Carboxylated lignin-based adsorbents for the efficient separation of Mg²⁺ from aqueous solutions containing Li

This study focuses on the preparation of bio-based adsorbents composed of carboxylated lignin (LC) and potassium polyacrylate (PAS) to remove Mg²⁺ from aqueous solutions in the presence of Li⁺. The successful incorporation of carboxylate groups into lignin was achieved, which was corroborated by FT-IR. Acid-base titration allowed the quantification of 11.7 ± 0.016 mmol of -COO^- groups per gram of LC. Radical polymerization was used to obtain the bio-based adsorbents. The structural composition was corroborated by FT-IR and their porous morphology was evaluated by SEM and BET isotherms. The incorporation of LC improved the adsorption capacity of the material for Mg²⁺, increasing it by 60 %. The adsorption process was more accurately described by the PFO and Redlich Peterson kinetic models. The effective adsorption and the ion-exchange adsorption mechanism of Mg²⁺ on the adsorbents were confirmed by XPS, FT-IR, and SEM/EDS analyses. Furthermore, the adsorbent systems were shown to be able to adsorb Mg²⁺ in the presence of Li⁺, reducing the concentration ratio from 8:1 to 6:1. However, Mg²⁺ ions showed higher affinity for PAS-20 %-LC adsorbents (α^Mg2+/Li+ = 3.78) as compared to PAS adsorbents (α^Mg2+/Li+ = 0.50), confirming that the incorporation of LC into the adsorbent enhances Mg²⁺ adsorption. A preferential affinity toward divalent ions was observed in multicomponent solutions after the adsorption process. The Mg²⁺ adsorption capacity on PAS-20 %-LC remained almost constant even with increasing ionic strength of the system.