Insight into the role of ionicity in the desalination and separation of a graphene oxide membrane

To provide a comprehensive understanding on the role of ionicity of solutes and polarity of the graphene oxide (GO) surface in permeabilities, a near-practical lab-scale cross-flow-type apparatus was used herein to systematically examine the separation behaviors of representative inorganic cations (alkaline and alkaline earth cations and the counter anions) and dye molecules through GO membrane (GOM). It was found that cation separations through a dried GOM of tens nanometer thickness receives strong influence from electrostatic interactions. As the result, both cationic permeation rate and water flux obey a linear increasing relationship with the strength of cationic power whereas each of the monovalent (Li+, Na+, K+, Rb+, Cs+) and bivalent (Mg2+, Ca2+, Sr2+) cation groups have a different increasing gradient. Accumulation adsorption through ion-exchange in the GO interlayers induces an increase in penetration rejection of organic dyes at the cost of water flux due to membrane blockade. The cation separation mechanism was understood through intercalation structure analysis and detailed using molecular dynamic simulations. The research results disclosed a dominant ion-exchange mechanism and the crucial role of ionicity of solutes in separation. These findings fill up the gap in understanding differences of salts and dyes migration within GOM, being worthwhile for future membrane material design and the references of various water treatment applications using GOM.