Separation Performance And Purification Mechanisms Of Stacked Graphene Oxide-based Nanofiltration Membrane

To overcome the difficulties in traditional treatment processes for the increasing types and quantities of new pollutants,it was particularly necessary to explore new membrane separation technologies.In general,the self-characteristics of membrane material are key factors affecting the membrane separation performance.Graphene oxide nanosheets?GO?had begun to receive widespread attention in the field of membrane separation due to their high surface to thickness aspect ratio,excellent mechanical properties and unique surface properties.GO nanosheets could be used as an additive of polymer filter membrane to improve the hydrophilic and mechanical properties of organic separation membrane,thus significantly improving its permeability and separation performance.GO nanosheets can also be stacked separately to form single membrane.Furthermore,the stacked graphene oxide separation membrane was expected to be a new generation separation membrane with high flux and selectivity owing to unique pore structure and mass transfer characteristics.However,excessive hydrophilic groups on the surface of the graphene oxide nanosheets may seriously affect the interlayer stability and separation performance.The interlayer hydration effect of GO nanosheets would be obvious especially when used in wet environment,which seriously shorted its service life.In order to construct an effective transmission path of GO stacking layers and promote its immobility,the stacking GO lamellas need to maintain a certain thickness.However,the increase in thickness would generally extend the film-forming time and reduce the water molecule permeability.Therefore,how to maintain the satisfied stability and selectivity of the stacked GO separation membrane without loss of flux was a key issue related to its practical application.With regard to adjusting the interaction between the GO nanosheets and the structure-activity relationship of the transport channels,this paper mainly prepared a series of stacked GO-alkenyl separation membrane,in which graphene oxide nanosheets as membrane-forming substrates,with high stability,high flux and excellent separation performance through adjusting and controlling the layer of inter-atomic forces and transport channel size by the method of chemical modification and physical doping.The entrapment behavior and purification mechanism of a series of graphene oxide-based composite separation membranes for dye molecules with different surface properties and metal ions with different valence states were discussed.The specific researches and conclusions of this paper could be summarized as follows:?1?Cross-linked graphene oxide nanofiltration membrane?GO-IPDI?was prepared by using isophorone diisocyanate?IPDI?as a bridging agent to control the interlayer structure of graphene oxide?GO?by chemical crosslinking.The interlayer spacing of GO nanosheets was fixed by covalent bonds to improve the stability and permeability of the modified graphene oxide stacked membranes.The pure water flux of the membrane was as high as 80-100 L m^-2h^-1 under the low pressure operation condition of 1 bar.The highest removal rates of methylene blue?MB?,rhodamine B?RB?,methyl orange?MO?and congo red?CR?by GO-IPDI cross-linked membrane were 97.6%,96.2%,96.9%and 98.2%,respectively.On the contrary,the surface charge of GO nanosheets was decreased due to cross-linked.The removal rates of Cu²⁺,Pb²⁺,Cr³⁺and Cd²⁺by GO-IPDI cross-linked separation membranes were 46.2%,66.4%,71.1%and 52.8%,respectively.The GO-IPDI cross-linked separation membranes had lower rejection performance for heavy metal ions than organic dyes molecules.The rejection rates of organic dyes molecules by GO-IPDI cross-linked nanofiltration membranes had almost no change under different pH conditions.However,with the increase of pH value,the rejection rate of heavy metal ions by GO-IPDI cross-linked nanofiltration membrane would be significantly increased.Therefore,increasing the surface charge intensity of GO-IPDI cross-linked nanofiltration membranes played an important role in improving the removal of heavy metal ions.?2?An ultra-thin UiO-66-?COOH?₂/prGO composite nanofiltration membrane was prepared by pre-pressure driven self-assembly method using metal-organic framework nanoparticles?UiO-66-?COOH?₂?as support between partial graphene oxide?prGO?layers.The thickness of the composite membrane was only³20 nm and the permeability was significantly improved.The pure water flux of UiO-66-?COOH?₂/prGO composite membranes(20.0±2.5 Lm^-2h^-1bar^-1)increased by more than 2.9 times compared with the pure prGO membranes(6.5±1.2 Lm^-2h^-1bar^-1).The highest rejection rates of charged dyes by UiO-66-?COOH?₂/prGO composite membranes were 95.1%for MB and 97.2%for CR,respectively.It indicated that UiO-66-?COOH?₂/prGO composite membranes had excellent rejection efficiency for organic dyes.The different concentrations and pH conditions had little effect on the rejection and permeability of negatively charged dyes?CR?by UiO-66-?COOH?₂/prGO composite membranes.In addition,the rejection rates of UiO-66-?COOH?₂/prGO membranes for Cu²⁺and Cd²⁺were 96.5⁸3.1%and 92.6⁸0.4%respectively,which were significantly higher than those of pure prGO membranes(Cu²⁺was 65.4⁴4.5%,Cd²⁺was 57.0³3.2%).Therefore,we believed that the electrostatic interaction on the surface and in the pore channels of the composite membrane played an important role in the removal of heavy metal ions.The charge intensity in the pore channels of the UiO-66-?COOH?₂/prGO composite membrane could be effectively enhanced by introducing electronegative UiO-66-?COOH?₂ crystalline particles into the prGO stacked layers.Thus,the rejection performance of positive charged organic pollutants and heavy metals ions by the UiO-66-?COOH?₂/prGO composite membranes were improved.?3?The GO/Gr composite nanofiltration membranes with compact structure and excellent stability were prepared by efficiently and orderly stacking GO and graphene?Gr?nanosheets via a hybrid and post-heat method.The retention performance of dyes molecules and salt ions by GO/Gr composite nanofiltration membranes was investigated.The results showed that the doping of ultra-low oxidation degree of Gr could effectively regulate the interaction between GO nanosheets and consequently improve the stability of the composite membrane.The retention efficiency of dyes molecule by GO/Gr composite membrane was also significantly improved.The van der Waals force and?-?interaction between the layers were enhanced with the introduction of Gr nanosheets,which resulted in the reduction of interlayer hydration effectively.Consequently,the overall stability of the composite membrane was improved.?4?The sulfonated graphene oxide nanosheet?S-GO?was prepared by surface chemical modification to change the electronegativity of GO nanosheets.The reduced graphene oxide?rGO?/sulfonated graphene oxide?S-GO?composite nanofiltration membranes were successfully prepared by pressure-assisted and thermal reduction methods.It has been found that the surface functional groups of graphene oxide could effectively adjust the distance between stacked sheets.In the complex stacked layers with gradient spacing,the flux could be effectively increased.The strong electronegativity enlarged the distance between the S-GO nanosheets and further enlarged the void size.The structure of reduced graphene oxide sheets remained tight,which improved the mechanical stability of the composite membrane.Both of them existed in complex stacked structures,which together improved the separation characteristics of the composite membranes.?5?A novel low-pressure nanofiltration composite membrane with loose structure was prepared by using molybdenum disulfide nanosheets?MoS₂?as the interlayer nano-support of graphene oxide?GO?.The GO/MoS₂ composite membrane exhibited excellent water flux?¹0.2 L/?m² h bar??and high rejection rate for organic dyes?>95%?,especially for negatively charged dyes?close to 100%?.Membrane fouling test results indicated that reversible fouling?36.5%for BSA and 41.3%for HA?and irreversible fouling?18.5%for BSA and 19.1%for HA?could occur in the GO/MoS₂ membranes.The low Rt?36.5%,41.3%?and high FRR?83.4%,80.2%?of the anti-fouling measurements?for BSA and HA?indicated that GO/MoS₂membranes had excellent anti-fouling properties.