Researchers from Pohang University of Science and Technology, Griffith University and King Khalid University have developed a graphene-based nanofiltration system that can selectively extract lithium ions from magnesium‑rich brines using sunlight as the driving force. The approach combines edge‑functionalized graphene nanoribbons (GNRs) with photothermally reduced graphene oxide (PrGO), forming sub‑nanometer ion‑coordination channels that enable efficient lithium transport while rejecting competing ions such as magnesium.
Recovering lithium from natural brines is difficult because lithium typically exists at much lower concentrations than other dissolved salts. In South American salt‑flat brines, for example, magnesium concentrations can exceed lithium by ratios of 20:1 or higher. The challenge arises from the similar chemical behavior of the ions, even though their hydration energies differ significantly. Magnesium ions bind water molecules roughly four times more strongly than lithium ions. The new membrane exploits this difference by creating functionalized transport pathways that encourage lithium ions to partially shed their hydration shells and migrate through the membrane while magnesium remains strongly hydrated and effectively blocked.
The membrane architecture relies on edge‑functionalized graphene nanoribbons, produced by unzipping multi‑walled carbon nanotubes to expose dense rows of oxygen‑ and nitrogen‑containing groups along the ribbon edges. These groups act as temporary coordination sites where lithium ions can adsorb, dehydrate, and hop between adjacent sites along the channel. Photothermally reduced graphene oxide serves as a structural scaffold that stabilizes the membrane and prevents swelling in highly saline environments. Density functional theory calculations show strong interfacial coupling between the two components with an interaction energy of −3.45 eV, accompanied by significant charge redistribution that lowers the lithium migration barrier to approximately 0.18–0.31 eV. Competing ions face substantially higher barriers, preventing efficient transport.
The optimized membrane, containing 15 wt% PrGO (GNRs/PrGO15), demonstrated a lithium permeation rate of 0.253 mol m⁻² h⁻¹ in static diffusion tests and achieved a Li⁺/Mg²⁺ selectivity of 21. The researchers further enhanced separation by integrating the membrane with a photothermal substrate that absorbs sunlight and converts it into localized heat. The substrate captures about 97% of incident solar radiation, raising the brine temperature near the membrane surface to roughly 48 °C under two‑sun illumination. This localized heating lowers solution viscosity and promotes lithium dehydration, further increasing the difference in transport rates between lithium and magnesium.
When tested with simulated brine resembling Bolivia’s Uyuni salt flat (total salinity above 350 g/L and an initial Mg²⁺/Li⁺ ratio of 19.8), the solar‑driven system significantly improved separation performance. Under two‑sun irradiation, the magnesium‑to‑lithium ratio in the permeate dropped to 0.7, corresponding to a 28‑fold lithium enrichment. The resulting lithium carbonate reached about 97% purity, approaching battery‑grade quality. Stability tests over 48 hours of repeated cycles showed consistent evaporation rates and stable membrane performance without significant structural degradation.
By combining selective ion‑coordination channels with photothermal solar operation, the GNRs/PrGO membrane platform demonstrates a potential pathway toward scalable lithium recovery systems that operate without electrical pumps or external power. If successfully scaled, such solar‑driven filtration technologies could offer a faster and more water‑efficient alternative to conventional evaporation ponds currently used in lithium brine extraction.
