Direct alginate recovery and solvent recycling from deep eutectic solvents via interfacial precipitation in aqueous two-phase systems

Conventional alginate extraction from brown seaweed relies on alkaline treatments that limit sustainability and solvent reuse. In this study, a novel deep eutectic solvent (DES)–salt aqueous two-phase system (ATPS) was developed for the integrated extraction and separation of alginate as a more sustainable alternative. By incorporating the precipitation principle of three-phase partitioning at elevated salt concentrations in an ATPS, alginate was selectively recovered in an intermediate phase between the DES and aqueous salt solution, enabling direct isolation while preserving the solvent for reuse. Salt selection significantly influenced phase formation, with K₃PO₄ exhibiting stronger phase-forming ability than K₂HPO₄ due to its enhanced salting-out effect, whereas alginate recovery was primarily governed by salt concentration. The highest recovery (76.4 ± 0.1%) was achieved using betaine: urea (1,2) with 50 wt% K₃PO₄, with most alginate accumulating in the intermediate phase. The DES maintained stable recovery over six cycles (63–86%), while preserving alginate integrity. These results demonstrate the potential of DES-based ATPS as an efficient platform for simultaneous extraction, separation, and solvent recycling, providing a promising route toward more sustainable and integrated seaweed biorefinery processes.