Controlling The Properties Of Graphene Oxide-Based Nanofiltration Membranes

In the nanofiltration membrane system,membranes serve as a thin barrier between feed and permeate liquids that allow preferential mass-transport to separate,concentrate,or fractionate for sea water desalination and waste water recycling under pressure driving condition.Recently,graphene oxide(GO)membranes have emerged as potential nanofiltration membranes.However,they suffer from low water permeance and low stability in aqueous solutions because of the presence of a great number of oxygen functional groups in GO sheets.In this dissertation,we systematically studied the influence of reduction degree on the water permeance,separation efficiency and stability of GO-based membranes,and designed and synthesized porous reduced GO nanosheets/nanodiamond particles composites for high-performance separation membranes.The main results are shown as follows:We realized the control on the reduction degree of GO nanosheets by hydrothermal treatment,and further studied the influence of reduction degree on the structure and separation performance of rGO membranes.It was found that weak reduction retains the good dispersion and hydrophilicity of GO nanosheets.More importantly,it can increase the number of pristine graphitic sp2 domains in rGO nanosheets while keeping the large interlayer spacing of the GO membranes in most regions at the same time.The resultant membranes show a high water permeance of 56.3 L m-2 h-1 bar-1,which is about 4 times and over 104 times larger than those of the GO and completely reduced rGO membranes,respectively,and high rejection over 95%for various dyes.Furthermore,they show better structure stability and more superior separation performance than GO membranes in acid and alkali environments.In order to further increase the water permeance of GO-based membrane,we synthesized porous reduced GO nanosheets/nanodiamond particles composite membranes by synergistic assembling method.The porous reduced GO nanosheets were synthesized by hydrothermal treatment of GO nanosheets in H2O2 solutions,which contains a great number of pores with a uniform pore size of 1-3 nm,and the nanodiamond particles have a size in the range of 3-10 nm.It was found that the nanodiamond particles are uniformly inserted between the porous reduced GO membranes,which efficiently prevent the re-stacking of the nanosheets,leading to the formation of nanochannels that are suitable for water transport.With increasing the content of nnaodiamond particles,the water permenase is dramatically increased but the rejection is significantly decreased.When the volume ratio of nanodiamond particles to reduced GO is 1:1,the composite membranes show the optimized separation performance,water permeance of 227.1 L m-2 h-1 bar-1 and excellent rejection rates over 90%for various dye molecules.