Equilibrium ion sorption in graphene oxide membranes
Melike Benan Altay, George Q. Chen, Dan Li, Sandra E. Kentish
Abstract
Graphene oxide (GO) has been identified as a promising 2D material for a wide range of applications including water treatment and energy storage. While ion adsorption plays a crucial role in these applications, there is a lack of comprehensive understanding regarding the uptake of ions within GO nanochannels. This knowledge gap hinders our understanding of the transport processes that are occurring and the development of membranes with desired transport properties. Here, we explore the equilibrium ion concentration distribution within these membranes. We show that the concentration of magnesium ions can be nearly 100 times that in the external solution. Similarly, the concentration of chloride ions can be nearly double that within the external solution, even though the GO sheets are negatively charged. While positively charged monovalent ions are associated with the same number of fixed charge groups based on the dry GO mass, magnesium ions can displace protons from more carboxylic acid groups to generate a greater number of these fixed charge sites. An idealised Donnan model is unable to describe the sorption behaviour based on electrostatic effects alone. Incorporation of a non-electrostatic, chemical attraction term is needed. The experimental data can be fitted extremely well through the use of a single parameter to represent this chemical attraction. Importantly, we draw together separate research studies on ion exchange polymers, nanofiltration membranes and porous carbons to show that this chemical attraction term is identical across a range of mathematical models, providing a unifying approach for the description of a very wide range of materials. • Mg ions sorb in GO membranes at 80 times their concentration in the external solution. • Chloride ions present in GO membranes at one to three times that in the external solution. • This equilibria modelled through a combination of electrostatic and chemical potentials. • The chemical attraction term can be characterised in any of three forms. • The equivalence of these forms draws together disparate approaches to equilibria modelling.