Graphene oxide membranes (GOMs) are promising materials for ion separation in energy, water, and environmental applications. However, quantitative understanding of how co-ion identity, concentration and ion association govern sorption and transport in GO nanochannels remains limited. Here, we combine equilibrium sorption measurements with concentration-gradient and electric-field-driven transport experiments to quantify individual ion diffusion coefficients in GOMs equilibrated with Na2SO4, MgSO4, and NaNO3 and benchmark the results against chloride electrolytes. The results show that Mg2+ uptake in MgSO4 solutions is lower than in MgCl2. In both magnesium- and sodium-based systems, the sulfate concentrations are lower than that of the corresponding monovalent co-ions. Conversely, nitrate ions adsorb preferentially in graphitic regions of the membrane, thus reducing counter-ion but not co-ion concentrations. pH measurements of the external salt solution during sorption experiments show that proton release in sulfate systems decreases with increasing external salt concentration, in contrast to that observed for chloride and nitrate electrolytes. These pH changes are, however, consistent with ion association to form NaSO4− and MgSO40 ion pairs and/or shifts in sulfate speciation (including HSO4− and NaHSO4 species). The results indicate that counter-ion diffusion coefficients are largely insensitive to co-ion identity and external concentration, while co-ion diffusion depends on external concentration. Incorporating ion pairing into the transport framework confirmed that ion pairs were increasingly dominant as the external concentration increased but had minimal impact on diffusivities. Overall, these findings provide a unified basis to interpret sorption-transport coupling in GOMs and support membrane design for water purification and electrochemical applications.
