Multilayer Graphene Oxide Membrane in Forward Osmosis: Molecular Insights

Abstract

We investigate the performance of layered graphene oxide (GO) membrane in forward osmosis (FO) process for seawater desalination via nonequilibrium molecular dynamics (MD) simulations. A 0.56 M NaCl solution is used as the feed solution and a mixture of 1.0 M MgCl2 and 0.05 M Al2(SO4)3 solution is used as a draw solution. The effect of internal structure of the layered GO membrane on its performance as a FO membrane is investigated by considering three different membrane configurations which differ in their pore offset distance W. The dynamics of the permeating water and ions through the layered GO membrane is also investigated. Increase in W reduces the water permeability of the layered GO membrane and increases its salt rejection. One of the reason for this could be the blockage of permeate pathways through the layered GO membrane due to the movement of the GO nanosheets. This becomes more prominent with increasing W. Increasing W also leads to increase in distance traversed by the permeating water molecule through the layered GO membrane while it decreases velocity of the same. This study also reveals that ion permeability is affected by the parameter W, type and magnitude of charge the ion possesses and the intensity of interaction between water molecules and the ion.

Effect of graphene oxide (GO) nanosheet sizes, pinhole defects and non-ideal lamellar stacking on the performance of layered GO membranes: an atomistic investigation

“…Recent atomistic simulation studies also support this notion. [36][37][38] The performance of a layered GO membrane also depends on the size of the constituent GO nanosheets. With the increase in the size of the GO nanosheets the water permeability of the layered GO membrane decreases and its salt rejection increases.…”

“…With the increase in the size of the GO nanosheets the water permeability of the layered GO membrane decreases and its salt rejection increases. 36,37,39 Very recently it was reported that with the inclusion of non-idealities in layered GO membranes, there is substantial reduction in the difference in water permeability of the size-differentiated GO laminates by the experimental and simulation (using nite element analysis) study of Saraswat et al 30 The two major factors (or non-idealities) contributing to this scenario are Pinhole defects on the constituent GO nanosheets: pinhole defects located on the GO nanosheets can play a pivotal role in water permeation through layered GO membranes by providing a shorter route for trans-sheet ow. 30,32,40 Imperfect stacking of the GO nanosheets: the ideal (or perfect) lamellar stacking of GO nanosheets in layered GO membranes may not be possible in practical scenarios.…”

“…This may lead to the presence of voids and disordered micro-structures in the layered GO membrane which can serve as alternate permeation pathways/blockages inside the layered GO membrane. 21,30,37,41,42 So, to get an meticulous insight into the water permeability of the layered GO membrane, the consideration of these nonidealities is very much crucial. However in most of the previously reported atomistic simulations on GO membranes, these non-idealities were not taken into consideration.…”

“…However in most of the previously reported atomistic simulations on GO membranes, these non-idealities were not taken into consideration. 36,37,[43][44][45][46] In the present study we investigate the effect of these non-idealities on the performance of the layered GO membrane using equilibrium molecular dynamics (MD) simulations. The effect of the non-ideality (i.e.…”

“…A larger value of W indicates layered GO membranes composed of GO nanosheets of larger lateral dimensions and vice versa. 36,37,39 For each of the three membrane congurations (which differ in the value of W) four different scenarios have been studied to get a comparative estimation of the effect of the non-idealities on the performance of layered GO membranes. The four different scenarios are as follows: (a) no pinhole defects and the nanosheets are ideally stacked together, (b) no pinhole defects with non-ideal lamellar stacking, (c) pinhole defects but the nanosheets are ideally stacked together, and (d) pinhole defects with non-ideal lamellar stacking.…”