Selective Adsorption of Magnesium Using Lithium Carboxylate-Based Covalent Organic Frameworks

Magnesium and lithium exhibit similar behaviors in aqueous solutions, making their separation from each other in saltlake brine challenging. Here, we report the design and synthesis of four lithium carboxylate-based covalent organic frameworks (COFs), ATSA-1 through ATSA-4, that selectively adsorb Mg2+ ions over Li+. Adsorption performance was investigated under varying initial Mg2+ concentrations, adsorbent dosages, and contact times. Among the COFs, ATSA-4 demonstrated the highest Mg2+ adsorption capacity, reaching 19 mg g–1. Adsorption data aligned with the Langmuir isotherm model, while kinetic analysis indicated a pseudo-second-order model best described Mg2+ uptake. Regeneration tests revealed that hydrochloric acid at pH 3 efficiently desorbed Mg2+, enabling the COF reusability. Additionally, a COF-supported ultrafiltration bed yielded a Mg2+ separation flux of 19 g h–1 m–2. The ATSA-COF series further displayed a high selectivity for Mg2+ in mixed Mg2+/Li+ solutions.