The demand for lithium has been steadily growing in recent years due to the boom of electric cars. High purity lithium is commonly used in the manufacture of battery grade lithium electrolyte. Sulfate residuals originating from acid leaching of lithium ores must be limited to below 20 mg·L−1 during refining. There are methods to remove sulfate such as membrane processing and chemical precipitation using barium salts. However, membrane separation is unable to achieve the required purity while chemical precipitation often causes secondary contamination with barium and requires extra filtration processes that lead to increased processing costs. In this study, we developed a polymeric matrix entrapped with barium ions as a novel adsorbent to selectively adsorb sulfate in aqueous solutions. The adsorbent was prepared by dropwise injection method where alginate droplets were crosslinked with barium to form hydrogel microcapsules. In a typical scenario, the microcapsules had a diameter of 3 mm and contained 5 wt-% alginate. The microcapsules could successfully reduce sulfate concentration in a solution from 100 to 16 mg·L−1, exceeding the removal target. However, the microcapsules were mechanically unstable in the presence of an excess amount of sulfate. Hence, calcium ions were added as a secondary crosslinking agent to improve the integrity of the microcapsules. The two-step Ca/Ba@alginate microcapsules showed an exceptional adsorption performance, reducing the sulfate concentration to as low as 0.02 mg·L−1. Since the sulfate selective microcapsules can be easily removed from the aqueous system and do not result in secondary barium contamination, these Ca/Ba@alginate adsorbents will find applications in ultra-refining of lithium in industry.
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