Microwave-assisted low-temperature hydrothermal treatment of red seaweed (Gracilaria lemaneiformis) for production of levulinic acid and algae hydrochar

In this study, red seaweed (Gracilaria lemaneiformis) food waste with high carbohydrate content was valorized into levulinic acid (LA) and algae hydrochar through microwave-assisted low-temperature hydrothermal treatment in dilute acid solution. Various parameters including treatment temperature (160–200 °C), reaction time (1–40 min), acid concentration (0–0.6 M), and biomass-to-liquid ratio (1%–10%, w/v) were examined. The energy efficiency and carbon recovery of the proposed process were investigated. Under the experimental conditions of 5% (w/v) biomass loading, 0.2 M H₂SO_4, 180 °C, and 20 min, the highest levulinic acid yield of 16.3 wt% was produced. The resulting hydrochar showed approximately 45–55% energy yield and higher heating values of 19–25 MJ kg⁻¹. The energy efficiency of the present study (1.31 × 10⁻⁶ g LA/J) was comparable to those of the conventional hydrothermal treatment of lignocellulosic biomass, while the reaction time (20 min) was much shorter with a high carbon recovery (73.3%).