Graphene Oxide Modified PVDF Membrane And Membrane Distillation Performance Study

Membrane distillation has the advantages of simple operation flow,low energy consumption and high separation efficiency.With the development of membrane material,membrane distillation has acted great interest in water treatment.The temperature defference between the two sides of the membrane leads the vapor pressure gradient which acts as the mass transfer driving force of membrane distillation.But the hydrophobic membrane increases the resistance of water,so the flux is lower than other membrane separation processes,which limits its large-scale application.It has been reported that graphene oxide could promote the water transport.The GO interlayer spacing ranges from 0.7 nm to 1 nm,and its size screening effect can effectively trap target contaminants.In this work,the polydopamine modified PVDF was coated by graphene oxide in the permeate side,and its effects on the flux and salt rejection of the membrane was investigated.The degree of oxidation was varied by changing the oxidation temperature.The relationship between the degree of oxidation and membrane distillation performance was studied.Therefore,this study was mainly divided into the following contents:（1）The GO was prepared by a modified Hummers method,and the degree of oxidation was controlled by varying reaction temperature.A series of characterizations such as FTIR,XRD,SEM and XPS indicated the characteristic structures of graphene oxide.The degree of oxidation increased with the increasing tempreture,resulting in increasing interlayer spacing of GO sheets.And higher reaction temperature led to incerasing content of C-O-C groups.（2）We chose the better modified method by comparing the direct coating method and composite surface coating method.And we studied the key factors which may affect the preparation: the soaking time of dopamine and the concentration of GO suspension.It was found that the direct coating method could not fix the GO sheets on the PVDF membrane while the composite surface coating method could firmly fix the GO sheets on PVDF membrane.The dopamine soaking time was chosenas 2 h and GO coating concentration 5 mg/m L.（3）GO was immobilized on the surface of the PVDF membrane by composite surface coating method.Different GO composite membranes（50GO/PVDF,60GO/PVDF,70GO/PVDF）were prepared.The results showed that the modification of GO enhanced the hydrophilicity of the PVDF membrane,and higher degree of GO,provided it higher hydrophilicity.The contact angle decreased from 131.5° of the original film to 85.8°,80.5°,74.8°of GO composite membrane.The membrane flux increased by 5.7%,10%,and 18.9% respectively,up to 18.3 kg.m^-2.h^-1 for 70GO/PVDF membrane;The rejection for salt was improved from 99.2% to 99.9%.