Extraction of Salicylic Acid Using Sustainable ATPSs and Respective Immobilization as API-IL at Small Pilot Scale

Active pharmaceutical ingredients (APIs), a class of emerging pollutants present in residual waters, are often found in hospital effluents and are known to pose profound consequences to aquatic environments, such as the feminization of fish, for which finding sustainable extractive methodologies for these chemical species is a must. Aqueous two-phase systems (ATPSs) constitute a liquid–liquid extraction technique with low environmental impact that often results in the use of a salting-out agent, such as tripotassium citrate (K₃Citrate), to separate two otherwise miscible phases. To increase the solubility of the extracted active pharmaceutical ingredients while preserving their specific properties, one can immobilize them as novel ionic liquids (ILs), and these complex chemical compounds are known as active pharmaceutical ingredients-ionic liquids (API-ILs). In this work, salicylic acid (Sa), a pharmaceutical component with wide medical and industrial applications, was extracted in ATPSs and immobilized as cholinium salicylate. Specifically, the systems {ethyl lactate (1) + Na₃Citrate or K₃Citrate or Na₂Tartrate or NaKTartrate or Na₂Succinate (2) + water (3)} were used at 298.15 K and 0.1 MPa. To advance large-scale application, the extraction studies were performed at small pilot scale using 500 g of ATPS and with the compositions of the shortest and longest tie-lines, having obtained the largest partition coefficient (K = (10 ± 2) × 10¹) and extraction efficiency (E/ % = 98.2 ± 0.7%) for the longest tie-lines of the ATPSs based on K₃Citrate and Na₂Tartrate, respectively. Then, the ATPS based on K₃Citrate was used to immobilize salicylic acid, and the structures of the synthesized ionic liquids (cholinium salicylate and cholinium citrate) were validated using FTIR measurements and IR predictions from computational chemistry.