Selective separation and recovery of strontium from seawater reverse osmosis brine using sequential oxalate precipitation and ion exchange

Seawater desalination effectively produces fresh water, yet brine byproducts raise significant environmental concerns. Strontium, a valuable resource in brine, is increasingly sought after due to diminishing terrestrial mineral sources. However, its recovery is challenging because strontium ion (Sr 2+ ) and calcium ion (Ca 2+ ) exhibit similar physicochemical behaviors in the aqueous phase, necessitating advanced recovery technologies. This study proposes an integrated recovery process for seawater reverse osmosis (SWRO) brine combining oxalate-based precipitation and ion exchange. In the pretreatment stage, NaOH and oxalate were employed to sequentially remove magnesium and calcium interferences. Specifically, oxalate addition at a 1.6 molar ratio relative to Ca 2+ preferentially precipitated calcium, drastically increasing the [Sr 2+ ]/[Ca 2+ ] ratio from 0.016 to 2.5, indicating a notable enhancement in Sr 2+ relative to Ca 2+ . For the enrichment of Sr 2+ from the pretreated solution, an ion-exchange column was employed. Two consecutive concentration cycles yielded a Sr-enriched solution exceeding 4 g/L. A post-IEX Ca 2+ removal step further reduced residual calcium, achieving a maximum [Sr 2+ ]/[Ca 2+ ] ratio of 28.6, with a Sr 2+ concentration of approximately 2 g/L. These results demonstrate that an integrated process using sequential oxalate-based precipitation and ion exchange achieves high efficiency in Ca 2+ removal, enhancing the overall feasibility of the Sr 2+ recovery process.