Isa Hiemstra

The transition towards a circular bioeconomy requires sustainable technologies capable of converting renewable biomass into multiple high-value products while reducing chemical consumption, waste generation, and environmental impact. Brown seaweed represents an attractive feedstock because it contains a wide range of valuable compounds, including alginate, fucoidan, laminarin, mannitol, and pigments such as fucoxanthin. However, conventional industrial alginate production relies on harsh acid and alkaline treatments, extensive water consumption, and multiple downstream processing steps, while recovering only a single product. Deep eutectic solvents (DES) have emerged as promising alternative solvents due to their tuneable physicochemical properties, selective extraction capability, and potential for solvent recycling. Nevertheless, although DES-based extraction has been widely investigated, product recovery and solvent recycling remain major bottlenecks that currently limit industrial implementation.

The aim of this thesis was to evaluate the feasibility of a circular DES-based multiproduct seaweed biorefinery by integrating computational solvent screening, extraction process development, product recovery, solvent recycling, and techno-economic and environmental assessment. Particular emphasis was placed on developing recovery strategies that enable both efficient alginate isolation and repeated DES reuse.

Chapter 2 investigated the molecular interactions between alginate and a wide range of DES using the computational screening tool COSMO-RS. Molecular descriptors including activity coefficients, capacity, selectivity and partition coefficients were evaluated to identify DES with favourable affinity towards alginate while maintaining selectivity over other seaweed polysaccharides. The results demonstrated that COSMO-RS is a valuable first screening tool for narrowing down promising solvent candidates prior to experimental validation.

Chapter 3 experimentally evaluated the laboratory extraction performance of selected DES. Response surface methodology was subsequently used to optimise the extraction conditions. DES achieved higher alginate extraction yields compared to conventional alkaline processing while preserving polymer characteristics. However, extraction performance did not always correspond directly with the computational predictions, demonstrating that extraction efficiency depends not only on solvent–solute interactions but also on the ability to interact with the seaweed biomass matrix.

Chapter 4 investigated pressure-driven membrane filtration as a proof-of-concept recovery strategy for separating alginate from DES. A combination of ultrafiltration, diafiltration and nanofiltration successfully recovered alginate from a simplified DES model solution, demonstrating the technical feasibility of membrane-based DES recovery. However, viscosity-related limitations, high water usage and incomplete DES retention highlighted the limitations of this approach for circular processing of complex seaweed extracts.

Chapter 5 explored three-phase partitioning (TPP) as an integrated extraction and recovery strategy for multiproduct seaweed biorefinery. A fatty acid-based hydrophobic DES simultaneously extracted pigments while pretreating the seaweed for alginate recovery in the aqueous phase. The hydrophobic DES could be reused over multiple extraction cycles, although alginate recovery yields did fluctuate upon reusing the DES and fucoxanthin remained dissolved within the DES phase. The chapter demonstrated the potential of integrating multiple process functions within a one-pot system.

Chapter 6 developed a thermo-separating aqueous two-phase system (ATPS) combining DES with a temperature-responsive EOPO copolymer. This approach enabled cyclic extraction, alginate recovery, and recycling of both DES and polymer phases. Although successful recycling was achieved, the process required multiple separation steps and relatively complex phase management, motivating the search for simpler recovery concepts.

Chapter 7 introduced temperature-responsive deep eutectic solvents (TRDES) as a one-solvent platform for alginate extraction, recovery and solvent recycling. The study demonstrated that reversible temperature-induced phase transitions can facilitate alginate recovery while allowing repeated solvent reuse. However, the optimal solvent formulation relied on amine-based components with limited suitability for food-related applications, and product recovery still required dialysis. The results indicated that the switching concept itself is promising, whereas future work should focus on developing greener temperature-responsive solvent formulations.

Chapter 8 combined the advantages of previous recovery systems (TPP and ATPS) by developing a DES–salt aqueous two-phase system with direct interfacial precipitation of alginate. This strategy enabled simultaneous product recovery and DES recycling while reducing downstream process complexity. Among all recovery strategies investigated, this process provided the most favourable balance between recovery efficiency, solvent circularity, processing time and operational simplicity.

Chapter 9 expanded the role of DES beyond single-product extraction by developing a biphasic fatty acid–betaine:urea (FABU) system for one-pot multiproduct extraction. The biphasic DES simultaneously recovered alginate and fucoxanthin while improving the fucoxanthin yield and integrity after FABU pretreatment. This chapter demonstrated that DES can function not only as extraction solvents but also as multifunctional processing platforms enabling integrated biomass fractionation.

Chapter 10 evaluated the techno-economic feasibility and environmental performance of the biphasic DES process through techno-economic assessment (TEA) and life cycle assessment (LCA). The analyses showed that DES production constitutes the largest contributor to both operating costs and environmental impacts. Nevertheless, the process becomes increasingly attractive when DES recycling is implemented, solvent consumption is reduced, fucoxanthin yields are increased, and higher-value alginate applications are considered. Compared with conventional alkaline extraction, the integrated DES process demonstrated improved environmental performance while highlighting the importance of solvent recycling for economic viability.

Chapter 11 integrates the findings of the preceding chapters and identifies three overarching conclusions. First, alginate recovery from DES is technically feasible but remains the primary process engineering bottleneck, requiring improved mechanistic understanding of DES–alginate interactions during repeated recycling. Second, DES selection should evolve from purely solvent–solute screening towards integrated dissolution–disruption design by combining molecular affinity with biomass interaction modelling. Third, DES should no longer be viewed solely as extraction solvents but as multifunctional process platforms capable of integrating biomass disruption, selective extraction, product stabilisation, fractionation and solvent recycling within a single process.

Overall, this thesis demonstrates that DES provide a promising foundation for the development of circular seaweed biorefineries. Although challenges remain in achieving robust product recovery and long-term solvent recycling, the results show that DES-based processing can intensify conventional extraction processes while creating opportunities for integrated multiproduct recovery. Future developments should therefore focus on designing DES as multifunctional processing environments that enable sustainable and economically viable biomass valorisation.