Conventional alginate extraction from brown seaweed typically relies on harsh, non-recyclable chemicals, limiting process sustainability. This study presents temperature-responsive deep eutectic solvents (TRDES) as circular, recyclable extractants for alginate recovery. Using computational screening with COSMO-RS and experimental validation of TRDES affinity and alginate partitioning, TRDES1 (o-cresol: ethanolamine) was identified as the most promising combination, and was optimised and reused over eight cycles, yielding up to 55.6 ± 14.4 mg/g DW. COSMO-RS modelling validated the observed increase in extraction efficiency over successive cycles, showing enhanced partition coefficients and reduced Gibbs free energy of transfer with reuse. The process enabled mild extraction of functional alginate with increasing efficiency over the cycles. The main solvent parameters for TRDES design found to govern extraction and recyclability were capacity (C), partition coefficient (K), and Gibbs free energy (ΔG). Optimal performance was achieved with moderate TRDES–water capacity (1.27 × 101 to 3.15 × 101), low TRDES capacity (<1.2), and K > 1. This work establishes a theoretical framework with design rules for future TRDES development based on computational and experimental analysis and highlights the need for novel, biocompatible TRDES systems. As demonstrated, combining computational screening with these design principles enables the use of recyclable solvents. Incorporating natural compounds into TRDES design enhances both process efficiency and sustainability, facilitating the integration of DES technologies into circular biorefineries and supporting environmentally responsible biomass valorisation.

This study evaluates the impact of using deep eutectic solvents (DES), specifically betaine-urea, as a novel extraction technology for alginate from brown seaweed and its effect on the techno-functionality of the final product. Ten eutectic mixtures were assessed, with betaine-urea (BU) demonstrating the highest performance, yielding 14.9 ± 2.0 % of the total amount of alginate in the first extraction step (BU-1). Although this yield was lower than the conventional acid-alkaline (AK) method (37.6 ± 4.1 %), a sequential extraction combining water (BU-2) and alkaline conditions (BU-3) achieved higher overall yields for the BU method. The study highlights significant differences in the chemical and rheological properties of alginates extracted with BU compared to those obtained via traditional methods. BU-extracted alginates exhibited the characteristic shear-thinning behaviour of alginate solutions but with markedly higher apparent viscosity at low shear rates. BU-2 displayed gel-like behaviour (tan δ = 0.1) with a consistency index (K) 62 times higher than the acid-alkaline extract. Chemical analysis revealed a higher fraction of high-molecular-weight alginates in BU-2 and BU-3, partially explaining their increased viscosity. However, the role of betaine-urea in shaping the alginate structure and function should be further studied. This research underscores the potential of DES technology in enhancing the functionality of alginate, offering a potentially more sustainable alternative to mineral acid-base methods.

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.