Insights into the enhanced flux of graphene oxide composite membrane in direct contact membrane distillation: The different role at evaporation and condensation interfaces

Graphene oxide (GO) coating has recently been reported as a novel approach to increase membrane flux of membrane distillation (MD), yet the phenomena underlying the process are still not fully understood. In this study, a mathematical model based on capillary-film assumption was developed and validated with the results (R2>0.99) from a series of MD experiments. According to the model, when GO layer was placed at the evaporation interface, the temperature difference across the membrane surface increases significantly (44.2%-92.0%) and the temperature polarization coefficient is increased greatly from 0.29-0.38 to around 0.55. This leads to a big increase of driving force for higher heat flow and subsequently mass flux (17.8-45.5%). However, the vapor pressure on membrane surface was decreased due to Kelvin effect of GO capillary pores, which has a negative influence on the driving force, accounting for about 26.9% to 52.6% drop in the achieved flux. In comparison, when GO layer was placed at the condensation interface, the temperature difference across the membrane surface decreases slightly (7.2-12.2%), but the reduced vapor pressure on GO capillary pores due to Kelvin effect become the dominant factor affecting membrane flux, resulting in an increase mass flux of 12.4-16.4%. The model developed in this study provides a theoretical foundation for understanding the role of GO coating on flux improvement, and can be used for further development of high flux membranes.