Confined intercalating sulfonated graphene quantum dots into GO laminates for fast alkali recovery

Graphene oxide (GO) membranes with uniform sub-nanometer interlayer channels have shown great promise in the ion sieving field. However, the lack of ion-transporting sites on the GO nanosheets usually leads to inferior ion permeation and selectivity. Although the intercalation of molecules with ionic groups into GO laminates is easily achieved to incorporate ion exchange sites, this approach inevitably enlarges the interlayer spacing, reducing the ionic sieving capacity. Thus, current methods are limited in breaking the permeation and selectivity “trade-off”. Herein, we report a confined intercalation strategy to inject ample sulfonic acid groups into GO laminates using sulfonated graphene quantum dots (SGQD) as intercalators. SGQD with abundant ion exchange sites assembles on the sp2 domains of GO via π-π interactions and confers the GO membranes with continuous ion conduction pathways. Meanwhile, benefiting from the ultra-small size SGQD as well as its sheet morphology, it can well accommodate in the GO laminates. This strategy fulfills the incorporation of ionic sites without expanding channel width and destructing the two-dimensional (2D) regularity for GO laminates, resulting in simultaneous improvements in ion permeability and selectivity. Correspondingly, the resulting membranes show synergistically improving OH- dialysis coefficients of 3.5 × 10−3-8.2 × 10−3 m h−1 and OH-/WO42- separation factors of 56–116 compared with the GO membrane (0.9 × 10−3 m h−1 and 8.4) for separating simulated NaOH/Na2WO4 alkaline effluents. We believe the confined intercalating regulation can act as a versatile and fine way to modify 2D membranes for intensifying ion separation.