Reduced Graphene Oxide Forward Osmosis Membranes for Lithium Brine Enrichment with Ultraviolet–Visible Spectroscopy for Monitoring

Abstract

The Smackover Formation brines in southern Arkansas contain a large quantity of lithium, a critical resource for electric vehicle batteries and the global energy transition. To extract the lithium, efficient downstream enrichment technologies are urgently needed. Methods for direct lithium extraction are being explored, followed by further purification and concentration of the lithium salt solution, such as using reverse osmosis, which is energy intensive. Here we use reduced graphene oxide (RGO) membrane-based forward osmosis (FO) as an environment-friendly and near zero-energy input method to concentrate lithium brine. In the FO tests, a saturated NaCl solution serves as a draw solution and either a dilute lithium nitrate (LiNO₃) solution (50.4  mM) or an artificial lithium brine (1.00 M NaCl + 12.0  mM LiNO₃) as a feed solution, where LiNO₃ is selected to mimics the typical LiCl component in lithium brine. Because nitrates have a unique absorption feature at ∼300  nm, their concentrations in both the feed and draw solutions can be monitored by a facile ultraviolet–visible (UV–Vis) absorption spectral method. For the dilute LiNO₃ solution, a rejection rate is determined to be 97.9 ± 0.1%, with a water flux of 6.2 ± 0.2 L/hm². For the artificial brine, a rejection rate of 88.4 ± 0.1% and a water flux of 5.0 ± 0.2 L/hm² are observed. With further optimization, this forward osmosis approach could provide a more energy-efficient method for lithium salt enrichment, supporting sustainable lithium extraction from Smackover brines.

Graphical abstract

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