride）（PVDF）membranes were obtained with alcohol solutions and saturated salt solution as coagulating bath.Then,a progressive cation-πassembly of polydopamine（PDA）was used to boost the antifouling property of the rough PVDF membrane.As a result,improved by the K⁺-πinteraction,the modified membrane P-DS8 showed higher hydrophilicity and lower negativity（zeta potential～84.3 m V at p H 7）.More importantly,the hydration layer was detected by the spin-spin relaxation time（T₂）.Assisted by the K⁺-πinteraction,more-NH₂ and-OH hydrophilic groups were introduced onto the membrane surface,forming strong hydrogen bonds with water molecules by trapping water molecules onto the membrane surface,resisting both proteins,natural organic polymers and oil pollutants in the filtration process.Fabrication of porous P-EG-DT foam and study of its adsorption/separation performance.In order to remove the small-molecule dye pollutants,a simple solvent-free process was used to fabricate a polyvinylidene fluoride（PVDF）-based porous foam supported by expandable graphite（EG）as the skeleton.Then,functional groups were introduced throughout the foam by deposition of tannic acid（TA）and dopamine（DA）.The adsorption mechanism of methylene blue（MB）is mainly the hydrogen bond,theπ-πinteraction and the attraction of positive and negative charges between the dye and the adsorbed group.The functionally porous foam P-EG-DT showed excellent dynamic adsorption capacity.In addition,it found that more sufficient contact between the cationic dyes and the functional sites throughout the foams led to the higher adsorption capacity of P-EG-DT.Low flow rate,low pollutant concentration and longer contact channel could improve the adsorption capacity of P-EG-DT.To degrade the dye pollutants in the filtration process,PB/GO heterostructured membrane was fabrication combining the Fenton and separation process.Using graphene oxide（GO）membrane with rich pore structures as the main part,Prussian blue（PB）were inserted into the nanosheet layers as a catalyst to prepare a PB/GO heterostructured membrane.The permeate capacity and the dye residue rate in the filtrate were detected by changing the ratio of GO and PB in the composite membrane and the applied pressure.As a result,reduced interlayer spacing of the heterostructured membrane resulting in lower water flux and higher degradation efficiency.Meanwhile,increased interlayer spacing resulting in higher water flux and lower degradation efficiency.During the filtration process of the membrane,water permeated the channels between the neighboring GO nanosheets,where the Fenton oxidation reaction induced by PB cubes embedded in the membrane.The strong oxidative species of·OH generated by Fe（Ⅱ）could degrade the organic microcontaminants in the filtrate.Thereby,the PB/GO heterostructured membrane cloud simultaneously degraded the dye pollutants in the filtration process.Photocatalysis technology reveals new strategies to further accelerate the catalytic degradation rate in the membrane separation process.Carbon quantum dots（CQD）,a solar energy conversion absorber and Cu Fe₂O₄（spinel structure catalyst）were introduced onto reduced graphene oxide（r GO）surface by hydrothermal method to prepare the catalyst G/CFQ.Then,photocatalytic composite membrane M-G/CFQ was prepared by a pressure-driven method.The blending G/CFQ and GO mixture would form a composite membrane with G/CFQ nanosheet and GO nanosheet.In conclusion,for M-G/CFQ composite membrane,it can directly remove the dyes in the filtrate for the MB solutions at p H 7,10 and 12 at 89.2 L m^-2 h^-1 in long-time running.During the filtration,M-G/CFQ showed excellent photocatalytic degradation performance by the synergistic effect of separation/adsorption/photocatalysis,ensuring the long-time filtration and degradation capacity.