Enhanced lithium extraction from high-sodium brines: Modification of a manganese-based ion sieve using hydroxylated graphene and graphene oxide

Salt-lake brines have emerged as a promising source of lithium; however, the separation of Li⁺ ions from other coexisting ions remains challenging. To surmount this issue, a three-dimensional wrinkled membrane that was based on a H_1.6Mn_1.6O₄ (HMO) ion sieve and comprised graphene oxide (GO) and hydroxylated graphene (GOH) was fabricated in this study. The designed structure provided abundant channels for ion migration. The hydroxyl groups of graphene allowed the membrane to exhibit improved Na⁺–Li⁺ sieving ability. The lithium adsorption capacity of HMO-GOH/GO (20.6 mg/g) was considerably higher than that of HMO (8.9 mg/g) in a low-concentration lithium solution. The adsorption capacity and separation coefficient of HMO-GOH/GO in a high-sodium brine of Na/Li = 49:1 (48.0 mg/g and 47.1) were higher than those of HMO (21.6 mg/g and 29.3, respectively). The adsorption capacity of HMO-GOH/GO remained at 90.8 % of its initial value after 10 adsorption–desorption cycles, thus demonstrating excellent cyclic stability. HMO/GO-GOH shows higher specific capacitance than HMO based on the cyclic voltammetry results. The density functional theory calculations on adsorption energy of Li⁺·4H₂O and the energy barrier across the GOH pore confirm the Na⁺/Li⁺ sieving capability. The adsorption mechanism was studied by in situ Raman spectroscopy, verifying the formation of LiO bond during lithium adsorption. Overall, this study provides guidance in the pursuit of a remedy for NaLi separation.