Separation of alginate and recycling of deep eutectic solvents using temperature-responsive aqueous two-phase systems

Abstract

Deep eutectic solvents (DES) have emerged as green alternative extraction solvents. However, challenges in DES recovery and recycling limit their broader application. In this study, a novel thermo-separating aqueous two-phase system (ATPS) was developed for the continuous, cyclic extraction and separation of alginate from Laminaria digitata using DES and temperature-responsive copolymers. The system enables a novel approach by repeatedly reusing both the DES phase and the EOPO copolymer phase five times, thereby reducing waste generation and enhancing process sustainability. This research demonstrated that DES can be efficiently recycled for ten cycles using temperature-responsive ethylene oxide-propylene oxide (EOPO) copolymers, remaining stable yields. For the extraction and subsequent separation of alginate, three different DESs were evaluated, all of which demonstrated to extract and recover alginate. DESs ChCl:Ethylene glycol and ChCl:Urea exhibited a preference for EOPO1000 (66 and 75 % recovery, respectively), whereas Bet:Urea achieved the highest recovery with EOPO3900 (66 % recovery). Subsequent recycling of the recovered DES showed that DES could be recycled for ten cycles, maintaining stable extraction yields between 74 and 86 mg alginate/g DW and alginate recovery yields of 55–65 %. Furthermore, combined DES and EOPO recycling could be performed for up to five cycles while maintaining an alginate recovery yield between 50 and 65 mg/g DW. This thermo-separating ATPS presents a novel, circular and sustainable approach for DES recycling compared to the non-circular conventional alkaline extraction. This proposed method can be applied in a simple and effective manner to both recover and recycle DES.