Study On The Preparation And Structure-function Relationship Of Mixed-matrix RO Membrane Incorporated With Carbon Nanomaterials

Thin film composite (TFC) polyamide membrane is the most successful commercial reverse osmosis membrane, which has been widely used in desalination and purification. However, there are still many issues that should be addressed in the polyamide membrane, such as poor chlorine resistance, vulnerable to pollution and limited flux. The rise of the nanoscience makes it possible to figure out these issues for the polyamide membrane. Graphene (Gr) and carbon nanotubes (CNTs) are the inorganic carbon-based nano-particles (CBNPs) with excellent properties and have received intensive attention from research community since their discovery. The combination of these nano-particles and the organic polymer could enhance the mechanical, electric, thermal and chemical properties of the nanocomposite, which has a glorious prospect in the academic research and the industrial applications. The hollow cavity of CNT and the layer gap between Gr sheets can serve as molecule passage, which make them gain popularity in the separation area. Incorporating CBNPs with molecule passage into the TFC polyamide membrane is expected to improve the separation performance and the stability. This process requires the good dispersibility, compatibility and orientation of CBNPs in the organic polymer matrix. In this thesis, starting from the thorough analysis of the structure of the TFC RO composite membrane, combined with the molecular dynamic simulation and experimental test, the separation performance of the RO membrane was investigated by incorporating CBNPs into the support layer and active layer of the TFC polyamide membrane via randomly or orientationally blending or crosslinking. Five parts are included as following:(1) In order to enhance the dispersibility and compatibility of CBNPs in the polymer matrix, CNTs was oxided by nitration mixture and graphene oxide (GO) was organo-functionalized by isocyanate. Results showed that the structure and the properties of GO and CNTs have been changed:isocyanate-treated GO (iGO) could be well dispersed in organic solvent, and acidulated CNTs had less aggregates.(2) Thorough understanding of the TFC composite membrane will favor the design of the novel RO membrane, so the hierarchical structure of the TFC polyamide RO membrane was studied via the molecular dynamic (MD) simulation and plasma etching. In the microscopic level, the polyamide layer has three residual benzene structure components, which intertwine with each other, build stable high-level structures and determine the average pore size of 0.2 nm. Somehow connected membrane pores form water channel, which plays an important role in the trans-membrane diffusion of water molecules. Moreover, the plasma etching technique was used to analyze the hierarchical structure of the TFC polyamide RO membrane from multi-dimension. In the mesoscopic level, the polyamide active layer is of porous structure. The top section of the active layer is quite loose and composed of range-like structure and protuberance structure, while the bottom is very condense and consist of valley-like structure and pores. In the polysulfone (PSF) support layer, the pores become larger and larger from the top to bottom, however the pore density first increased and then decreased.(3) Regulating the support membrane is a good strategy to realize the precise design of the polyamide active layer. Firstly, iGO was blended into the PSF support to improve the hydrophilicity and stability of the composite membrane. Afterwards, CNTs were orientationally aligned on the PSF sport by hop-press and peel-off (HPPO) process and in situ plasma etching (ISPE) technique. In HPPO process, shear force and mechanic stretch were triggered to pull nanotubes from the interlayer and vertically align them on the support surface, in which the macroscopic manipulation was used to achieve the microcosmic alignment of the nanotubes. In ISPE technique, after the CNTs/PSF support was etched the CNTs terminal was exposed to be considered as partially-aligned CNTs (PACNTs).(4) In order to create more water channels in the polyamide active layer, GO was used to prepare GO/polyamide thin film nanocomposite (TFN) membrane via interfacial polymerization by dispersion GO in the MPD aqueous solution. Experimental test showed that GO barely improved the water flux though it can increase the hydrophilicity, electronegativity and chlorine resistance of the TFN membrane. MD simulation revealed that Gr cannot form effective water channels in polyamide layer.(5) To make CNTs cavity serve as water channels in polyamide active layer more effectively, CNTs should be introduced with orientation. After PACNTs was achieved by ISPE technique, PACNTs/polyamide was prepared by in situ interfacial polymerization. Structure and performance studies indicated that the flux of PACNTs/polyamide membrane is two times higher than that of polyamide membrane, but the rejection of PACNTs/polyamide membrane is slightly lower.Conclusively, water channel plays an important role in water transport of the TFC polyamide membrane, and thus this thesis introduced two porous CBNPs into the polyamide active layer to build new water channels. Results suggested that Gr can barely form water channels in the polyamide layer while CNTs cavity can effectively serve as water channels to enhance the separation performance of the TFC membrane.