Integrated biorefinery approach for seaweed Sargassum fusiforme: A step towards green circular economy

Abstract

This study aims to maximize the utilization of Sargassum fusiforme (SF), embedding the concept of a green circular economy. We developed a green extraction process using water and ultrasound assistance to extract high-value SF polysaccharides (SFPS). A 2k factorial design coupled with center points evaluated the curvature effect in the extraction process. Optimal extraction conditions were identified to be a mass-liquid ratio of 1:59 (w/v), an extraction temperature of 70 °C and time of 88 min, yielding 15.30 wt% SFPS, validated through experiments. SFPS demonstrated significant antioxidant capacity, indicated by their ferric ion reduction ability and superoxide anion scavenging ability. Post-extraction, the waste ethanol solution and SF residue were employed as feedstock for hydrothermal liquefaction (HTL), aiming to produce biocrude oil. We explored various parameters affecting the yield and quality of the resultant biocrude. The optimal HTL conditions for SF residue included a temperature of 280 °C and a reaction time of 45 min. The biocrude obtained using the waste solution exhibited higher carbon content (69.83%) and higher heating value (33.42 MJ/kg) compared to freshwater, indicating enhanced energy recovery (87.65% vs. 15.39%). Also, the presence of ethanol in waste solution facilitated a decrease in fatty acid content from 48.50% to 24.50%, while simultaneously increasing ester content from 6.37% to 35.54%, presumably due to the trans-esterification of fatty acids into esters. Utilizing the waste ethanol solution as HTL processing medium also led to a much lower greenhouse gas emissions (1714.0 kg CO2-eq) compared to that of freshwater (7735.4 kg CO2-eq) as evidenced by the life cycle assessment. This study's innovative approach not only valorizes the waste streams from SFPS extraction but also adds value by generating high-quality biocrude oil, offering practical solutions for waste reduction and resource recovery, fostering sustainable development and circular economy initiatives.