Lignosulfonate-based adsorbents for selective Mg²⁺ removal from Mg²⁺/Li⁺ mixture in water with high efficiency and reusability

This study focuses on the removal of Mg²⁺ from aqueous solutions during Li⁺ recovery. The objective was to capture Mg²⁺ ions by adsorption using based adsorbents of sulfonated lignin (LS) and poly-3-sulfopropyl potassium acrylate (PSPAK). The bio-based adsorbents were obtained in high mass yields by radical polymerization. Their structural composition and porous morphology were corroborated by infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). Lignin reinforces the mechanical properties of the material by increasing its toughness under a constant load, making it a suitable choice for adsorption processes. During the experiments, approximately 98.0 % of Mg²⁺ ions were removed using aqueous solutions with Mg²⁺/Li⁺ mass ratios (mg/L) of 0.5/1, significantly reducing the magnesium concentration in the aqueous medium. More than 90.0 % of Li⁺ was recovered from solution in the presence of Mg²⁺ at high ratios 8/1 and 10/1. The adsorbents retain their affinity for Mg²⁺ ions even in the presence of interfering ions such as Li⁺, Na⁺, K⁺, and Ca²⁺. Furthermore, they can be reused for more than five consecutive adsorption–desorption cycles, without a significant decrease in their adsorption efficiency and structural stability. The computational results indicate that the presence of Mg²⁺ ions has an impact on the stability and conformational behavior of the system. The polymer-Mg²⁺/Li⁺ system demonstrates a stronger affinity for Mg²⁺ compared to Li⁺. The increased adsorption energy and greater stability of the polymer-[Mg²⁺] combination provide evidence for the improved selectivity and effectiveness of polymer-based adsorbents in removing Mg²⁺ in the presence of Li⁺.