Chemically crosslinked rGO laminate film as an ion selective barrier of composite membrane

• rGO film has the suitable interlayer spacing and hydrophilic region by sulfonation.
• Water stable rGO layers were assembled on substrate followed by crosslinking.
• rGO film shows favourable water flux, high rejection of inorganic salts.
• Salt rejection was subject to the net surface charges Donnan exclusion mechanism.
• Confirm well prepared and assembled rGO forms an effective ion selective barrier.

In this research, a novel approach of sulfonation followed by mild reduction was employed first time to prepare well separated rGO laminate composite membranes. The combined multistep approach offers hydrophilic regions as well as suitable interlayer spacing of 0.63 nm and 0.74 nm for ion selectivity and water permeation. In-situ Crosslinking reaction has been conducted on the assembled rGO film to successfully improve the stability of the rGO film in water. The promising water permeability of 61.7 LMH/kPa was achieved from optimized rGO interlayer distance by carefully controlled sulfonation and partial reduction conditions (sample Low-R). It was found that the net surface charges Donnan exclusion mechanism played a significant role in the salt rejection efficiency, which was subject to the ratio of cationic and anionic species valency present in the solution. The prepared rGO membrane High-R achieved 80.5% rejection of Na2SO4, followed by 52.0% rejection of NaCl, and 13.7% rejection of MgCl2. It can be seen that there was an interplay between water flux and salt rejection performance, the balance can be achieved by adjusting the reduction conditions. Stronger reduction condition resulted in higher salt rejection rate, while the water flux would decrease to some extent, it was due to the less remaining surface functional groups and resulted in smaller interlayer distance. This research has tested and confirmed of using a novel approach, a relatively small quantity of well prepared and assembled two-dimensional rGO laminates could form an integrated barrier for effective ion selectivity.