Integrated Valorization of Macroalgae in a Recyclable Ionic Liquid: Hydrothermal Production of Formic and Levulinic Acids with Product Recovery and Biohydrogen Generation

Marine macroalgae are drawing growing interest as underutilized feedstocks for chemicals and fuels and as high-productivity, carbon-assimilating biomass that could contribute to carbon capture and sequestration strategies. Here, we investigate an ionic-liquid-assisted hydrothermal conversion of Gracilaria verrucosa (Rhodophyta; red alga) to formic acid (FA) and levulinic acid (LA) using 1-methyl-3-(4-sulfobutyl)imidazolium hydrogen sulfate ([SBMim][HSO4]). Under optimized conditions (210 °C, 5 h), FA and LA were produced in 18% (3.0 g/L) and 38% (6.36 g/L) yields, respectively. In a 10 g scale experiment, FA was recovered as an aqueous solution in 4% isolated yield via vacuum distillation, while LA was obtained as a solid in 29% isolated yield via ethyl acetate extraction. The ionic liquid was reconditioned using an activated carbon–Celite–S108H protocol and reused over ten consecutive cycles while maintaining FA/LA yields. Preliminary PMI/E-factor and break-even analyses further clarified the sustainability advantages and lab-scale economic limitations of the process. Importantly, the recovered FA was evaluated as a substrate for biohydrogen production using the hyperthermophilic archaeon Thermococcus onnurineus NA1, which couples formate oxidation to growth. Macroalgae-derived FA supported a hydrogen production rate comparable to that obtained with chemical-grade FA in batch culture. Overall, this work demonstrates a recyclable, ionic-liquid-assisted hydrothermal platform for converting marine macroalgae into value-added organic acids, integrating product recovery with downstream biohydrogen generation and underscoring the potential of macroalgal biorefineries for sustainable chemicals and energy.