| As the novel carbon nanomaterials,multi-wall carbon nanotubes?MWCNTs?and graphene oxide?GO?have some special physicochemical properties,such as high surface activity,controlled group composition and easy composite incorporation etc.,and are more applied in the field of water pollution control recently.Although carbon nanomaterials show a good adsorption performance for some organic pollutants at low concentration,it is noteworthy that their suspensions in the water are difficult to be reclaimed and hold high potential ecological risk.So,it is crucial to develop efficient solid-liquid separation process for the industrial application of carbon nanomaterials in this field.Microfiltration?MF?is a typical membrane separation technique,which could be operated at a low trans-membrane pressure with high water flux and suspended solid removal,but a poor rejection of soluble substances.In this study,the MF was employed as the separator for carbon nanomaterials,for developing a novel composite membrane with filtration-coating methods.The effects of coating dosage,external dimension and mass proportion etc of carbon nanomaterials on mats structure were investigated,and the response relationship between carbon mats structure and rejection performance of composite membranes were also built up initially.It would provide important theoretical support for combing carbon nanomaterials adsorption and membrane separation process,which included:?1?It is generally considered that to ensure the full dispersion of MWCNTs in water is conducive to the formation of a uniform carbon layer structure during loading on the surface of the membrane.Therefore,the effects of carbon tube size,surface group,ultrasonic time,ultrasonic power density,and dosage of polymer dispersants?BCPs?on carbon nanotube dispersion were investigated firstly.It was found that when the concentration of carbon nanotube suspension was 0.1 mg·mL-1,the ultrasonic density was set to 5 W·mL-1,the ultrasonic duration was 30 minutes and the mass ratio of dispersant to carbon tube?m?BCPs?/m?CNTs??was 0.2 to 0.6,the carbon tube dispersions could keep homogeneous and stable within 2 hours.Among them,the amount of BCPs added was related to the size of the carbon tube and the surface groups.?2?During the preparation of MWCNTs composite membranes,the effects of carbon nanotube loading,carbon tube size,surface groups and basement membrane types on the structure of the loaded carbon layer were investigated systematically.It was shown that the carbon layer thickness increased linearly with the increase of carbon tube load and the permeability of composite membrane decreased obviously.At the same time,both increasing the outer diameter of carbon tube and reducing the length of carbon tube were conducive to form a homogeneous,dense and flat carbon layer structure.The scanning electron microscope?SEM?results using software showed that the surface pore size of the carbon layer followed the Lognormal distribution and it was mainly concentrated in the range of 20-30 nm.Compared with the surface groups of carbon nanotube,the basement membrane type had a more pronounced effect on the carbon layer structure and it was easier to obtain a relatively flat loaded carbon layer by using an inorganic alumina?AAO?membrane.?3?The effects of the carbon nanotube loading,carbon tube size,surface groups,and basement membrane type were systematically investigated for the retention performance of multi-walled carbon nanotube composite membranes by using the small molecule fulvic acid?FA,about 227.17 Da?as the target.The research showed that increasing the load of carbon tube was beneficial to enhance the ability of composite membranes to remove FA.Under the same load condition,the larger the outer diameter of the carbon tube,the smaller the specific surface area,the permeability and the FA rejection rate of the composite membrane.Carbon tubes with longer lengths tended to form a non-uniform dough-like layer on the surface of the membrane,resulting in poor retention of FA.Based on the fitting of the pseudo-second-order kinetic model,the adsorption of carbon layer was the dominant mechanism for the retention of FA by composite membrane.Compared with the original carbon tube,the composite membrane corresponding to the hydroxyl and carboxyl carbon tubes had a higher permeability,and the equilibrium adsorption capacity of the unit carbon tube was relatively lower.From the types of basement membrane,the pore distribution of the carbon layer on the surface of the AAO membrane was more uniform,which helped to improve the utilization of the carbon layer adsorption sites.?4?In order to further improve the retention performance of the composite membrane for small-molecule FA,a small amount of GO was mixed in the carbon tube to construct a MWCNTs/GO composite carbon layer.Compared with the results of SEM,it could be seen that the mixture of GO made the carbon layer on the surface of the membrane change from a honeycomb of pure MWCNTs to a smooth layered structure.The element composition of MWCNTs/GO composite carbon layer was very close to that of pure GO by using a variety of surface element analysis methods.This meant that MWCNTs in the carbon layer had been fully coated by GO and may form complex multidimensional nanochannels.Although the mixing of a small amount of GO could lead to a decrease of the permeability of the composite membrane and an increase of the specific resistance of the carbon layer,which in turn reduced the initial adsorption rate,the equilibrium adsorption capacity qe per unit carbon layer was obviously higher than in the case of pure carbon tube.It was closely related to the change of flow state in the carbon layer. |
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