Graphene oxide (GO) membrane has been reported to have ultrafast water permeability. However, the reverse salt flux in osmotic-driven processes such as forward osmosis (FO) can be very low to maintain the driving force (osmotic pressure difference) across the membrane. To better understand this phenomenon, a mathematical mass transfer model is developed to simulate the draw solute transport in the nanoporous membrane operated in FO mode. Our hypothesis is that the ultrafast transport of water from feed solution to draw solution can prevent the salt transport from draw solution to feed solution. The NaCl reverse salt flux at upper and lower bounds for our lab-made GO membrane operating under FO mode can be calculated with the theoretical GO membrane tortuosity at 1 and 690, respectively. Under a 0.5M draw solution concentration, the measured NaCl reverse flux is 1.23 g/m2h that successfully falls in between the calculated upper (41170 g/m2h) and lower (4.24 g/m2h) bound values under the same conditions. To further illustrate it, the streaming potential caused by the flow of salt solution across charged GO channel is also calculated from measured zeta potential value and it can retard the ion passage potentially. In addition, density functional theory (DFT) calculation is conducted at the molecular level, which shows that water can attach on GO surface through a binding energy of -49.04 KJ/mol, forming a water film on the membrane and hindering the NaCl passage. Our study provides a new perspective in understanding water and ion transport mechanism for GO membranes.

Study of the transport mechanism in a freestanding graphene oxide forward osmosis membrane

2019 Spring National Meeting

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