Cationic stabilized layered graphene oxide (GO) membrane for shale gas wastewater treatment: An atomistic insight

In recent years graphene oxide (GO) membranes have emerged as one of the most promising candidates for wastewater treatment. However, one major issue with layered GO membranes is their tendency to swell in an aqueous environment. Cation intercalation inside the interlayer gallery of layered GO membrane is one of the promising solutions to mitigate this problem. However, the effect of cationic concentration inside membrane channels on the mechanism of water permeance and salt rejection is yet to be investigated in detail. This study investigates shale gas wastewater (NaCl, CaCl2, and crude oil) treatment via nonequilibrium molecular dynamics (MD) simulations with cation (K+, Mg2+) intercalated GO membranes. The effect of cationic concentration on water permeance and salt rejection has been presented in detail. This study reveals that the presence of a higher concentration of cations inside the GO membrane induces higher water resistance in the membrane. In addition to the concentration of intercalated ions, interlayer distance also plays a significant role in determining water permeance. We observed that Mg2+ ion intercalated and K+ ion intercalated GO membranes maintain an interlayer distance of around 13.6 Å and 11.4 Å, respectively. As a result, Mg2+ ion intercalated GO membranes exhibit superior water permeance than K+ ion intercalated GO membranes. We also observed a difference in the crude oil aggregate formation (a composition of alkanes, cycloalkanes, and aromatics) in the two systems (more stable aggregate in K+ ion intercalated GO membrane system as compared to Mg2+ ion intercalated GO membrane system) during the forward osmosis (FO) process. This can be attributed to weaker interactions of K+ ions with the crude oil and lower water permeance of K+ ion intercalated GO membranes than Mg2+ ion intercalated GO membranes. This aggregate formation is an important factor for crude oil rejection. Moreover, aromatics are more concentrated at the crude oil-water interface than other hydrocarbons. On the other hand, more cations inside the GO membrane layers results in better Na+ rejection but decreased Cl− rejection.