most intensively studied research areas in the development of high-performance separation membranes.Most 2D materials typically have a layered geometry where the atoms are linked by strong in-plane covalent bonds, while the two adjacent layers are held together by van der Waals forces. [7] Exfoliated nanosheets typically show a high surface-area-tothickness ratio, which is instrumental in improving the adsorption capacity or the ion selectivity, both of which can lead to a better separation performance. In addition to highly effective ion-rejection properties, 2D membranes are also known to possess excellent water permeability. Due to these properties, many 2D materials such as nm-thick graphitic carbon nitride (g-C 3 N 4 ) nanosheets, [8] MoS 2 sheets, [9] WS 2 nanosheets, [6] graphene, [10] Mxene nanosheets, [11] and graphene oxide (GO), [12] have been recently used as the building blocks for fabricating ultrathin layered membranes for separation applications. These 2D membranes have shown highly efficient size-selective ion separation and high water permeance.Hexagonal boron nitride (h-BN), so-called "white graphene," is however one of the promising 2D materials that has not been utilized to its full potential in ion-separation applications. For efficient ion separation, the membrane should show high ion selectivity, which is primarily dependent on the channel size and surface charge on the membrane. The h-BN material is known to have a high surface charge density resulting from the adsorption of the hydroxyl ions on the surface defect sites. [13] It also shows excellent oxidation and corrosion resistance, which is an important property for wastewater treatment applications. The high chemical stability of h-BN also enables it to be resistant to chemical cleaning, which is frequently needed during separation processes. Qin et al. reported high ionic conductivities for h-BN nanofluidic channels, prepared by the one-step BN exfoliation method and amine functionalization. [14] More recently, Chen et al. developed a 2D h-BN membrane by functionalizing h-BN flakes (h-BNF) with amino groups to overcome its poor water dispersibility. The functionalized h-BNF membranes demonstrated fast solvent transport and good ion-rejection properties, based on molecular sieving mechanism. [15] However, these membranes do not show charge-based (Donnan) exclusion, due to the amine functionalization of the 2D layered nanomaterials have attracted considerable attention for their potential for highly efficient separations, among other applications. Here, a 2D lamellar membrane synthesized using hexagonal boron nitride nanoflakes (h-BNF) for highly efficient ion separation is reported. The ion-rejection performance and the water permeance of the membrane as a function of the ionic radius, ion valance, and solution pH are investigated. The nonfunctionalized h-BNF membranes show excellent ion rejection for small sized salt ions as well as for anionic dyes (>97%) while maintaining a high water permeability, â1.0 Ă 10 â3 L m m â2 h â1 bar â1...