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.

Conventional alginate extraction methods from brown seaweed typically rely on harsh chemicals that are not reused, and valuable pigments are lost during this process. This study applied a novel approach utilising reusable natural deep eutectic solvents (DES) in three-phase partitioning (TPP) to simultaneously extract alginate and pigments from Saccharina latissima . The hydrophobic DES effectively released alginate from the algal cell wall in the aqueous phase and served as a solvent for pigment extraction. Computational screening confirmed that all selected DES had an affinity for pigments chlorophyll a and fucoxanthin, while alginate extraction confirmed their role in disrupting the algal cell wall. Extraction conditions were optimised, resulting in an alginate yield of 101.8 ± 3.1 mg/g DW compared to 55.3 ± 14.1 mg/g DW for conventional alkaline extraction. According to physicochemical characterisation through FT-IR and M/G ratio (mannuronic to guluronic) analysis, the extracted alginate was comparable to that obtained via alkaline extraction, exhibiting similar functional groups and M/G ratios. The DES was reused successfully, showing that it could be reused for up to seven extraction cycles, during which pigments accumulated. After the seventh cycle, alginate yield declined, likely due to partial transfer into the DES phase, possibly driven by reverse micelle formation in the system. This study highlights a novel, mild multiproduct approach of a DES-based TPP system, enhancing economic feasibility by employing gentler and quicker extraction conditions. It facilitates the concurrent recovery of alginate and pigments while allowing for the repeated reuse of the DES.