Study On Preparation Of Two-dimensional Layered Composites And Their Applications In Water Treatment

As an essential element for human survival,water resources are presenting an increasingly serious crisis with the continuous growth of the global population and the rapid development of society.In response to this crisis,some new materials and/or technologies have been developed to treat the polluted water resources for reuse.Due to unique physical and chemical properties,and excellent performance,two-dimensional materials are one of the promising candidate materials for water treatment.As the typical two-dimensional materials,MXene(Ti₃C₂T_xx)and graphene oxide(GO)are attracting increasing attention for a facile synthesis and absorption characteristics.They can not only be formed into membrane by simple stacking,but also can be combined with other materials to make composite films.In this dissertation,MXene/mixed cellulose ester(MCE)composite membrane,MXene/attapulgite(ATP)composite film,GO composite membrane grafted with polyaniline(PANI)and MXene/TiO₂ composite materials have been prepared,respectively.The performance of composite membranes in wastewater treatment have been tested and analyzed in details,and the working mechanisms have been further explored as the followings(1)The MXene/MCE composite membranes were prepared by stacking assembly based on the cellulose ester mixture under vacuum filtration,which could separate methylene blue(MB)efficiently and rapidly.And the permeation flux,retention rate and separation mechanism of MXene composite membranes for MB solution were studied and analyzed.The results showed that MXene composite membranes possess excellent mechanical properties,hydrophilicity and unique interlayer transport channels,making them have excellent separation properties,while that of MXene nanosheets is poor.When the unit loading of MXene is 0.2mg·cm^-2,the pure water permeation flux can be as high as 119.83 L·m^-2·h^-1.While that of MXene is 0.6 mg·cm^-2,the water flux can be as high as 28.94 L·m^-2·h^-1,the membranes show excellent and stable separation performance for MB solution,and the exclusion rate can also be as high as 90.59%.The main separation mechanism of MXene composite membranes for methylene blue is the adsorption effect in the early stage.As the adsorption site is gradually occupied by MB,the repulsion effect gradually increases Once the adsorption reaches saturation,the repulsion of size and electrostatic dominate the main effect.(2)The MXene/ATP composite membranes were prepared by compounding MXene nanosheets and ATP nanorods.During stack assembly of the MXene nanosheets,the ATP nanorods could insert into the stacked layers of MXene.The permeation flux and rejection rate of the composite membranes for MB solution were studied,and the working mechanism was analyzed.The MXene membrane was intercalated with ATP nanorods,which effectively increased the size of the nano-channel between layers,and the low permeation flux of the MXene membrane was effectively improved.When the mass ration of MXene and ATP was 1:2,the composite membranes showed the best separation performances,such as the permeation flux of pure water increased by 97.31%.For MB solution,the permeability flux increased by 67.17%,and the interception rate still reached 90.67%.The synergistic effect of size repulsion and electrostatic repulsion was observed after the composite membranes stabilized during the MB separation(3)TiO₂/MXene composite materials were synthesized by hydrothermal reaction for the degradation of MB dye.The catalytic degradation effect under different conditions were tested,and the degradation mechanism of composite materials was analyzed.The removal effect of MB was effectively improved by combining TiO₂ with MXene materials.When the mass ratio of TiO₂ and MXene is 4:1,the degradation effect was the best.The degradation effect of TiO₂/MXene composite material for MB under UV light was 96.44%,which was 29.83%higher than that of pure TiO₂.Under visible light,the degradation effect of MB was 8.26 times higher(40.19%)than that of pure TiO₂.Therefore,the utilization efficiency of visible light was greatly improved,and the degradation process of MB dye was promoted.The mechanism of catalytic enhancement were explained as the followings:1)The recombination of electrons and holes on nano-TiO₂ was significantly suppressed by adding MXene,so that the electrons transfer from TiO₂ to MXene and the energy barrier was reduced,2)The TiO₂/MXene composite materials could provide abundant surface active sites due to the multi-layer structure,resulting the adsorption effect for MB were effectively improved.3)The black MXene nanosheets could also further improve the efficiency of light utilization,thereby the photocatalytic effect of MB could be enhanced.(4)The PANI/GO composite membranes were prepared by grafting and modifying the GO film with PANI.The permeation flux of pure water and the rejection rate of different heavy metal ion solution were studied,and the separation mechanism of composite membranes was discussed in details.The composite membranes were enriched with positive functional groups through the modification of GO with PANI.The surface potential of membranes changed from negative to positive,so that the excellent pure water flux and retention rate could be found.The loading of GO played important role in the separation performance of the composite membranes.When the loading was 150mg·m^-2,the water flux of 21.45 L·m^-2·h^-1·bar^-1 could be reached for the composite membranes.In terms of heavy metal removal,such as CuCl₂,ZnCl₂,BaCl₂,CdCl₂ and NiCl₂,the retention could be over 90%.The surface Zeta potential of the PANI/GO composite membranes was positive,resulting in the cations with two positive charges can be effectively rejected.In addition,the excellent separation performance of PANI/GO composite membranes is mainly attributed to Donnan effect and size exclusion.