Enhancing stability and ionic sieving efficiencies of GO membranes via copper ion crosslinking and tannic acid intercalation

Graphene oxide (GO) membranes have exhibited excellent molecular sieving properties in several areas, such as water treatment and gas separation, but unsatisfactory permeability and/or selectivity for ionic separation. Meanwhile, the instability of pure GO membranes in acidic solution has significantly impeded the ionic sieving applications. In this study, a kind of GO composite membranes (GO-Cu-TAa) with exceptional aqueous stability and superior ionic sieving properties are successfully prepared by employing a unique natural deposition strategy to introduce copper ions and tannic acid (TA). Based on the synergistic effect of the two cross-linking agents of copper ion and TA, the structural stability and ionic sieving performance of GO-Cu-TAa membranes are obviously enhanced. The resultant membrane with 20% TA loading displays excellent ionic sieving performance in a mixed solution containing K+, Mg2+ and Cr3+ three metal cations, and the selectivities of K+/Mg2+ and K+/Cr3+ are 46.43 and 185.32, respectively. Especially, the permeation rate of K+ ion reaches up to 0.75 mol m-2 h-1. Meanwhile, the ionic sieving property of such GO membranes possesses outstanding reusability and long-term stability. Such a membrane can also effectively separate mono-/multivalent metal cations from the mixed solution contained of the nine metal cations. The selectivities of monovalent/divalent metal cations (such as K+/Pb2+, K+/Ca2+, K+/Mg2+) are as high as 7.69–51.49, and those of monovalent/trivalent metal cations (such as K+/Fe3+, K+/Cr3+, K+/Al3+) are as high as 247.62–346.67. These results indicate that the GO-Cu-TAa membranes fabricated by the method of natural deposition strategy are expected to be used in the practical applications of lithium extraction from Salt Lake and wastewater purification, etc.